*** Apologies for such a long post but there is much to discuss ***
I am hoping to get some brilliant minds in here to help facilitate a scientific approach I suppose in exploring different turbo options that may have not been tried yet. My criteria for searching for a turbo is to find a large enough turbo to lower backpressure and get as close to 1:1 EMP:IMP ratios or lower but not to exceed +1.5:1 and still be fun to drive in the lower RPMs (1500-2000rpm) which we know high EMP:IMP (exhaust : intake manifold pressure ratio) say 1.7-2 and higher is not efficient for an engine, I realize some may not care about this as much as others. Going by what I see on the forums this may be considered TOO big by most here for our engines to spool up as most want all the power as early as possible which usually leads to a turbo so small it chokes on top end, different strokes for different folks. To me, there are a lot of different options out there for finding a modern efficient turbo that could work well on these engines that may have not been attempted out of either lack of knowledge in turbo selection, or if it has, not in a way where it is public knowledge on some of these forums we have or even because the cost will be higher and more complicated to fit rather than just bolt in turbos based off the CT26. When I say different options specifically I have been interested in Borg Warner turbos. As most will know they offer some rather advanced technology when it comes to turbos. The EFR line has things like twin-scroll exhaust housings, Ti-Gamma extremely lightweight turbine wheels, and even very well designed internal wastegate ports among other things. Twin scroll housings incorporated with a true split manifold is proven technology that works decreasing spool time while maintaining high RPM power. The diesel guys in USA are pretty sold on this in the Cummins community but we don't see it much here for whatever reason of which I can't figure out, which leads me down a path to try it. As we know the EFR got a bad wrap with some turbine wheel failures early on but that can be a conversation for later, my own conclusion was these turbos did exceed their shaft limit, just my well thought out opinion.
BW also has a very neat calculator that helps you select a turbo which I have used quite a bit and I believe I'm in a good ballpark with it. It is very in-depth and needs basic values such as engine displacement, fuel type, altitude, but it has more complicated values of things such as Volumetric Efficiency, AFR, turbine and compressor efficiency, intercooler efficiency, and many others all broken down at 6 RPM points. The values I struggle to calculate accurately is mainly Volumetric Efficiency at given rpm points. This is a major contributor to matching a turbo to an engine for the desired HP goal. I have come very near to what I thought was a way to estimate VE through calculation but feel I'm missing a piece to the puzzle somewhere. This is where I am and looking for help...
My truck is a 1HDT 5 speed, G turbo Green, front mount intercooler, big airbox, 3.5in exhaust with 1 muffler. Using a Garret tech link to learn to plot a point on a compressor map of a turbo in choice for the desired HP. In my case, I'm using 290 crank hp to hopefully get me in the ballpark of 250WHP. This is a rough estimate calculated by multiplying 250 and 1.15 (typical drivetrain loss for rear-wheel drive). Which gives me 287 and I rounded to 290.
Step 1) we need the Pressure Ratio (PR) for our desired target Boost level. PR=(PSIg + absolute atmospheric pressure)/(atmospheric pressure(14.7) - 1 system depression or air ducting restriction). My values would be 25 PSIg + 14.7 psi being at sea level divided by 13.7 = 2.9 Pressure Ratio. PSIg is the gauge pressure above ambient.
Step 2) Calculate Air Flow Demand we will call it AD to make it simple. AD=(target hp)x(air-fuel ratio)x(BSFC/60). For BSFC garret gives an estimation for diesel at (.36). This could be inaccurate.
So AD = (290) (20) (.36)
AD = 35lb/min to make 290 crank hp
Step3) Calculate Manifold Absolute Pressure (MAP/req). MAP/req=(airflow demands from above x gas constant x 460+Intake air tempt)/(VE
x Max engine rpm/2 x engine displacement in CID). This is where it gets tricky for me. The gas constant is just a value garret gives as 639.6 and I may be confused but wonder if there is a diesel constant value but my logic may be wrong. VE is also an estimation of .80 from garret for a 2 valve per cylinder engine. For intake air temp I am using 150F as I am intercooled. These values could be inaccurate. For VE I used 75% so .75 for the value of VE in the equation. I'll explain why I used 75% later.
MAP/req=(35lb/min x 639.6 x 610)/(.75 x 2100 x 256) = 33.87
Subtracting 14.7 from this value also gives us our Gauge PSI to make 290HP which is 19.17psi on gauge. This value seems off to me compared to what most run here to get 200+hp.
