random thoughts and questions on EGTs and BTUs and boost (1 Viewer)

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I've seen people discuss cracked 3B heads, even for naturally aspirated engines. If this is the case, how are people getting away adding boost and such? Is head longevity simply a matter of EGTs? I would think that total volume of heat matters too (BTUs). Surely a completely stock 3B at 550 C EGT is having an easier time (thermally speaking) than one pushing a significant amount of fuel and boost but with the same EGTs.

Another thought: It is said that an increase in boost (without altering the fuel supply) will generally lower EGTs, can this reduction in EGTs translate to lower engine temperatures or less load on the cooling system? I would think that if the engine is burning the same amount of fuel and doing the same amount of work, that the cooling system still has the same amount of work to do.
 
I would think that if the engine is burning the same amount of fuel and doing the same amount of work, that the cooling system still has the same amount of work to do.

Combustion temperature is dependent on air fuel ratio, RPM, injection timing, quality of fuel atomisation, overall volumetric efficiency.

A NA diesel might run at 50 to 60% volumetric efficiency. Add a turbo and that might increase to 80 or 90%

NA diesel with mechanical injection has crude tuning capability.
It's gonna run rich and smokey a lot of the time. Rich AFR in a diesel means high combustion temperatures.
Add boost, your volumetruc efficiency improves, there's more air in the air fuel mix, you get more complete combustion, so more power from the same amount of fuel. The AFR is leaner, and combustion temperature is lower.

The turbo will add some heat to air going in, but there's less heat coming out from the same amount of fuel.
 
agreed
i feel my 3b is much less stressed with the turbo, heatwise
however... now comes the next issue, can the crank deal with peak cylinder pressure now at the higher and faster combustion pressure levels...
the only way to counter this is retarded injection timing, this can be achieved two ways, retard global injection timing by clocking the pump itself (away from the engine)
and increasing injector pop pressure (effectively retarding injection again)
....things a 3b guy thinks about with a drink in his hand
 
Combustion temperature is dependent on air fuel ratio, RPM, injection timing, quality of fuel atomisation, overall volumetric efficiency.

A NA diesel might run at 50 to 60% volumetric efficiency. Add a turbo and that might increase to 80 or 90%

NA diesel with mechanical injection has crude tuning capability.
It's gonna run rich and smokey a lot of the time. Rich AFR in a diesel means high combustion temperatures.
Add boost, your volumetruc efficiency improves, there's more air in the air fuel mix, you get more complete combustion, so more power from the same amount of fuel. The AFR is leaner, and combustion temperature is lower.

The turbo will add some heat to air going in, but there's less heat coming out from the same amount of fuel.
great explanation!!!

don't forget a turbo quiets down the diesel clacking noise too
 
I'm definitely no expert but I would have assumed that given additional boost is increasing the mass of air flowing through an engine, more of the heat produced by the combustion will leave the engine through the tail pipe therefore the cooling system has less work to do. If the fuel is increased also, then this would presumably counteract the above through the increased heat of the combustion of the extra fuel.

I recently installed an intercooler in my 2LTE Surf and was initially surprised that max EGTs only dropped 150F or so. However, when I increased boost from 10PSI to 12PSI, I saw an additional 60F drop. As I understand it, the ECU in this engine starts to defuel above 10 PSI so I'm unsure whether some fuel was being added along with the boost or none, but either way EGTs dropped. More cool air into the cylinders = lower EGTs it seems.

Another interesting question is whether, if in the process of modifying an engine, power output is increased through boost and fuel, but engine temperatures are lowered, (with an intercooler for example) does the reduction in thermal stress compensate for the increase in mechanical stress? Probably this would be component specific I guess. Chat GPT had this to say on the topic "as a general trend, metals become less resistant to mechanical stress as they get hotter"
 
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I would have assumed that given additional boost is increasing the mass of air flowing through an engine, more of the heat produced by the combustion will leave the engine through the tail pipe therefore the cooling system has less work to do.
I don't know for sure but I've been thinking this may be the case! I don't have much to add but everything you and others have said has been good food for thought.
 
component specific for sure,
as the pistons are what we are mostly concerned about with egt as aluminum has a much lower fatigue/melting point than steel
where as power increase will stress the rotating assembly more even if heat is under control
this is exacerbated by not taking into account the need for reduced injection timing advance (diesel) to limit the combustion event happening before top dead centre and creating extreme cylinder pressures (as the piston is still on the way up)
 
More cool air into the cylinders = lower EGTs it seems.

Yes, for two reasons.

Cooler intake air is going to reduce total heat load in comparison to hotter intake air.
A cool intake charge cools piston crowns, head, valves, cylinder walls etc.
This all increases the safety margin when tuning for more power.

