TURBO FAQ AND Info...... (1 Viewer)

Introduction

The Turbo Tech Center is designed for those of you looking for a better understanding on turbocharging systems in general. This section also includes information that is found in several reference books.

You've heard the word "turbo" tossed around a lot, especially by performance car enthusiasts. But all you know is that it means an engine has more "oomph" to it than normal. But what exactly is going on underneath that hood? The tutorials sections is divided into 3 sections; Beginner, Intermediate and Advanced.

The Basic tutorials (101) are for those with little or no prior knowledge of turbochargers and their functions.

The Advanced section (102) is designed for those who have a basic knowledge of turbocharging principles and are looking to expand their knowledge.

The Expert tutorials (103) are for the die-hard Gearheads covering topics such as Pressure Ratio, Mass Flow Rates, and Surge Lines. The faint of heart need not apply, enter this section at your own risk.

Information for this FAQ was obtained and compiled from Garrett industries.

Acronyms / Terminology

Acronyms / Terminology

A/R
A/R describes a geometric characteristic of all compressor and turbine housings. It is defined as the inlet cross-sectional area divided by the radius from the turbo centerline to the centroid of that area.

Compressor A/R - Compressor performance is largely insensitive to changes in A/R, but generally larger A/R housings are used to optimize the performance for low boost applications, and smaller housings are used for high boost applications. Usually there are not A/R options available for compressor housings.

Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing. Turbine A/R is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel, causing the wheel to spin faster at lower engine RPMs giving a quicker boost rise. This will also tend to increase exhaust backpressure and reduce the max power at high RPM. Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise, but the lower backpressure will give better high RPM power. When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband.

Choke Line
The choke line is on the right hand side of a compressor map and represents the flow limit. Properly sizing a turbo is important to prevent the compressor from operating past the choke line. When a turbocharger is run deep into choke, turbo speeds increase dramatically while compressor efficiency plunges (very high compressor outlet temps). Additionally, the turbo's durability is compromised by the resulting high thrust loads.

CHRA
(Center Housing & Rotating Assembly)
The CHRA is essentially a turbocharger minus the compressor and turbine housings

Clipped Turbine Wheel
When an angle is machined on the turbine wheel exducer (outlet side), the wheel is said to be "clipped". Clipping causes a minor increase in the wheel's flow capability; however, it dramatically lowers the turbo efficiency. This reduction in efficiency causes the turbo to come up on boost at a later engine speed (ex. increased turbo lag). High performance applications should never use a clipped turbine wheel. All Garrett GT turbos use modern unclipped turbine wheels.

Corrected Air Flow
When plotting actual airflow data on a compressor map, the flow must be corrected to account for different atmospheric conditions that affect air density.

Example:
Air Temperature (Air Temp) - 60°F
Barometric Pressure (Baro) - 14.7 psi
Engine air consumption (Actual Flow) = 50 lb/min
Corrected Flow= Actual Flow SQR([Air Temp+460]/545)/ Baro/13.95
Corrected Flow= 50*SQR([60+460]/545)/ 14.7/13.95 = 46.3 lb/min

Efficiency Contours
The efficiency contours depict the regional efficiency of the compressor stage. When sizing a turbo, it is important to maintain the proposed lugline with a high efficiency range on the map.

Free-Float
A free-floating turbocharger has no Wastegates device. This turbocharger can't control its own boost levels. For performance applications, the user normally must install an external Wastegates.

GT
The GT designation refers to Garrett's state-of-the-art turbocharger line. GT-series turbos use redesigned bearing systems and modern compressor/turbine aerodynamics. These new compressor and turbine wheels represent huge efficiency improvements over the old T2, T3, T3/T4, T04 products. The net result is increased durability, higher boost, and more engine power over the older T-series product line.

On-Center Turbine Housings
On-center turbine housings refer to an outdated style of turbine housing with a centered turbine inlet pad. The inlet pad is centered on the turbo's axis of rotation instead of being tangentially located. Using an on-center housing will significantly lower the turbine's efficiency. This results in increased turbo lag, more backpressure, lower engine volumetric efficiency, and less overall engine power. No Garrett OEM's use on-center housings.

Pressure Ratio
Ratio of absolute outlet pressure divided by absolute inlet pressure
Example:
Intake manifold pressure (Boost) = 12 psi
Pressure drop, intercooler (DPIntercooler) = 2 psi
Pressure drop, air filter (DPAir Filter) = 0.5 psi
Atmosphere (Atmos) = 14.7 psi at sea level
PR = (Boost + DPIntercooler+ Atmos) / (Atmos-DPAir Filter)
PR = (12 + 2 + 14.7) / (14.7 -.5) = 2.02

Surge Line
The surge region, located on the left-hand side of the compressor map, is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur.

Trim
Trim is an area ratio used to describe both turbine and compressor wheels. Trim is calculated using the inducer and exducer diameters.
Example:
Inducer diameter = 88mm
Exducer diameter = 117.5mm
Trim = Inducer2/Exducer2
Trim = 882/117.52 = 56 Trim
As trim is increased, the wheel can support more air/gas flow.

Wastegates
A Wastegated turbocharger includes an integral device to limit turbo boost. This consists of a pneumatic actuator connected to a valve assembly mounted inside the turbine housing. By connecting the pneumatic actuator to boost pressure, the turbo is able to limit its maximum boost output. The net result is increased durability, quicker time to boost, and adjustability of boost.

TURBO 101 (Basic)

Turbo Tech 101 ( Basic )
How a Turbo System Works
Engine power is proportional to the amount of air and fuel that can get into the cylinders. All things being equal, larger engines flow more air and as such will produce more power. If we want our small engine to perform like a big engine, or simply make our bigger engine produce more power, our ultimate objective is to draw more air into the cylinder. By installing a Garrett turbocharger, the power and performance of an engine can be dramatically increased.

So how does a turbocharger get more air into the engine? Let us first look at the schematic below:












The components that make up a typical turbocharger system are:

The air filter (not shown) through which ambient air passes before entering the compressor (1)
The air is then compressed which raises the air’s density (mass / unit volume) (2)
Many turbocharged engines have a charge air cooler (aka intercooler) (3) that cools the compressed air to further increase its density and to increase resistance to detonation
After passing through the intake manifold (4), the air enters the engine’s cylinders, which contain a fixed volume. Since the air is at elevated density, each cylinder can draw in an increased mass flow rate of air. Higher air mass flow rate allows a higher fuel flow rate (with similar air/fuel ratio). Combusting more fuel results in more power being produced for a given size or displacement
After the fuel is burned in the cylinder it is exhausted during the cylinder’s exhaust stroke in to the exhaust manifold (5)
The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure
A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas’ energy to provide the power necessary to drive the compressor

What are the components of a turbocharger?



The layout of the turbocharger in a given application is critical to a properly performing system. Intake and exhaust plumbing is often driven primarily by packaging constraints. We will explore exhaust manifolds in more detail in subsequent tutorials; however, it is important to understand the need for a compressor bypass valve (commonly referred to as a Blow-Off valve) on the intake tract and a Wastegates for the exhaust flow.

Other Components

Blow-Off (Bypass) Valves
The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.
Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure; helping to eliminate the phenomenon of surge.





Wastegates
On the exhaust side, a Wastegates provides us a means to control the boost pressure of the engine. Some commercial diesel applications do not use a Wastegates at all. This type of system is called a free-floating turbocharger.

