Properly sized turbos

[AK1RA07]

How do we choose? Please someone correct me where I'm wrong.

For ease I will just divide the engine in half and analize 1 turbo.. so a 1.35L motor with one turbo..

1.35L = .047 cubic ft.
1 lb/min = 14.27cfm
Assuming this volume is displaced once per revolution... ( rpm x .047 )

@ 3000 rpm = 141 cfm or 9.8 lb/min
@ 4000 rpm = 188 cfm or 13.17 lb/min
@ 5000 rpm = 235 cfm or 16.46 lb/min
@ 6000 rpm = 282 cfm or 19.76 lb/min
@ 7000 rpm = 329 cfm or 23.05 lb/min

To flow the turbo to its max capacity... ( turbo cfm / .047 = rpm? )
K03 turbo rated at 283 cfm would be at 6021 RPM
K04 turbo rated at 370 cfm would be at 7872 RPM
GT25 turbo rated at 408 cfm would be at 8680 RPM
GT28 turbo rated at 448 cfm would be at 9531 RPM

So first off, why would a K03 ever be an option for this car if it can rev beyond 6000 RPM? Sure, the turbo should never see this because the engine is not perfectly efficent, nor is the turbo.. but still its close, and we all know its weaksauce cause we run more boost and blow them I don't have maps for these off hand..

So .. better idea:

Looking at a compressor map for say a GT2554:



Its greatest efficency island seems to be around a PR of 1.4 (~6psi boost) and 13 lb/min (~4000 RPM on our motor) and range to a PR of 2.0 (14.7 psi boost) and 21 lbs/min (~6400 RPM on our motor) which might equate to a total of 390hp at that RPM. ( assuming 1hp equates to .108cfm x 2 turbos) This turbo seems ideal and you could obviously run more boost and make more power to redline. This setup seems pretty good.. so..

Why would we run a GT2860RS on a stock displacement motor?



Seems its range is about a PR of 1.5 ( ~7.35psi of boost ) to 2.3 (~19psi of boost) but at 17lb/min (about 5160RPM) to 28 lb/min (about 8500rpm) Sure there is the potential to make more power ( ~520hp @8500rpm ), but it seems like it would require more boost and take longer to make it..assuming we could safely rev to 8500rpm..

Is there any turbo size in between these two? Am I calculating this correctly? Anything else I should be considering?

[generationjdm]

gt2560 is in between which im running, and no you really cant go by the graph its to complicated to explain why.. plus the k03 doesnt peak at 6k the k04 does just a little over 6k more like 6500rpm...On the gt28 part your completely wrong

[AK1RA07]

How can I be completely wrong.. I calculated them all the same way. Is the data supplied by Garrett wrong? And if I cant go by the graphs, what else should I be using?

[generationjdm]

its not the data just the way your using it... First off gt2554s on r cars make 24+lbs before 4k and 28ss make over 24lbs under 4600rpmss.. Theres way to many variables in when selecting a turbo, so using the graph is almost useless. if your looking for when and what a turbo will do on r cars ask that question, that will be a lot easier

[AK1RA07]

I understand how this is possible, I am saying, if you are trying to properly size a turbo, you should pick the one where it is most efficent for the size of the motor you are using.. This is the only data I know that is available to do this.. In the case of the K03s, it seems like the motor could potentially outflow the turbos.. the GT2554 would be more efficent for a broader range of RPMs while requiring requiring less boost than a GT2860RS.. If I was looking to make higher peak HP dyno queen with a built bottom end to handle higher cylinder pressures at higher RPMs, I might run the GT28s.. right? What other variables are you taking into consideration when you selected the GT2560?

[generationjdm]

gt2560 is the best of both worlds, gt28 comp wheel and a gt25 turbine side.. R motors are very effeicent and with the motor being a 30 valve its much harder to go by the graph.. R motors really perform really well when selecting a larger turbo that u couldn't get a way with on other motors..

[revhards4]

I've always liked the GT2554 for our cars...especially on stock displacement. Plus it's very difficult to calculate because real world has so many variables. I think our motor is really efficient especially on the intake side with 3 valves per cylinder. Your post melted my brain btw...to much math for me

[gmann]

I was up until wee hours of the night a few days ago on Garrett's website also. I am looking for ~400 BHP that can "quickly" spool for great mid range power as I'll be tracking the car mostly. I didn't get as quite as far as you did though, I couldn't get the volumetric efficiency correct which was throwing everything off. But like already stated I knew there were way too many variables. I'm still working out the variables but from what I could tell the GT2560R is the best for the application. So I'll let you buy them first, try them out and let me know how it goes

[generationjdm]

I was up until wee hours of the night a few days ago on Garrett's website also. I am looking for ~400 BHP that can "quickly" spool for great mid range power as I'll be tracking the car mostly. I didn't get as quite as far as you did though, I couldn't get the volumetric efficiency correct which was throwing everything off. But like already stated I knew there were way too many variables. I'm still working out the variables but from what I could tell the GT2560R is the best for the application. So I'll let you buy them first, try them out and let me know how it goes

Thats what i just used on my personally car... follow the links below for info or pics

[AK1RA07]

I already have a different size turbo setup purchaced which I think I am going with.. but I am contemplating a GT as well.. My only issue is, with all things being ideal and 100% efficent / (which is what I would assume graphs and calculations would represent).. a GT25 seems more streetable in my opinion. I can't find it now, since youtube is blocked at my work, but there was a video of a GT28 S4 on a dyno and it took like a minute to wind it all the way up.. sure it made like 520hp and maybe they were dynoing in 5th gear or something, but unless you're porting heads and have some RS4 intake components, I'm not sure the motor effeciently flows that much in its current form, under ideal/100% efficent conditions..so why would I put them on assuming an engine is say 90-95% efficent? Not sure.

