choosing the correct turbo per motor

[dug]

I was putting this together to find out if the turbo I have is going to be too small for my goals, maybe this will help some of you guys out also...

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:::: Engine Airflow Requirements :::
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(CID × RPM) ÷3456 = CFM

CFM × 0.85 = CFM @85%VE

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:::::::::: pressure ratio ::::::::::
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(psi + 14.7) ÷ 14.7 = pressure ratio

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:::::: Temperature rise ::::::::::::
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T2 = T1 (P2 ÷ P1)^0.283

Where:
T2 = Outlet Temperature °R
T1 = Inlet Temperature °R
°R = °F + 460
P1 = Inlet Pressure Absolute
P2 = Outlet Pressure Absolute

T1 = temp°F + 460 = °R
P2 = 14.7 + psi = psi

°R - 460 = °F Ideal Outlet Temperature

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:::::::: Adiabatic Efficiency ::::::
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(ideal outlet temp - inlet temp) ÷ 0.7 =°F Actual Outlet Temperature Rise
temp + actual rise = °F Actual Outlet Temperature

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::::::::: Density Ratio ::::::::::::

(Inlet °R ÷ Outlet °R) × (Outlet Pressure ÷ Inlet Pressure) = Density Ratio

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::: Compressor Inlet Airflow ::::::
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Outlet CFM × Density Ratio = Actual Inlet CFM
CFM × 0.069 = lbs/min

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HERE'S WHAT I CAME UP WITH
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:::: Engine Airflow Requirements :::
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(98 × 6600) ÷3456 = 187.15 CFM

187.15 × 0.85 = 159.08 CFM @85%VE

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:::::::::: pressure ratio ::::::::::
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(15 + 14.7) ÷ 14.7 = 2.02 pressure ratio

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__________________________________

:::::: Temperature rise ::::::::::::
__________________________________
T2 = T1 (P2 ÷ P1)^0.283

Where:
T2 = Outlet Temperature °R
T1 = Inlet Temperature °R
°R = °F + 460
P1 = Inlet Pressure Absolute
P2 = Outlet Pressure Absolute

T1 = 75°F + 460 = 535°R
P2 = 14.7 + 15 = 29.7psi

T1 = 535
P1 = 14.7
P2 = 29.7

T2 = 535 * (29.7 ÷ 14.7)^0.283 = 652°R

652°R - 460 = 192°F Ideal Outlet Temperature

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:::::::: Adiabatic Efficiency ::::::
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117 ÷ 0.7 = 167.14°F Actual Outlet Temperature Rise

75 + 167.14 = 242.14°F Actual Outlet Temperature

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::::::::: Density Ratio ::::::::::::

(535°R ÷ 652°R) × (29.7 ÷ 14.7) = 1.66 Density Ratio

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::: Compressor Inlet Airflow ::::::
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159.08 × 1.66 = 264.07 Actual Inlet CFM

264.07 × 0.069 = 18.22lbs/min

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let's take a look at how this all adds up, here is a compressor map for the T3 "40" trim


2.02 pressure ratio
18.22lbs/min


if you intersect the lines it lands outside its efficiency range


this shows that when the engine is hitting peak HP @ 15psi the turbo will be operating outside of it's efficiency range, so you will need to go bigger with the turbo, in this case the T3 "50 trim" would be perfect

[dug]

bump, this is good info, especially for anyone wanting to go custom when it comes to choosing the right turbo or even if your looking to buying a pre fab kit, you all should take this info and find the best possible turbo for you...

[uoboarder]

Awesome post Dug! This is killer!

[dug]

thanks, I was thinking about redoing it, explaining every part in detail, but I don't think too many people care about matching the right turbo with the right engine...