However, the vast majority of gasoline performance applications require a Wastegates. There are two (2) configurations of Wastegates, internal or external. Both internal and external Wastegates provide a means to bypass exhaust flow from the turbine wheel. Bypassing this energy (e.g. exhaust flow) reduces the power driving the turbine wheel to match the power required for a given boost level. Similar to the BOV, the Wastegates uses boost pressure and spring force to regulate the flow bypassing the turbine.

Internal Wastegates are built into the turbine housing and consist of a “flapper” valve, crank arm, rod end, and pneumatic actuator. It is important to connect this actuator only to boost pressure; i.e. it is not designed to handle vacuum and as such should not be referenced to an intake manifold.




External Wastegates are added to the exhaust plumbing on the exhaust manifold or header. The advantage of external Wastegates is that the bypassed flow can be reintroduced into the exhaust stream further downstream of the turbine. This tends to
improve the turbine’s performance. On racing applications, this Wastegated exhaust flow can be vented directly to atmosphere.







Oil & Water Plumbing

The intake and exhaust plumbing often receives the focus leaving the oil and water plumbing neglected.

Garrett ball bearing turbochargers require less oil than journal bearing turbos. Therefore an oil inlet restrictor is recommended if you have oil pressure over about 60 psig. The oil outlet should be plumbed to the oil pan above the oil level (for wet sump systems). Since the oil drain is gravity fed, it is important that the oil outlet points downward, and that the drain tube does not become horizontal or go “uphill” at any point.

Following a hot shutdown of a turbocharger, heat soak begins. This means that the heat in the head, exhaust manifold, and turbine housing finds it way to the turbo’s center housing, raising its temperature. These extreme temperatures in the center housing can result in oil coking.

To minimize the effects of heat soak-back, water-cooled center housings were introduced. These use coolant from the engine to act as a heat sink after engine shutdown, preventing the oil from coking. The water lines utilize a thermal siphon effect to reduce the peak heat soak-back temperature after key-off. The layout of the pipes should minimize peaks and troughs with the (cool) water inlet on the low side. To help this along, it is advantageous to tilt the turbocharger about 25° about the axis of shaft rotation.

Many Garrett turbos are water-cooled for enhanced durability.

Which Turbocharger is Right for Me or more affectionately known as My Turbo & Me
Selecting the proper turbocharger for your specific application requires many inputs. With decades of collective turbocharging experience, the Garrett Performance Distributors can assist in selecting the right turbocharger for your application.

The primary input in determining which turbocharger is appropriate is to have a target horsepower in mind. This should be as realistic as possible for the application. Remember that engine power is generally proportional to air and fuel flow. Thus, once you have a target power level identified, you begin to hone in on the turbocharger size, which is highly dependent on airflow requirements.

Other important factors include the type of application. An autocross car, for example, requires rapid boost response. A smaller turbocharger or smaller turbine housing would be most suitable for this application. While this will trade off ultimate power due to increased exhaust backpressure at higher engine speeds, boost response of the small turbo will be excellent.

Alternatively, on a car dedicated to track days, peak horsepower is a higher priority than low-end torque. Plus, engine speeds tend to be consistently higher. Here, a larger turbocharger or turbine housing will provide reduced backpressure but less-immediate low-end response. This is a welcome tradeoff given the intended operating conditions.

Selecting the turbocharger for your application goes beyond “how much boost” you want to run. Defining your target power level and the primary use for the application are the first steps in enabling your Garrett Performance Distributor to select the right turbocharger for you.



Journal Bearings vs. Ball Bearings
The journal bearing has long been the brawn of the turbocharger, however a ball-bearing cartridge is now an affordable technology advancement that provides significant performance improvements to the turbocharger.

Ball bearing innovation began as a result of work with the Garrett Motorsports group for several racing series where it received the term the ‘cartridge ball bearing’. The cartridge is a single sleeve system that contains a set of angular contact ball bearings on either end, whereas the traditional bearing system contains a set of journal bearings and a thrust bearing


Journal Bearings



Ball Bearings



Turbo Response – When driving a vehicle with the cartridge ball bearing turbocharger, you will find exceptionally crisp and strong throttle response. Garrett Ball Bearing turbochargers spool up 15% faster than traditional journal bearings. This produces an improved response that can be converted to quicker 0-60 mph speed. In fact, some professional drivers of Garrett ball-bearing turbocharged engines report that they feel like they are driving a big, normally aspirated engine.

Tests run on CART turbos have shown that ball-bearings have up to half of the power consumption of traditional bearings. The result is faster time to boost which translates into better drivability and acceleration.

On-engine performance is also better in the steady-state for the Garrett Cartridge Ball Bearing



Reduced Oil Flow – The ball bearing design reduces the required amount of oil required to provide adequate lubrication. This lower oil volume reduces the chance for seal leakage. Also, the ball bearing is more tolerant of marginal lube conditions, and diminishes the possibility of turbocharger failure on engine shut down.

Improved Rotordynamics and Durability – The ball bearing cartridge gives better damping and control over shaft motion, allowing enhanced reliability for both everyday and extreme driving conditions. In addition, the opposed angular contact bearing cartridge eliminates the need for the thrust bearing commonly a weak link in the turbo bearing system.

Competitor Ball Bearing Options – Another option one will find is a hybrid ball bearing. This consists of replacing only the compressor side journal bearing with a single angular contact ball bearing. Since the single bearing can only take thrust in one direction, a thrust bearing is still necessary and drag in the turbine side journal bearing is unchanged. With the Garrett ball bearing cartridge the rotor-group is entirely supported by the ball bearings, maximizing efficiency, performance, and durability.

Ball Bearings in Original Equipment – Pumping up the MAZDASPEED Protegé’s heart rate is a Garrett T25 turbocharger system. With Garrett technology on board, the vehicle gains increased acceleration without sacrificing overall efficiency and it has received many rave reviews from the world’s top automotive press for it’s unprecedented performance.

TURBO 102 (Advanced)

Turbo Systems 102 (Advanced)
Please thoroughly review and have a good understanding of Turbo Systems 101- Basic prior to reading this section. The following areas will be covered in the Turbo System 102 - Advanced section:

1. Wheel trim topic coverage

2. Understanding turbine housing A/R and housing sizing

3. Different types of manifolds (advantages/disadvantages log style vs. equal length)

4. Compression ratio with boost

5. Air/Fuel Ratio tuning: Rich v. Lean, why lean makes more power but is more dangerous

1. Wheel trim topic coverage

Trim is a common term used when talking about or describing turbochargers. For example, you may hear someone say "I have a GT2871R ' 56 Trim ' turbocharger. What is 'Trim?' Trim is a term to express the relationship between the inducer* and exducer* of both turbine and compressor wheels. More accurately, it is an area ratio.

* The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air exits the wheel.

Based on aerodynamics and air entry paths, the inducer for a compressor wheel is the smaller diameter. For turbine wheels, the inducer it is the larger diameter (see Figure 1.)



Figure 1. Illustration of the inducer and exducer diameter of compressor and turbine wheels




Example #1: GT2871R turbocharger (Garrett part number 743347-2) has a compressor wheel with the below dimensions. What is the trim of the compressor wheel?