Then we correct for pressure loss in the IC by adding 2 as a value of typical loss from Garret. MAP + 2 = 35.87 MAP/req.
If we calculate the boost gauge now after the new MAP of 35.87 we get 21.17psi on the gauge which is making more sense.
Now we calculate a new PR based on these values we have found. PR = MAP / 13.7(system depression from before)
=2.6 pressure ratio.
Last we can now start plotting different rpm points below max rpm. The Equations is (MAP x VE x RPM/2 x Discplacement) / (Gas constant x (460 + IAT)
Lets plot 3100 rpm because this seems to be our peak power RPM on a 1hdt. (35.87 x .75 x 3100/2 x 256) / (639.6 x 610) = 27.36 LB/MIN PR still 2.6
Next 2100 RPM as this seems to be peak torque. = 18.53 LB/MIN PR 2.2?
Last point of 1800 rpm just to see air flow at lower rpm. = 15.88 LB/MIN I used PR of 2 as just a guess?
Garret states after plotting peak power and torque we can use less Pressure Ratio values to see how the turbo will act with normal driving at lower RPMs.
This is all from How To Select A Turbo Part 2: Calculations - Garrett Motion if you want to check my work.
Now we can plot a point on a compressor map to see if it will flow our required air at a given pressure ratio and at what efficiency. This is a compressor map of an EFR 7064 and below that a Garret GT3071. EFR 7064 70/52 exducer/inducer and a 64mm turbine wheel lightweight Ti Gamma. To plot this point we find 2.6 on the Y or left axis and MAP/req of 35.87 on bottom X-axis. You will see we are nearly directly in the middle of the efficiency islands but this is just a consideration of a turbo I have been looking at. There are many more.
Ignore the black lines. I suck at editing.
Using the Match Bot on BW I have dialed in the 7064 and it seems quite a good fit. It would have a .92 A/R twin scroll rear housing. BW advises raising turbine efficiency in the values set by 15% for point 1 10% for point 2 and 5% for point three because of the added efficiency of the twin-scroll housing so this is reflected on match bot. I will upload the link here so other more intelligent minds can play with it. I struggle to figure out all of the values but have made very educated guesses on certain values of pressure losses or gains in piping, intake, and exhaust with the BW tutorial videos and hints in the menus. Volumetric Efficiency being the most elusive. I have a theory though.
In the MAP/req equation (airflow x gas constant x 460 + IAT) / (VE? x RPM/2 x Displacement). Well, we discover through this equation what MAP is which is 33.87. So if we set the equation equal to this and replace the estimated value of .80 VE with X and then solve for X wouldn't this be our Volumetric Efficiency? Doing this I come up with 75% VE. My logic could be inaccurate.
What I hope to achieve with this post is to gain and spread knowledge about a more in-depth look at selecting an alternate turbo for our engines so it will help me make a decision on a unique turbo setup. Hopefully, someone more intelligent than me can offer help or just people out there using what would be considered an ODD turbo selection outside the norm with real-world experience. Even if it's just correcting my math or logic please do I'm here to learn. One concept I really struggle with is mapping a turbine map. Luckily the Match Bot does that for us.
Here is the Match Bot link I have completed to the best of my current abilities. My engine RPMs reflect the ranges I'm more concerned about but anything can be adjusted and if you do you just need to copy and paste the link that you made adjustments on. VE starts at 75% tapers up to 80% for peak power and back down to 75% in the higher rpm. The EFR7064 is more of a middle of the pack between low down response and not choking on the top end. Also, it seems backpressure is near always less than 1:1 EMP vs IMP which is great. There are non EFR options like the S252SXE its a 70/52 compressor and a 61/70 turbine. For all of these, I'm trying my best to run it with a twin-scroll housing. The issue with the SXE line is you lose the internal wastegate and you get a traditional Inconel turbine wheel but it's much much cheaper than the EFR 7064. So sorting out twin external wastagtes for the SXE is a problem if we want to run a true divided setup and all the money you saved goes to fabbing a manifold and what not. Open to single scroll but then it becomes much harder to get a turbo that I believe will give me the characteristics I'm looking for. A properly sized single with decent bottom end and great top end. Compounds would also solve this but sizing those correctly is even more difficult.