[drifted]

Originally posted by dug
thanks, I was thinking about redoing it, explaining every part in detail, but I don't think too many people care about matching the right turbo with the right engine...

they do... but unfortunately they either cant grasp the math, or they're too ricey to afford a turbo system anyways after they get their Pepboys bill. above all else, they'd rather have their parents buy them a prefabbed kit.


also, here's a site that has various compressor maps for various turbos... plus a turbo calculator that took all of your formulas and applied it into an easy to use Java component. just plug in your engine specs and out pops the closest matches in turbo size for your setup.

http://www.turbofast.com.au/TFmatch.html

[Sunder]

I like

I did some calcs myself based off airflow and power expectations, and came to the conclusion that I needed a T3 or a T3/4...

I plugged in my numbers in the Java script thingy. I got back that I need a:

T3 w/ Compressor Trim of 50, and a Turbine HSG A/R of 0.82

If I want to up the boost a little, I want to increase my compressor trim to 60 and Turbine HSG A/R to 1.06

Anyone care to explain?

EDIT: Just having a quick look, the Garrett GT3040, the one I had picked before is the closest match to the specs the Java script gave me.

[drifted]

Originally posted by Sunder
I plugged in my numbers in the Java script thingy. I got back that I need a:

T3 w/ Compressor Trim of 50, and a Turbine HSG A/R of 0.82

If I want to up the boost a little, I want to increase my compressor trim to 60 and Turbine HSG A/R to 1.06

Anyone care to explain?

the increase in compressor size is to help support a bit more boost with acceptable overall air volume.

the increase in turbine a/r is to compensate for more air being forced in... more air in = more air out, so you need more flow or the housing becomes an obstruction.

in your case, you're motor is probably right on edge, maxing out with the smaller turbo, and any more power gains would have to be achieved with a larger turbo for more efficiency.


also, the java calc doesnt have hybrid turbo compressor maps loaded in.

[Sunder]

The GT3040 is mean to be capable of providing air for up to 600bhp... My target HP is more like 350... So why would the turbo be so close to the edge?

[drifted]

Originally posted by Sunder
The GT3040 is mean to be capable of providing air for up to 600bhp... My target HP is more like 350... So why would the turbo be so close to the edge?

realise that the 3 main things you need to watch for in a turbo are: air in, volume of air in, and air out.

you have to have a turbo that flows enough air to suit your needs. more volume of air helps by not having irregular feeding, as the conditions change on the motor from throttle angle and RPM, the turbine produces enough total air to keep feeding the motor. as the exhaust gas exits, it spins the turbo... restricting the exhaust thru the turbine spins it up faster, but too restrictive will cause backpressure and compressor surge.

in the case of a T3/TO4 hybrid, considering i dont know the A/R ratios of either the hot or cold side, nor the compressor trim, it's impossible to haphazard a guess... but with an output capable of 650hp, that's a lot of flow. but with that flow, is it tuned for your redline? your redline being ~9000rpm on the S2K, the turbine housing has to be large enough to flow all that exhaust gas at peak rpm, yet be small enough to spool the turbo efficiently.

also consider rotating mass... if the compressor side supports 650hp, and you only want 350hp, obviously you could get away with using a smaller turbo, thus allowing boost to come on sooner in the rpm band due to less rotational mass for your exhaust gas to have to push.

in your case, i would guess a 60 trim compressor section with at least a 60 A/R compressor housing, mated to a 1.06 A/R turbine housing, all T3 Garrett. this would provide you the power level you desire.

now i did notice that a difference in 2 psi changes compressor maps to a T4 O trim setup with an S3 compressor. it's always better to slightly upsize so you're not running excessively high temps in the turbine, and also from compressing. to give you slightly longer "legs", a TO4 S3 compressor mated to a T3 1.06A/R center and turbine section, resulting in a T3/TO4 hybrid, would probably yield you better spooling results and still give you enough headroom should you need it.

full boost at 4000+rpm is kinda useless in my opinion... it's nice compared to nothing, but it should come on at lower levels to be even more useable. a turbo too large for your application, or improperly sized and set up will result in boost coming on at almost worthless rpm levels. a properly matched system will provide full boost around 3000rpm or less.

now in the case of dramatic power levels, like 650hp... the turbo can only spool so fast with the amount of exhaust gas provided. so obviously a 650hp civic will reach full boost later than a 350hp civic... any turbine housing smaller to encourage faster spoolup will be a cause of backpressure at higher rpm due to much more exhaust gas from all the boost.

a very careful balance, as you can see, but still based on the 3 points to address on a turbo as i stated at the beginning.