Inducer diameter = 53.1mm
Exducer diameter = 71.0mm





Example #2: GT2871R turbocharger (part # 743347-1) has a compressor wheel with an exducer diameter of 71.0mm and a trim of 48. What is the inducer diameter of the compressor wheel?
Exducer diameter = 71.0mm
Trim = 48



The trim of a wheel, whether compressor or turbine, affects performance by shifting the airflow capacity. All other factors held constant, a higher trim wheel will flow more than a smaller trim wheel.
However, it is important to note that very often all other factors are not held constant. So just because a wheel is a larger trim does not necessarily mean that it will flow more.


2. Understanding housing sizing: A/R

A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings. Technically, it is defined as:

the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area (see Figure 2.).



Figure 2. Illustration of compressor housing showing A/R characteristic



The A/R parameter has different effects on the compressor and turbine performance, as outlined below.

Compressor A/R - Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications. However, as this influence of A/R on compressor performance is minor, there are not A/R options available for compressor housings.

Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.

Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.

When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.

Here's a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system.

Imagine two 3.5L engines both using GT30R turbochargers. The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:
1. Engine #1 has turbine housing with an A/R of 0.63
2. Engine #2 has a turbine housing with an A/R of 1.06.

What can we infer about the intended use and the turbocharger matching for each engine?

Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more "fun" to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high backpressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.

Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 where the engine will be operating at high engine speeds most of the time.


3. Different types of manifolds (advantages/disadvantages log style vs. equal length)

There are two different types of turbocharger manifolds; cast log style (see Figure 3.) and welded tubular style (see Figure 4.).



Figure 3. Cast log style turbocharger manifold



Figure 4. Welded tubular turbocharger manifold



Manifold design on turbocharged applications is deceptively complex as there many factors to take into account and trade off

General design tips for best overall performance are to:
Maximize the radius of the bends that make up the exhaust primaries to maintain pulse energy
Make the exhaust primaries equal length to balance exhaust reversion across all cylinders
Avoid rapid area changes to maintain pulse energy to the turbine
At the collector, introduce flow from all runners at a narrow angle to minimize "turning" of the flow in the collector
For better boost response, minimize the exhaust volume between the exhaust ports and the turbine inlet
For best power, tuned primary lengths can be used
Cast manifolds are commonly found on OEM applications, whereas welded tubular manifolds are found almost exclusively on aftermarket and race applications. Both manifold types have their advantages and disadvantages. Cast manifolds are generally very durable and are usually dedicated to one application. They require special tooling for the casting and machining of specific features on the manifold. This tooling can be expensive.

On the other hand, welded tubular manifolds can be custom-made for a specific application without special tooling requirements. The manufacturer typically cuts pre-bent steel U-bends into the desired geometry and then welds all of the components together. Welded tubular manifolds are a very effective solution. One item of note is durability of this design. Because of the welded joints, thinner wall sections, and reduced stiffness, these types of manifolds are often susceptible to cracking due to thermal expansion/contraction and vibration. Properly constructed tubular manifolds can last a long time, however. In addition, tubular manifolds can offer a substantial performance advantage over a log-type manifold.

A design feature that can be common to both manifold types is a " DIVIDED MANIFOLD" , typically employed with " DIVIDED " or "twin-scroll" turbine housings. Divided exhaust manifolds can be incorporated into either a cast or welded tubular manifolds (see Figure 5. and Figure 6.).



Figure 5. Cast manifold with a divided turbine inlet design feature



Figure 6. Welded tubular manifold with a divided turbine inlet design feature



The concept is to DIVIDE or separate the cylinders whose cycles interfere with one another to best utilize the engine's exhaust pulse energy.

For example, on a four-cylinder engine with firing order 1-3-4-2, cylinder #1 is ending its expansion stroke and opening its exhaust valve while cylinder #2 still has its exhaust valve open (cylinder #2 is in its overlap period). In an undivided exhaust manifold, this pressure pulse from cylinder #1's exhaust blowdown event is much more likely to contaminate cylinder #2 with high pressure exhaust gas. Not only does this hurt cylinder #2's ability to breathe properly, but this pulse energy would have been better utilized in the turbine.

The proper grouping for this engine is to keep complementary cylinders grouped together-- #1 and #4 are complementary; as are cylinders #2 and #3.



Figure 7. Illustration of divided turbine housing




Because of the better utilization of the exhaust pulse energy, the turbine's performance is improved and boost increases more quickly.

4. Compression ratio with boost

Before discussing compression ratio and boost, it is important to understand engine knock, also known as detonation. Knock is a dangerous condition caused by uncontrolled combustion of the air/fuel mixture. This abnormal combustion causes rapid spikes in cylinder pressure which can result in engine damage.

Three primary factors that influence engine knock are:

Knock resistance characteristics (knock limit) of the engine: Since every engine is vastly different when it comes to knock resistance, there is no single answer to "how much." Design features such as combustion chamber geometry, spark plug location, bore size and compression ratio all affect the knock characteristics of an engine.
Ambient air conditions: For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases, thus increasing the tendency to knock. Charge air cooling (e.g. an intercooler) addresses this concern by cooling the compressed air produced by the turbocharger
Octane rating of the fuel being used: octane is a measure of a fuel's ability to resist knock. The octane rating for pump gas ranges from 85 to 94, while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for the application. Generally speaking, the more boost run, the higher the octane requirement.
This cannot be overstated: engine calibration of fuel and spark plays an enormous role in dictating knock behavior of an engine. See Section 5 below for more details.

Now that we have introduced knock/detonation, contributing factors and ways to decrease the likelihood of detonation, let's talk about compression ratio. Compression ratio is defined as:



or



where
CR = compression ratio
Vd = displacement volume
Vcv = clearance volume



The compression ratio from the factory will be different for naturally aspirated engines and boosted engines. For example, a stock Honda S2000 has a compression ratio of 11.1:1, whereas a turbocharged Subaru Impreza WRX has a compression ratio of 8.0:1.

There are numerous factors that affect the maximum allowable compression ratio. There is no single correct answer for every application. Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems.

Factors that influence the compression ratio include: fuel anti-knock properties (octane rating), boost pressure, intake air temperature, combustion chamber design, ignition timing, valve events, and exhaust backpressure. Many modern normally-aspirated engines have well-designed combustion chambers that, with appropriate tuning, will allow modest boost levels with no change to compression ratio. For higher power targets with more boost , compression ratio should be adjusted to compensate.

There are a handful of ways to reduce compression ratio, some better than others. Least desirable is adding a spacer between the block and the head. These spacers reduce the amount a "quench" designed into an engine's combustion chambers, and can alter cam timing as well. Spacers are, however, relatively simple and inexpensive.

A better option, if more expensive and time-consuming to install, is to use lower-compression pistons. These will have no adverse effects on cam timing or the head's ability to seal, and allow proper quench regions in the combustion chambers.

5. Air/Fuel Ratio tuning: Rich v. Lean, why lean makes more power but is more dangerous

When discussing engine tuning the 'Air/Fuel Ratio' (AFR) is one of the main topics. Proper AFR calibration is critical to performance and durability of the engine and it's components. The AFR defines the ratio of the amount of air consumed by the engine compared to the amount of fuel.

A 'Stoichiometric' AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric , A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel. The stoichiometric AFR depends on fuel type-- for alcohol it is 6.4:1 and 14.5:1 for diesel.

So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture.

For Example:
15.0:1 = Lean
14.7:1 = Stoichiometric
13.0:1 = Rich

Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.

Let's take a closer look. As the air-fuel mixture is ignited by the spark plug, a flame front propagates from the spark plug. The now-burning mixture raises the cylinder pressure and temperature, peaking at some point in the combustion process.