Cooler air is more dense. More oxygen, more complete fuel burn. More air means leaner AFRs which means lower combustion temperatures.
More complete fuel burn means more power from the same amount of fuel (if there was excess fuel).

a lot of this is achieved just with boost, provided the turbo is working within its efficient range, and boost isn't really high. Temperature of compressed air charge increases as boost increases.

There's diminishing gains at some point
 
With all this talk it’s quite the balancing act I know everyone’s setup will be different but what is the golden numbers in a perfect world? Is there a perfect target to hit?

Boost=25 Psi
EGT= Under 1350F
AFR= Anywhere between 17 - 22:1


???
 
different for different diesel engines
and even variables same engine to same engine
and everyone's opinion for what is safe is different too, or should i say their tolerance for higher levels of risk

i found my boost was providing cooling to a certain extend up to 18 to20 psi (from a base of 10-12)
much after that the heat in the compressed air seemed to be contributing to egt's more aggressively
i have an air to water intercooler and retarded timing and higher pop pressure which all have their own effect on egt's

I turned my fuel up conservatively till it felt how i wanted it to drive, then added boost until it kept the egt's better in check, then intercooled it for some safety
the extra boost above 10-12psi didnt add as much power as the extra fuel at 10-12 psi did. intercooler was just icing

1350 is too hot for a 3b..1150-1200F, maybe 1225 for a very short stint
a 1hd-t will take 1350f (anecdotally) but most dont run their truck that hot
18/20 psi in a diesel is the right place to be as far as im concerned... after you take care of a few things first of course
no afr monitoring on mine
 
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With all this talk it’s quite the balancing act I know everyone’s setup will be different but what is the golden numbers in a perfect world? Is there a perfect target to hit?

Boost=25 Psi
EGT= Under 1350F
AFR= Anywhere between 17 - 22:1


???

What engine?

In regard to EGT, if that's your peak, measured pre-turbo, and you only hit that when really pushing hard for brief periods, I'd personally have no issues with that (engine depending).

You actually have to be pushing hard to see that in some cases.

AFRs are also constantly changing.

There's no single answer to this. Too many variables.

Boost for example, 18psi from a small turbo that's working hard, vs 18psi from a larger turbo that's lazily pushing a greater volume of air at the same boost is completely different.
 
Boost for example, 18psi from a small turbo that's working hard, vs 18psi from a larger turbo that's lazily pushing a greater volume of air at the same boost is completely different.
good point
 
It's not though. The only thing that matters for boost is how efficient the compressor is. The compressor that is closest to the centre of it's compressor map is going to be the most efficient.

The idea that bigger turbos are more efficient and/or put out cooler air is completely wrong but refuses to die.
 
It's not though. The only thing that matters for boost is how efficient the compressor is. The compressor that is closest to the centre of it's compressor map is going to be the most efficient.

The idea that bigger turbos are more efficient and/or put out cooler air is completely wrong but refuses to die.

In reference to my post?

Show me where I suggest a larger turbo puts out cooler air?

The advantage of a larger turbo is greater mass air flow.

Greater volume of air either means a leaner AFR, or opportunity to burn more fuel and make more power.
 
Boost for example, 18psi from a small turbo that's working hard, vs 18psi from a larger turbo that's lazily pushing a greater volume of air at the same boost is completely different.
I'm guessing he assumed the difference you're referring to here is temperature which TBF I think I did too
 
It's not though. The only thing that matters for boost is how efficient the compressor is. The compressor that is closest to the centre of it's compressor map is going to be the most efficient.

The idea that bigger turbos are more efficient and/or put out cooler air is completely wrong but refuses to die.

So what's better Dougal? A ct26 running at 18psi? Or a gturbo at 18psi?

My point was 18psi is an arbitrary number. It's too vague.

A ct26 pushing 18psi is way beyond its efficient range.
It's adding more heat into the air than a gturbo that's not only moving a greater volume of air, it's creating less heat during the compression process because it's more efficient.

If you have a small turbo that's 75% efficient, and a larger turbo that's also 75% efficient, theoretically the both heat the air the same amount, but that's not the only thing that's relevant.
The larger turbo shifts a larger volume of air which has a positive effect on performance, and provides an opportunity to tune in a way that produces as much or more power than is possible with a smaller turbo AND with lower EGTs.
 
If it's not too off-topic, can anyone explain the difference between boost and flow? I've seen it claimed that a more efficient turbo can create the potential for more power at the same boost pressure compared to a less efficient turbo.

Surely we only have two variables here: pressure and temperature. So am I right in assuming that the only benefit of a more efficient turbo at a given boost pressure, is that it can create that boost with less heat as a by product? This would mean denser air potentially accomodating increased fueling (or more complete combustion of existing fuel) therefore more power. So the benefit would be much the same as adding an intercooler?