[Sunder]

Thats going to take me a while to chew through that one!

Looking at pure HP only, it looks like I can get away with a T28 Coupled with a t3 compressor. However, I'd rather oversize and spool a little later than undersize and starve at the top end.

One thing I hate people saying about the S2000 is that "To generate real power, you need to rev the high hell out of it".

Well, true, but in this engine, who cares? You commonly cruise at 3500rpm and life doesn't start till 5700rpm. To get from 3000rpm 6000rpm is less than a second on full throttle.

If I could get spooling by 3000rpm and full boost by 4000rpm, I'd be more than happy!

[dug]

Originally posted by drifted

full boost at 4000+rpm is kinda useless in my opinion... it's nice compared to nothing, but it should come on at lower levels to be even more useable. a turbo too large for your application, or improperly sized and set up will result in boost coming on at almost worthless rpm levels. a properly matched system will provide full boost around 3000rpm or less.

thank you

[Sunder]

Originally posted by dug
thank you

On an S2000, you could say any boost applied below 4000rpm is wasted, because you never spend any time down there!

NA dump at 6000rpm, drops to 4000rpm, back to 6000 within 3/4 of a second, shift at 9000rpm, which drops back to 5500rpm. By the time you finish the quarter mile, you never spent any time below 4000rom!

With boost, I'd expect you'd launch at 3000, and be in full boost + VTEC, by 5700!

[dug]

picking a turbo for your application is going to be tricky, I did some rough calculations and your car flows
28.45lbs/min @85%volumetric effiency (8300rpm) and only
13.74lbs/min @85%volumetric effiency (4000rpm)

now your wanting to be at full boost at 4000 rpm, so you have to find a turbo that will not be going above the surge limit at lower rpm's but yet is still effiecent at higher rpm's...

you can find some compressor maps here

I've already supplied you with half the numbers you need, I'm not sure what kind of boost your wanting to run, but to find the pressure ratio, use this formula
(boost + 14.7) / 14.7 = pressure ratio
say you want to run 10 psi it would be
(10 + 14.7) / 14.7 = 1.68

[Sunder]

Dug,

Talk to me about reading these compressor maps. They look interesting

The thing with the S2000 is that at 8300rpm, the car actually has a volumetric efficiency of 101% Yes, that's right, a VE of over 100%. Something to do with resonance forcing air in at higher than atmospheric pressure.

Thanks for your help so far by the way. As you can tell, I'm new to the Forced Induction world, after several years of looking down my nose at the "cheat's" way of making a car go faster

(when I realised that getting massive numbers out of a NA engine was extremely expensive and made the car unstreetable, I kind of changed my mind)

[uoboarder]

Originally posted by Sunder
Looking at pure HP only, it looks like I can get away with a T28 Coupled with a t3 compressor. However, I'd rather oversize and spool a little later than undersize and starve at the top end.

Well, true, but in this engine, who cares? You commonly cruise at 3500rpm and life doesn't start till 5700rpm. To get from 3000rpm 6000rpm is less than a second on full throttle.

If I could get spooling by 3000rpm and full boost by 4000rpm, I'd be more than happy!

Sunder, its good to follow the 15% rule. That is... don't hyrid a T4 and a T25. At first thought I'd think that putting a T3 exhaust and a T28 comp. together would be a little too extreme.

Its good to read that you don't care that you wouldn't have full boost till 4k. I cruise on the freeway in 5th between 3200 and 4200 rpm and not having boost onset is till 3300 is good. Especially if when launching. After all...turbo lag is your friend when it comes to traction.