The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.

There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power.

For further in-depth calculations of pressure ratio, mass flow, and turbocharger selection, please read Turbo Systems 103 Expert tutorial.

TURBO 103 (Expert)

Turbo 103
This article is a bit more involved and will describe parts of the compressor map, how to estimate pressure ratio and mass flow rate for your engine, and how to plot the points on the maps to help choose the right turbocharger. Have your calculator handy!!

1 Parts of the Compressor Map:
◊ The compressor map is a graph that describes a particular compressor’s performance characteristics, including efficiency, mass flow range, boost pressure capability, and turbo speed. Shown below is a figure that identifies aspects of a typical compressor map:



◊ Pressure Ratio

Pressure Ratio ( ) is defined as the Absolute outlet pressure divided by the Absolute inlet pressure.



Where:
= Pressure Ratio
P2c = Compressor
Discharge Pressure
P1c = Compressor Inlet Pressure
It is important to use units of Absolute Pressure for both P1c and P2c. Remember that Absolute Pressure at sea level is 14.7 psia (in units of psia, the a refers to “absolute”). This is referred to as standard atmospheric pressure at standard conditions.
Gauge Pressure (in units of psig, the g refers to “gauge”) measures the pressure above atmospheric, so a gauge pressure reading at atmospheric conditions will read zero. Boost gauges measure the manifold pressure relative to atmospheric pressure, and thus are measuring Gauge Pressure. This is important when determining P2c. For example, a reading of 12 psig on a boost gauge means that the air pressure in the manifold is 12 psi above atmospheric pressure. For a day at standard atmospheric conditions,


12 psig + 14.7 psia = 26.7 psi absolute pressure in the manifold





The pressure ratio at this condition can now be calculated:


26.7 psia / 14.7 psia = 1.82
However, this assumes there is no adverse impact of the air filter assembly at the compressor inlet.
In determining pressure ratio, the absolute pressure at the compressor inlet (P2c) is often LESS than the ambient pressure, especially at high load. Why is this? Any restriction (caused by the air filter or restrictive ducting) will result in a “depression,” or pressure loss, upstream of the compressor that needs to be accounted for when determining pressure ratio. This depression can be 1 psig or more on some intake systems. In this case P1c on a standard day is:


14.7psia – 1 psig = 13.7 psia at compressor inlet


Taking into account the 1 psig intake depression, the pressure ratio is now:


(12 psig + 14.7 psia) / 13.7 psia = 1.95.


That’s great, but what if you’re not at sea level? In this case, simply substitute the actual atmospheric pressure in place of the 14.7 psi in the equations above to give a more accurate calculation. At higher elevations, this can have a significant effect on pressure ratio.
For example, at Denver’s 5000 feet elevation, the atmospheric pressure is typically around 12.4 psia. In this case, the pressure ratio calculation, taking into account the intake depression, is:

(12 psig + 12.4 psia) / (12.4 psia – 1 psig) = 2.14
Compared to the 1.82 pressure ratio calculated originally, this is a big difference.


As you can see in the above examples, pressure ratio depends on a lot more than just boost.

◊ Mass Flow Rate
Mass Flow Rate is the mass of air flowing through a compressor (and engine!) over a given period of time and is commonly expressed as lb/min (pounds per minute). Mass flow can be physically measured, but in many cases it is sufficient to estimate the mass flow for choosing the proper turbo.
Many people use Volumetric Flow Rate (expressed in cubic feet per minute, CFM or ft3/min) instead of mass flow rate. Volumetric flow rate can be converted to mass flow by multiplying by the air density. Air density at sea level is 0.076lb/ft3
What is my mass flow rate? As a very general rule, turbocharged gasoline engines will generate 9.5-10.5 horsepower (as measured at the flywheel) for each lb/min of airflow. So, an engine with a target peak horsepower of 400 Hp will require 36-44 lb/min of airflow to achieve that target. This is just a rough first approximation to help narrow the turbo selection options.
◊ Surge Line
Surge is the left hand boundary of the compressor map. Operation to the left of this line represents a region of flow instability. This region is characterized by mild flutter to wildly fluctuating boost and “barking” from the compressor. Continued operation within this region can lead to premature turbo failure due to heavy thrust loading.
Surge is most commonly experienced when one of two situations exist. The first and most damaging is surge under load. It can be an indication that your compressor is too large. Surge is also commonly experienced when the throttle is quickly closed after boosting. This occurs because mass flow is drastically reduced as the throttle is closed, but the turbo is still spinning and generating boost. This immediately drives the operating point to the far left of the compressor map, right into surge.



Surge will decay once the turbo speed finally slows enough to reduce the boost and move the operating point back into the stable region. This situation is commonly addressed by using a Blow-Off Valves (BOV) or bypass valve. A BOV functions to vent intake pressure to atmosphere so that the mass flow ramps down smoothly, keeping the compressor out of surge. In the case of a recirculating bypass valve, the airflow is recirculated back to the compressor inlet.

A Ported Shroud compressor (see Fig. 2) is a feature that is incorporated into the compressor housing. It functions to move the surge line further to the left (see Fig. 3) by allowing some airflow to exit the wheel through the port to keep surge from occurring. This provides additional useable range and allows a larger compressor to be used for higher flow requirements without risking running the compressor into a dangerous surge condition. The presence of the ported shroud usually has a minor negative impact on compressor efficiency.



Fig. 2



Fig. 3




◊ The Choke Line is the right hand boundary of the compressor map. For Garrett maps, the choke line is typically defined by the point where the efficiency drops below 58%. In addition to the rapid drop of compressor efficiency past this point, the turbo speed will also be approaching or exceeding the allowable limit. If your actual or predicted operation is beyond this limit, a larger compressor is necessary.

◊ Turbo Speed Lines are lines of constant turbo speed. Turbo speed for points between these lines can be estimated by interpolation. As turbo speed increases, the pressure ratio increases and/or mass flow increases. As indicated above in the choke line description, the turbo speed lines are very close together at the far right edge of the map. Once a compressor is operating past the choke limit, turbo speed increases very quickly and a turbo over-speed condition is very likely.

◊ Efficiency Islands are concentric regions on the maps that represent the compressor efficiency at any point on the map. The smallest island near the center of the map is the highest or peak efficiency island. As the rings move out from there, the efficiency drops by the indicated amount until the surge and choke limits are reached.

2. Plotting Your Data on the Compressor Map
In this section, methods to calculate mass flow rate and boost pressure required to meet a horsepower target are presented. This data will then be used to choose the appropriate compressor and turbocharger. Having a horsepower target in mind is a vital part of the process. In addition to being necessary for calculating mass flow and boost pressure, a horsepower target is required for choosing the right fuel injectors, fuel pump and regulator, and other engine components.

◊ Estimating Required Air Mass Flow and Boost Pressures to reach a Horsepower target.

· Things you need to know:
· Horsepower Target
· Engine displacement
· Maximum RPM
· Ambient conditions (temperature and barometric pressure. Barometric pressure is usually given as inches of mercury and can be converted to psi by dividing by 2)

· Things you need to estimate: · Engine Volumetric Efficiency. Typical numbers for peak Volumetric Efficiency (VE) range in the 95%-99% for modern 4-valve heads, to 88% - 95% for 2-valve designs. If you have a torque curve for your engine, you can use this to estimate VE at various engine speeds. On a well-tuned engine, the VE will peak at the torque peak, and this number can be used to scale the VE at other engine speeds. A 4-valve engine will typically have higher VE over more of its rev range than a two-valve engine.