At any rate, that GENJDM GT kit is amazing looking, and I'd like to see how it runs once its finished, espeically with the 2560

[generationjdm]

Actually with the larger turbos you don't need any of the rs4 stuff to make power thats the beautiful thing about boost.. The Gt25ss are the way to go for all around particle purposes..

[gmann]

so why would I put them on assuming an engine is say 90-95% efficent? Not sure.

Because my 12V V6 has about 92% efficiency stock, and the S is a 30V V6. I'd expect it to be around 97-99% Volumetric Efficiency on paper. With boost well into the 150% range.


genjdm, I just finished reading both your build threads... Damn, that's about 99% of what I had thought about doing. Except I was going to paint my valve covers blue and intake mani black.

[MiniRS4]

its not the data just the way your using it... First off gt2554s on r cars make 24+lbs before 4k and 28ss make over 24lbs under 4600rpmss.. Theres way to many variables in when selecting a turbo, so using the graph is almost useless. if your looking for when and what a turbo will do on r cars ask that question, that will be a lot easier

isnt your data a lil generous/off?

24psi before 4k/4.6k? that's a lil too low IMHO.

[jibberjive]

Hey check out this thread
http://www.audizine.com/forum/showthread.php?t=203909
The GT2860r-707160-7 seems like the perfect medium and was what I was going to use for daily driving and racing at the road track

[bradyzq]

How do we choose? Please someone correct me where I'm wrong.

For ease I will just divide the engine in half and analize 1 turbo.. so a 1.35L motor with one turbo..

1.35L = .047 cubic ft.
1 lb/min = 14.27cfm
Assuming this volume is displaced once per revolution... ( rpm x .047 )

That's what one would think, but we have 4 stroke engines.

suck :
squish :---- 1 revolution
bang :
blow :---- 2 revolutions

This means that your 1.35L engine is ingesting air only every other revolution.

@ 3000 rpm = 141 cfm or 9.8 lb/min
@ 4000 rpm = 188 cfm or 13.17 lb/min
@ 5000 rpm = 235 cfm or 16.46 lb/min
@ 6000 rpm = 282 cfm or 19.76 lb/min
@ 7000 rpm = 329 cfm or 23.05 lb/min

So that makes the above lb/min figures twice as high as they really are.

BUT, those numbers are still for a normally aspirated engine. To get the air flow, you have to multiply the normally aspirated flow rate by the density ratio.

Density ratio is pressure ratio minus the losses due to turbo heating the air and intercooler pressure drop and intercooler inefficiency.

Pressure ratio is (at sea level) the absolute pressure in bar that you want to run.

To simplify things, let's assume you have a perfect intercooler (no pressure drop and 100% efficiency). Let's also assume you want 15psi boost (a pressure ratio of 2).

This mean that your density ratio is also 2.

So, NOW your calculated air flow table is correct for 15psi boost.

For the GT25, it shows it can push 15psi in a range of about 9-27 lb/min, most happily around 20lb/min. (just look across the pressure ratio 2 line). Or, from around 3000rpm to 8000rpm.

For the GT28, it can push 15psi from about 11lb/min to 37lb/min, most happily in the 22-27lb/min range. Or, from about 3400rpm upward.

Of course, these are theoretical, but are in the ballpark, if a little optimistic.

Based on the above, you can rethink what you've asked below. I think your questions will change a bit!!

To flow the turbo to its max capacity... ( turbo cfm / .047 = rpm? )
K03 turbo rated at 283 cfm would be at 6021 RPM
K04 turbo rated at 370 cfm would be at 7872 RPM
GT25 turbo rated at 408 cfm would be at 8680 RPM
GT28 turbo rated at 448 cfm would be at 9531 RPM

So first off, why would a K03 ever be an option for this car if it can rev beyond 6000 RPM? Sure, the turbo should never see this because the engine is not perfectly efficent, nor is the turbo.. but still its close, and we all know its weaksauce cause we run more boost and blow them I don't have maps for these off hand..

So .. better idea:

Looking at a compressor map for say a GT2554:



Its greatest efficency island seems to be around a PR of 1.4 (~6psi boost) and 13 lb/min (~4000 RPM on our motor) and range to a PR of 2.0 (14.7 psi boost) and 21 lbs/min (~6400 RPM on our motor) which might equate to a total of 390hp at that RPM. ( assuming 1hp equates to .108cfm x 2 turbos) This turbo seems ideal and you could obviously run more boost and make more power to redline. This setup seems pretty good.. so..

Why would we run a GT2860RS on a stock displacement motor?



Seems its range is about a PR of 1.5 ( ~7.35psi of boost ) to 2.3 (~19psi of boost) but at 17lb/min (about 5160RPM) to 28 lb/min (about 8500rpm) Sure there is the potential to make more power ( ~520hp @8500rpm ), but it seems like it would require more boost and take longer to make it..assuming we could safely rev to 8500rpm..

Is there any turbo size in between these two? Am I calculating this correctly? Anything else I should be considering?

The efficiency islands of these turbos don't have the same scales. To accurately compare the GT28 to the GT25, I would suggest using the same compressor efficiency as your guide.

For the GT25 the most efficient area appears to be 72%.

For the GT28, 72% or better is PR 1.2 at 9-12lb/min to PR 2.4 @34lb/min - PR 2.6 @25lb/min

Hope this helped!

[AK1RA07]

Hope this helped!

Sure did! Thanks for the help.

[gmann]

Hope this helped!

You're a fawking wizard!

How long did it take you to figure that out? I sat and pissed and moaned because I couldn't figure out how to make the chart for the car being N/A and not turboed... and you did it like that.