Additionally, does the greater efficiency / flow achieve a given boost pressure faster, thereby creating more power in a sort of indirect way by ramping up the whole boost / fuel curve more aggressively?

Or is there some other sort of fairy dust involved that I'm not aware of?
 
Surely we only have two variables here: pressure and temperature. So am I right in assuming that the only benefit of a more efficient turbo at a given boost pressure, is that it can create that boost with less heat as a by product?
yes.
pressure and temperature are connected by laws of thermodynamics (amateur science hat on)
for every psi increase in pressure, there's a known increase in temperature. Lots of Caicos are done ASSUMING ideal conditions, but confessors are never 100% efficient.

As compressor efficiency drops, more heat is added. This can be calculated too.

Efficiency in this sense is a reference to adabiatic efficiency or, how efficiently a compressor compresses air. Less efficient compressor, more heat created.

TD04-comp-map2.jpg


My scenario of a ct 26 at 18psi, it would be operating toward the far right of the efficiency map. Working too hard.

A gturbo at 18psi would be in the centre, well within its design parameters.

This would mean denser air potentially accomodating increased fueling (or more complete combustion of existing fuel) therefore more power. So the benefit would be much the same as adding an intercooler?
To flip that, hotter air is less dense. If you can avoid heating the air by compressing it, then yeah, its a win.

Additionally, does the greater efficiency / flow achieve a given boost pressure faster, thereby creating more power in a sort of indirect way by ramping up the whole boost / fuel curve more aggressively?

how quickly a turbo spools up is dependent on turbine and turbine housing design, size, and efficiency, andis also affected by compressor design and size. A large compressor will take longer to overcome inertia and spool up.
A smaller turbine housing will help spool a large compressor sooner, but will choke exhaust flow sooner too, so you lose out on top end performance.

At a given turbo speed, a larger compressor is going to be pushing more air. It may take longer to get up to speed, but at every point along the way it's pushing more air too. It's not as simple as saying one spools faster than another.
As a turbo spools up, at slow shaft speeds, initially it's off the map to the left, as it spools up it will do most of its work in the verge of the map.
Under hard acceleration, full throttle, lots of fuel, mid to high engine RPM, the turbo is gonna spin faster and efficiency moves toward the right of the map. If it's poorly sized, or incorrectly wastegated, at some point its gonna push all the way to the right of the map , like the ct26 at 18psi.
I blew up a ct26 in a scenario like above, but it boost was peaking at 22psi. Way off the map, and in choke flow (the compressor spinning so fast it just chops at the air, and can no longer compress it).

Or is there some other sort of fairy dust involved that I'm not aware of?
Yes. Black magic mostly.
 
In reference to my post?

Show me where I suggest a larger turbo puts out cooler air?

The advantage of a larger turbo is greater mass air flow.

Greater volume of air either means a leaner AFR, or opportunity to burn more fuel and make more power.

You didn't say cooler. You said they're "completely different" but they're not. The only difference is compressor efficiency.

Above you've said a larger turbo has greater mass air flow. But that's not correct either. The bigger turbo can only flow more air mass if it's more efficient or on a bigger engine.
A bigger or smaller turbo running at the same efficiency push exactly the same mass of air into the same engine.

An engine fed 18psi at 70C doesn't know or care what size turbo is feeding that air.

So what's better Dougal? A ct26 running at 18psi? Or a gturbo at 18psi?

My point was 18psi is an arbitrary number. It's too vague.

A ct26 pushing 18psi is way beyond its efficient range.
It's adding more heat into the air than a gturbo that's not only moving a greater volume of air, it's creating less heat during the compression process because it's more efficient.

If you have a small turbo that's 75% efficient, and a larger turbo that's also 75% efficient, theoretically the both heat the air the same amount, but that's not the only thing that's relevant.
The larger turbo shifts a larger volume of air which has a positive effect on performance, and provides an opportunity to tune in a way that produces as much or more power than is possible with a smaller turbo AND with lower EGTs.

The CT26 is an old turbo design with no compressor or turbine maps available. Gturbo make a range of turbos that are more modern and efficient but also have no compressor or turbine maps available.

There is no data to prove and answer, but it's very safe bet the Gturbo has the turbine and compressor operating more efficiently over the whole range.
 
The only difference is compressor efficiency.

Which is kind of the whole point.

A small turbo that's working hard (or an outdated design) and outside its efficient range produces more heat than a larger turbo that's within its efficient range.

Doesn't change the fact that using an arbitrary number such as 18psi above is only part of an incomplete picture.
It would be more useful in conjunction with intake air temperature information.

Swap out ct26 and grurbo for any other turbo name in the same scenarios if you like. The name of turbos is irrelevant, they were to provide an example of known turbos on this forum.
 

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