· Intake Manifold Temperature. Compressors with higher efficiency give lower manifold temperatures. Manifold temperatures of intercooled setups are typically 100 - 130 degrees F, while non-intercooled values can reach from 175-300 degrees F.

· Brake Specific Fuel Consumption (BSFC). BSFC describes the fuel flow rate required to generate each horsepower. General values of BSFC for turbocharged gasoline engines range from 0.50 to 0.60 and higher. The units of BSFC are
Lower BSFC means that the engine requires less fuel to generate a given horsepower. Race fuels and aggressive tuning are required to reach the low end of the BSFC range described above.

For the equations below, we will divide BSFC by 60 to convert from hours to minutes.

To plot the compressor operating point, first calculate airflow:




Where:
· Wa = Airflowactual (lb/min)
· HP = Horsepower Target (flywheel)
· = Air/Fuel Ratio
· = Brake Specific Fuel Consumption ( ) ÷ 60 (to convert from hours to minutes)

TURBO 103 (Advanced con't)

continued from previous post due to IMG limitations........

EXAMPLE:

I have an engine that I would like to use to make 400Hp, I want to choose an air/fuel ratio of 12 and use a BSFC of 0.55. Plugging these numbers into the formula from above:



of air.

Thus, a compressor map that has the capability of at least 44 pounds per minute of airflow capacity is a good starting point.

Note that nowhere in this calculation did we enter any engine displacement or RPM numbers. This means that for any engine, in order to make 400 Hp, it needs to flow about 44 lb/min (this assumes that BSFC remains constant across all engine types).

Naturally, a smaller displacement engine will require more boost or higher engine speed to meet this target than a larger engine will. So how much boost pressure would be required?

◊ Calculate required manifold pressure required to meet the horsepower, or flow target:




Where:

· MAPreq = Manifold Absolute Pressure (psia) required to meet the horsepower target
· Wa = Airflowactual(lb/min)
· R = Gas Constant = 639.6
· Tm = Intake Manifold Temperature (degrees F)
· VE = Volumetric Efficiency
· N = Engine speed (RPM)
· Vd = engine displacement (Cubic Inches, convert from liters to CI by multiplying by 61.02, ex. 2.0 liters * 61.02 = 122 CI)

EXAMPLE:

To continue the example above, let’s consider a 2.0 liter engine with the following description:

· Wa = 44 lb/min as previously calculated
· Tm = 130 degrees F
· VE = 92% at peak power
· N = 7200 RPM
· Vd = 2.0 liters * 61.02 = 122 CI



= 41.1 psia (remember, this is absolute pressure. Subtract atmospheric pressure to get gauge pressure (aka boost):

41.1 psia – 14.7 psia (at sea level) = 26.4 psig boost

As a comparison let’s repeat the calculation for a larger displacement 5.0L (4942 cc/302 CI) engine.

Where:

· Wa = 44 lb/min as previously calculated
· Tm = 130 degrees F
· VE = 85% at peak power (it is a pushrod V-8)
· N = 6000 RPM
· Vd = 4.942*61.02= 302 CI


= 21.6 psia (or 6.9 psig boost)

This example illustrates in order to reach the horsepower target of 400 hp, a larger engine requires lower manifold pressure but still needs 44lb/min of airflow. This can have a very significant effect on choosing the correct compressor.

With Mass Flow and Manifold Pressure, we are nearly ready to plot the data on the compressor map. The next step is to determine how much pressure loss exists between the compressor and the manifold. The best way to do this is to measure the pressure drop with a data acquisition system, but many times that is not practical.

Depending upon flow rate, charge air cooler characteristics, piping size, number/quality of the bends, throttle body restriction, etc., the plumbing pressure drop can be estimated. This can be 1 psi or less for a very well designed system. On certain restrictive OEM setups, especially those that have now higher-than-stock airflow levels, the pressure drop can be 4 psi or greater.

For our examples we will assume that there is a 2 psi loss. So to determine the Compressor Discharge Pressure (P2c), 2 psi will be added to the manifold pressure calculated above.



Where:

· P2c = Compressor Discharge Pressure (psia)
· MAP = Manifold Absolute Pressure (psia)
· ΔPloss = Pressure Loss Between the Compressor and the Manifold (psi)

For the 2.0 L engine:



= 43.1 psia

For the 5.0 L engine:



= 23.6 psia

Remember our discussion on inlet depression in the Pressure Ratio discussion earlier, we said that a typical value might be 1 psi, so that is what will be used in this calculation. For this example, assume that we are at sea level, so ambient pressure is 14.7 psia.

We will need to subtract the 1 psi pressure loss from the ambient pressure to determine the Compressor Inlet Pressure (P1).



Where:

· P1c = Compressor Inlet Pressure (psia)
· Pamb = Ambient Air pressure (psia)
· ΔPloss = Pressure Loss due to Air Filter/Piping (psi)


P1c = 14.7 - 1

= 13.7 psia

With this, we can calculate Pressure Ratio () using the equation.



For the 2.0 L engine:



= 3.14

For the 5.0 L engine:



= 1.72

We now have enough information to plot these operating points on the compressor map. First we will try a GT2860RS. This turbo has a 60mm, 60 trim compressor wheel.



Clearly this compressor is too small, as both points are positioned far to the right and beyond the compressor’s choke line.

Another potential candidate might be the GT3076R. This turbo has a 76mm, 56 trim compressor wheel:



This is much better; at least both points are on the map! Let’s look at each point in more detail.

For the 2.0L engine this point is in a very efficient area of the map, but since it is in the center of the map, there would be a concern that at a lower engine speeds that it would be near or over the surge line. This might be ok for a high-rpm-biased powerband that might be used on a racing application, but a street application would be better served by a different compressor.

For the 5.0L engine, this looks like a very good street-biased powerband, with the lower engine speeds passing through the highest efficiency zone on the map, and plenty of margin to stay clear of surge. One area of concern would be turbo overspeed when revving the engine past peak power. A larger compressor would place the operating point nearer to the center of the map and would give some additional benefit to a high-rpm-biased powerband. We’ll look at a larger compressor for the 5.0L after we figure out a good street match for the 2.0L engine.

So now lets look at a GT3071R, which uses a 71mm, 56 trim compressor wheel.



For the 2.0L engine, this is a much more mid-range-oriented compressor. The operating point is shifted a bit towards the choke side of the map and this provides additional surge margin. The lower engine speeds will now pass through the higher efficiency zones and give excellent performance and response.

For the 5.0L engine, the compressor is clearly too small and would not be considered.

Now that we have arrived at an acceptable compressor for the 2.0L engine, lets calculate a lower rpm point to put on the map to better get a feel for what the engine operating line will look like. We can calculate this using the following formula:



We’ll choose the engine speed at which we would expect to see peak torque, based on experience or an educated guess. In this case we’ll choose 5000rpm.

Where:

· Wa = Airflowactual (lb/min)
· MAP = Manifold Absolute Pressure (psia) =35.1 psia
· R = Gas Constant = 639.6
· Tm = Intake Manifold Temperature (degrees F) =130
· VE = Volumetric Efficiency = 0.98
· N = Engine speed (RPM) = 5000rpm
· Vd = engine displacement (Cubic Inches, convert from liters to CI by multiplying by 61, ex. 2.0 liters * 61 = 122 CI)



= 34.1 lb/min

Plotting this on the GT3071R compressor map gives the following operating points.



This gives a good representation of the operating line at that boost level, which is well suited to this map. At engine speeds lower than 5000rpm the boost pressure will be lower, and the pressure ratio would be lower, to keep the compressor out of surge.

Back to the 5.0 L engine. Let’s look at a larger compressor’s map. This time we will try a GT3582R with an 82mm, 56 trim compressor.



Here , compared to the GT3076R, we can see that this point is not quite so deep into choke and will give better high-rpm performance than the 76mm wheel. A further increase in wheel size would give even better high-rpm performance, but at the cost of low- and mid-range response and drivability.

Hopefully this has given a basic idea of what a compressor map displays and how to choose a compressor. As you can see, a few simple estimations and calculations can provide a good basis for compressor selection. If real data is available to be substituted in place of estimation, more accurate results can be generated.

FAQ in regard to TURBO

1. What are the main tuning problems when dealing with Turbos?

Engine calibration - fueling and ignition timing. Under boost, it is crucial that there is no engine-killing detonation occurring within the cylinder. This is done by fine tuning the air/fuel ratio a bit rich to help cool the combustion gas, and by tuning the ignition advance curve to ensure that combustion chamber pressures stay below the level that causes unburned fuel to ignite ahead of the advancing flame front.



2. What are the main differences between a Single and Twin Turbo setup?

A single turbo receives exhaust flow from and supplies air to all cylinders.
The most common type of twin turbo setup is the parallel system where each turbo is fed by ½ of the engine's cylinders. Here, both compressors supply air to the intake manifold simultaneously.
There are also sequential twin turbo systems, which run on one small turbo at low engine speeds and switch to two parallel turbos at a predetermined engine speed and/or load.
Furthermore, there are series twin turbo systems where one turbo feeds the other turbo. These are primarily used on diesel engines due to the extremely high boost levels that can be generated.
For this FAQ, we will just refer to the first two setups identified above.
Choosing between a single or parallel twin turbo setup is primarily based on packaging constraints in the engine bay, or a personal choice by the tuner. In most cases, for top performance, a single turbo is preferable because larger turbos are generally more efficient than smaller turbos. However, often there is not room for one large single, or the tuner wants the visual impact of twin turbos. The notion that two smaller turbos will build boost faster than one large turbo is not always accurate because even though the turbos are smaller, each one is only getting half of the exhaust flow. Sequential systems seem to have the capacity to support big power. In theory, the sequential twin turbo setup is a potent combination. A few O.E.s have produced systems of this type but control issues have proven significant, making them challenging to function seamlessly. One slight draw back to a sequential twin turbo system is that sometimes during daily driving (specifically, in cornering) if the driver is not constantly aware, the second turbo will spool and result in a lot of unpredicted power.



3. What is Turbo Lag?

Turbo lag is the time delay of boost response after the throttle is opened when operating above the boost threshold engine speed. Turbo lag is determined by many factors, including turbo size relative to engine size, the state of tuning of the engine, the inertia of the turbo's rotating group, turbine efficiency, intake plumbing losses, exhaust backpressure, etc.




4. What is Boost Threshold?

Boost threshold is the engine speed at which there is sufficient exhaust gas flow to generate positive manifold pressure, or boost.



5. What is a boost leak?

A boost leak means that somewhere in the turbo or intake, there is an area where the air (boost) is escaping. Typically a boost leak is caused by a loose or bad seal, cracked housing, etc. When a boost leak is present, the turbo will be able to generate boost, but it may not be able to hold it at a constant level and pressure will drop off proportionally to the size of the leak.



6. How can I adjust the turbo boost?

Adjusting the boost is straightforward. However, it depends on the type of boost controller.

For a standard Wastegates actuator, simply recalibrate the actuator to open (more or less) for a given pressure. Changing the length of the rod that attaches to the Wastegates lever accomplishes this adjustment.
For mechanical boost control systems, adjustments may involve changing the setting on a regulator valve(s).
For electronic boost control systems, adjustments may need to be made to the vehicle's engine management system.
For an external Wastegates, adjusting the boost often requires turning the adjustment screw (when equipped) to increase/decrease spring load, changing Wastegates springs, or shimming Wastegates springs.
IMPORTANT: WHILE ADJUSTING THE BOOST IS STRAIGHTFORWARD, OFTEN THIS CHANGE REQUIRES MODIFICATIONS TO THE ENGINE FUEL MANAGEMENT SYSTEM!



7. What is boost spike?

A boost spike is a brief period of uncontrolled boost, usually encountered in lower gears during the onset of boost. Typically spikes occur when the boost controller cannot keep up with the rapidly changing engine conditions.



8. What is boost creep?

Boost creep is a condition of rising boost levels past what the predetermined level has been set at. Boost creep is caused by a fully opened Wastegates not being able to flow enough exhaust to bypass the housing via the Wastegates itself. For example, if your boost is set to 12psi, and you go into full boost, you will see a quick rise to 12 or 13psi, but as the rpm's increase, the boost levels also increase beyond what the boost controller or stock settings were. Boost creep is typically more pronounced at higher rpm's since there is more exhaust flow present for the Wastegates to bypass.
Effective methods of avoiding or eliminating boost creep include porting the internal Wastegates opening to allow more airflow out of the turbine, or to use an external Wastegates.



9. What is compressor surge?

The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side.
Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself.
Common conditions that result in compressor surge on turbocharger gasoline engines are:
A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body
The outlet plumbing for the bypass valve is too small or restrictive
The turbo is too big for the application



10. How does a Wastegate work?

A Wastegate is simply a turbine bypass valve. It works by diverting some portion of the exhaust gas around, instead of through, the turbine. This limits the amount of power that the turbine can deliver to the compressor, thereby limiting the turbo speed and boost level that the compressor provides.

The Wastegate valve can be "internal" or "external". For internal Wastegates, the valve itself is integrated into the turbine housing and is opened by a turbo-mounted boost-referenced actuator.
An external Wastegate is a self-contained valve and actuator unit that is completely separate from the turbocharger.
In either case, the actuator is calibrated (or set electronically with an electronic boost controller) by internal spring pressure to begin opening the Wastegate valve at a predetermined boost level.
When this boost level is reached, the valve will open and begin to bypass exhaust gas, preventing boost from increasing.




11. What is the difference between a BOV and a Bypass Valve? How do they work, and are they necessary?

A Blow Off Valve (BOV) is a valve that is mounted on the intake pipe after the turbo but before the throttle body. A BOV's purpose is to prevent compressor surge. When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge.
Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.



12. How should I break in a turbo?

A properly assembled and balanced turbo requires no specific break-in procedure. However, for new installations a close inspection is recommended to insure proper installation and function. Common problems are generally associated with leaks (oil, water, inlet or exhaust).



13. What is/causes Shaft Play?

Shaft play is caused by the bearings in the center section of the turbo wearing out over time. When a bearing is worn, shaft play, a side to side wiggling motion of the shaft occurs. This in turn causes the shaft to scrape against the inside of the turbo and often produces a high-pitched whine or whizzing noise. This is a potentially serious condition that can lead to internal damage or complete failure of the turbine wheel or the turbo itself.



14. What is causing my turbo to sound like a sewing machine's whistle?

The "sewing machine whistle" is a distinct cyclic noise cause by unstable compressor operating conditions known as compressor surge. This aerodynamic instability is the most noticeable during a rapid lift of the throttle, following operation at full boost.



15. I want to make x horsepower, which turbo kit should I get? or Which turbo is best?

Select a turbocharger to achieve desired performance. Performance includes boost response, peak power and total area under the power curve. Further decision factors will include the intended application. The best turbo kit dictated by how well it meets your needs. Kits that bolt on without any modification are best if you don't have fabrication capabilities. Less refined kits can be cost effective if you access to fabrication capabilities.

FAQ in regard to Intercooler's

1. What is intercooler heat soak?

Heat soak is when the intercooler can't dissipate the heat that it absorbs from the turbo fast enough. When an intercooler can't cool the charge air by removing the heat from it, it loses its effectiveness. This explains why turbo cars tend to run slower or have slightly less power when the weather is warm.



2. What is the purpose of an Intercooler?

An intercooler's primary function is to cool the charge air after it has been heated due to boosting and the heat that is produced by the turbo before sending the air into the engine. As the air is cooled, it becomes denser, and denser air makes for better combustion (more power). Additionally, the denser, cooler air helps reduce the chances of knock.



3. FMIC, TMIC, SMIC - what do they mean?

(Front Mounted Intercooler, Top Mounted Intercooler, Side Mounted Intercooler) terms which refer to the placement of the charge air cooler in the engine bay and in reference to the engine. Typically FMICs provide the best cooling capability, as they are located in front of the radiator for optimum airflow. SMICs and TMICs are commonly found on factory-turbo'd cars. TMICs are more prone to heatsoak as they are placed over the engine directly in the path of the rising heat and very close to the hood. However, when a TMIC is used in conjunction with a hood scoop, they can provide adequate cooling.



4. Will an intercooler help make more horsepower?

Yes, although it is only indirectly responsible for helping make more power. Since the intercooler increases the charge air density, an intercooled engine will typically make more power than a non-intercooled engine with the same setup by allowing more air to be crammed into each cylinder.



5. What is the difference between an air-to-air intercooler and a water-to-air (liquid-to-air) intercooler?

An Air-to-Air intercooler uses ambient air flowing over the fins to cool the charge air, while an Air-to-Water intercooler uses coolant (water) with a system similar to that of a radiator's cooling system. Traditionally, air-to-air intercoolers are used for street applications because of their lower cost and reduced complexity, while air-to-water intercoolers are used in race and packaging-constrained applications.



6. How do I select the proper intercooler core size?

A major limiting factor in choosing an intercooler size is space constraints within the engine bay. If there is not enough room for an intercooler with adequate flow, then often a water-to-air intercooler is used instead to maximize the cooling capability of the surface area of the core.
You want to make sure that the intercooler you choose is large enough to effectively handle the air. Too small of a core, and you will restrict the potential of the turbo by not allowing the charge air to be cooled adequately.



7. What is the best placement of my intercooler?

The best place for your intercooler is directly in the path of the inflow of ambient air. Traditionally this has been right in front of the radiator in the front of the car, hence the term Front Mount Intercooler.



8. Will a FMIC block flow to my radiator?

No. Since the intercooler allows air to pass through it, airflow to the radiator will not be blocked. However, using an intercooler core that is too thick and does not allow air to pass through it quickly or completely and airflow to the radiator can be restricted which can lead to potential overheating problems.



9. What is intercooler effectiveness and how do I measure it?

Effectiveness is defined as the ratio of how many degrees of temperature that were removed from the charge air by the intercooler to the original temperature that is put into the charge air by the turbo.

Example:
If the turbo compresses the charge air to a temperature of 140° F, but after passing through the intercooler the air is 115° cooler (resulting in a 25° F charge air temperature), the efficiency would be:
Effectiveness: 115/140 = 0.82 or 82% efficiency
Typically, air-to-air intercoolers for normal street applications range between 60% and 70% efficiency. Often, liquid-to-air intercoolers have effectiveness ratings from 75% to 95%. One common method of improving the cooling of the charge air dramatically in an air-to-water intercooler is the inclusion of ice as a coolant.



10. What exactly is 'pressure drop/loss' / 'flow loss' and how are they measured?

Pressure loss, or pressure drop, refers to the change in pressure when comparing the air entering the intercooler with the exiting air. This change is mostly affected by the internal flow area of the intercooler. Flow loss, however, is measured not just with pressure loss but with how much restriction to airflow exists.
Maximum performance can be obtained if the pressure loss is kept below 1.0 to 1.5 psi. Anything in excess of these numbers, especially higher than 3.8 psi, and the intercooler is not properly fitted for the application which results in hindered performance and dramatically decreased functionality of the intercooler system itself.



11. I want to turn up my boost, is a larger intercooler necessary?

Usually, it is not necessary to upgrade the intercooler when raising boost levels. The pressure drop contributed by the intercooler is proportional to it's flow (CFM) squared. This relationship shows that it is highly unlikely the change resulting in loss from higher boost levels will require a larger intercooler. If there is a significant change however, such as 40% or 50%, then a larger intercooler may be necessary.



12. Is there a maximum amount of boost I can run on my intercooler?

While it is possible that an intercooler can fail from boost levels being too high, it is a very rare scenario. However, if not properly designed to handle high boost, cracking along seams and of the endtanks can occur.



13. How significant is a leak in my intercooler?

For an air-to-air intercooler, a leak, as long as it is not a significantly large one, will not hinder performance at all. However, if an air-to-water intercooler develops a leak in the main core, it could lead to other more significant problems with the engine itself. Be sure to fix these problems as soon as they occur to prevent other damage.



14. I want my car to remain a sleeper/stealth. Can I paint or anodize my intercooler so it is not easily visible?

Yes! It is not uncommon at all for an intercooler and endtanks to be anodized black to keep attention away from the car and help it maintain a sleeper appearance. A very light coat of paint on the core and endtanks is also another option, usually much cheaper and easier than anodizing, with a negligible performance loss.



15. Is there any maintenance required for my intercooler? Are there any special things to do to keep it working longer?

In a water-to-air intercooler, check the water level often as this is crucial for the intercooler to operate properly. In cold weather, just like a car, it will need antifreeze in order to function effectively and properly. With an air-to-air intercooler, there really is no maintenance that needs to be done other than just the routine checking of hoses and clamps to make sure everything is tight. Additionally, the intercooler fins may be picked-out or de-smashed to ensure maximum cooling. Every 20,000 miles or so it is recommended that the intercooler core be inspected, and if necessary, flushed/washed out to remove any accumulated oil or buildup.

FAQ Other Various

1. What other systems are affected by turbocharging? (Fuel, Oil, Cooling, Drivetrain, etc)

There are several factors that must be addressed when deciding to turbocharge a previously naturally aspirated engine, such as: Is the current fuel delivery system capable of providing increased, adequate amounts of fuel? Is the cooling/oiling system capable of handling the extra power and consequently, extra heat that is generated by the turbo? Is the clutch/transmission/drivetrain up to the task of handling the extra power? Etc



2. How is boost measured? (Bar, mmHg, PSI) and How do you convert from one to another?

Boost is measured as the pressure that the turbo creates above atmospheric pressure.
Normal Atmospheric Pressure (1 atm) = 14.7 psi = 760 mm Hg
1 Bar is not actually equal to 14.7 psi, but rather it is equal to 14.5 psi, = 0.9869 atm = 750.062 mm Hg



3. The turbo gauges measures turbine speed, right?

The "turbo gauge", commonly called a boost gauge, does not measure turbine speed. It measures the intake manifold pressure. Under light loads the boost gauge will indicate a vacuum due to the turbocharger shaft not rotating fast enough to create positive pressure (boost). Once load (throttle position) increases, the boost gauge will indicate a positive pressure.



4. What is a boost controller?

A boost controller is a device that bleeds or blocks the boost pressure signal entering the Wastegates actuator. The idea is to keep the Wastegates closed to allow higher boost pressures than the actuator would otherwise allow. These can be simple mechanical or sophisticated electronic devices, with price tags to match.



5. Which boost controller should I get? (Manual or Electronic)

Boost controllers vary widely in performance, price, and functionality. For a comprehensive breakdown of some of the more popular options, see the July 2002 issue of Sport Compact Car Magazine.


6. How much boost can I run on pump gas?

The primary limitation to maximum boost is engine knock. It is also not advisable to run the maximum amount of boost your car can handle on a daily driven basis as a precaution against if the boost spikes.



7. What is Knock/Detonation?

Knock is a condition caused by abnormal combustion of the air/fuel mixture and can result in damage to an engine.
The three factors that result in engine knock are: 1) knock resistance characteristics (knock limit) of the engine, 2) ambient air conditions, and 3) octane rating of the fuel being used.


Since every engine is vastly different when it comes to knock resistance, there is no single answer to "how much." Design features such as combustion chamber shape, spark plug location, bore size and compression ratio affects the knock characteristics of an engine. In addition, engine calibration of fuel and spark plays an enormous role in dictating knock behavior.
For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases thus increasing the tendency to knock. Charge air cooling (e.g. an intercooler) addresses this concern by cooling the compressed air produced by the turbocharger.
The octane rating of fuel is a measure of a fuel's ability to resist knock. The octane rating for pump gas ranges from 85 to 94 while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant it is to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for your application. Generally speaking, the more boost you run, the higher the octane requirement.




8. Should I run a Turbo Timer?

A turbo timer enables the engine to run at idle for a specified time after the ignition has been turned off. The purpose is to allow the turbo to cool down thus avoiding "coking" ("coking" is burned oil that deposits on surfaces and can lead to blocked passages).
The need for a turbo timer depends on how hard the turbo and engine is used. Running at full speed and full load then immediately shutting down (heat soak) can be extremely hard on a turbo. Water-cooling of the turbocharger's center housing has essentially eliminated the need for turbo timers or extended idling periods.



9. Do I really need the cool down procedure on my turbo?

The need for a cool down procedure depends on how hard the turbo and engine is used, and whether or not the turbo is water-cooled. All Garrett turbochargers must pass a heat soak test and the introduction of water-cooling has virtually eliminated the need for a cool down procedure. Garrett is one of the few turbocharger manufactures that subjects their turbos to several OE qualification tests. When you buy a Garrett turbo you can be sure it's a reliable one!



10. How can I remove and clean the oil condensation box/oil catch can?

The oil condensation box, or catch can, can be cleaned once it is removed with any cleaning solvent. Simply fill the box with a cleaner and slosh it around until oil deposits are gone. Removing the oil condensation box can be a challenge and varies by vehicle.
NOTE: some vehicles are not equipped with an oil condensation box.



11. What is the purpose of an oil catch can?

An oil catch can's purpose is to catch oil blow-by gasses that can eventually create a carbon and oil sludge build-up in the intake and turbo.



12. What additional maintenance is required for the turbo?

Good, clean oil is extremely important to the turbocharger. It is best to change the oil and filter at least as often as the automobile manufacturer recommends. FRAM produces replacement oil filters for all levels of server use. Visit www.FRAM.com to select the right filter for your application!
Turbo performance is sensitive to turbo inlet conditions. A clogged air filter can drastically affect the turbo inlet. Air filters should be inspected at every oil change and replaced at 12,000 to 15,000 mile intervals. FRAM produces replacement air filters including a new performance filter the FRAM AirHog.
NOTE: Never exceed the vehicle manufacturer's recommended filter change intervals.





13. What compression ratio should I run with my turbo engine?

Allowable compression ratio depends on many factors, and there is no one right answer for every application. Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Setting the compression ratio too low will result in an engine that is a bit sluggish in off-boost operation. Setting it too high however, can lead to serious engine problems due to knock.
Factors that influence the compression ratio can include: fuel anti-knock properties (octane rating), boost pressure, intake air temperature, combustion chamber design, ignition timing, and exhaust backpressure. Many modern engines have well designed combustion chambers that will allow modest boost levels with no change to compression ratio, assuming appropriate tuning. For higher power targets with more boost, compression ratios should be adjusted to compensate.



14. How do I adjust my compression ratio?

The easiest and most effective way to accomplish this is through the use of either higher/lower compression pistons, and/or using a head gasket of a different thickness.



15. Should my turbo/exhaust manifold glow red after driving?

Yes, the turbo/exhaust manifold can glow red under certain driving conditions. The exhaust gas temperature can reach over 1600F under high load operating conditions; i.e. towing, extended uphill driving, or extended high rpm/boost conditions.



16. What should I look out for when buying a turbo?

Condition of the turbine housing - inspect for cracks on the exterior and inside the inlet of the housing. If the housing has cracks then the housing needs to be replaced.
Condition of the turbine and compressor wheels - inspect for cracks and damaged blades. If either of the wheels are damaged then the wheel (s) need to be replaced and the center section balanced.
Condition of the bearings - spin the turbocharger shaft and check for roughness. If roughness is detected then the turbocharger needs to be disassembled and the internal components inspected and replaced if necessary.
The most important factor is to make sure the turbo is the proper one for your application. A properly matched turbo will provide better performance and more reliable operation. A properly matched turbo includes matched turbine and compressor wheel sizes and appropriate housings.



17. Are oil deposits indicative of impending turbo failure? There is blue/black smoke, is my turbo going bad?

Blue/black smoke can be caused by numerous conditions, and one of them could be a turbocharger worn past its useful service life. The following are potential reasons that blue/black smoke could occur:

Clogged air filter element or obstructed air intake duct. This condition creates a vacuum due to high differential pressure resulting in oil drawn into the compressor and subsequently burned during engine combustion.
Engine component problems; i.e. worn piston rings or liners, valve seals, fuel pump, fuel injectors, etc.
Obstructed oil drain on turbocharger resulting in pressure building inside the center housing and forcing oil past the turbocharger seals
Damaged turbocharger or turbocharger worn past its useful service life
Black smoke is also sometimes indicative of too rich an air/fuel mixture.



18. How fast will my car go with xyz?

This question cannot be answered as how fast any given car will go depends on the unique individual setup, road/weather conditions, and of course, the driver's skill.



19. What is the Inducer?

Looking at a compressor wheel, the inducer is the "minor" diameter. For a turbine wheel, the inducer is the "major" diameter. The inducer, in either case, is where flow enters the wheel.



20. What is the Exducer?

Looking at a compressor wheel, the exducer is the "major" diameter. For a turbine wheel, the exducer is the "minor" diameter. The exducer, in either case, is where flow exits the wheel.

Compressor wheel exducer diameter is incorporated into Garrett GT-series nomenclature: The "60" in the GT2860RS for example, has a 60mm compressor wheel exducer diameter.