Since I'm new to this site i figured I would contribute a little tech. Before I went turbo I decided to find out what size turbo is capable of supporting around 1200 HP (flywheel), roughly 1000 rwhp. This was my HP goal for a stock 4V cobra motor. I used a VE of .95 and a BFSC of.85 to determine my final numbers. Those are pretty important numbers so I wanted to be as accurate as possible. I got a pressure ratio of 3.30 and a PsiA of 45.25. The math is the same for any engine, so just plug in your data (cubic inches, etc.) to get the appropriate map plot. Then find some compressor maps and shoot for the proper efficiency island. This also works for centrifugal superchargers.

ROUGH ESTIMATE FIRST:
1200HP @ 10 hp per pound of air (used commonly for smaller turbos) = 120/2 = 60 ppm per compressor for a twin configuration. For single turbos, do not divide by 2.

Actual Math.

Wa = HP x A/F x BSFC / 60 SO 1200 x 11.7 x .85/60 = 198.8

Wa = Airflow actual (lb/min) 198.8 /2 = 99.45 (twin turbo application)
HP = Target flywheel HP
A/F = Air Fuel Ratio
BSFC(Brake Specific Fuel Cons.) = convert hours to minutes divide by 60

Divide by two in this step for twins.

MAPreq = Wa x R x (460 + Tm) SO 99.45 x 639.6 x 590 All that equals = 43.25 Psi(A) when:

divide by VE x N/2 x VD SO .95 x 3250 x 281



· 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) (130 typical intercooled temp, use 220-230 for non-intercooled)
· VE = Volumetric Efficiency
· N = Engine speed (RPM)
· Vd = engine displacement

43.25 (Absolute press.) -14.7 (atmospheric press.) = 28.5 PsiG (PsiG = Gauge pressure)

One should assume that there is a 2 psi inefficiency loss. So to determine the Compressor Discharge Pressure (P2c), 2 psi will be added to the manifold pressure calculated above.

P2c = MAP + ΔPloss 43.25 + 2 = 45.25

Where:

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

P1c = Pamb - ΔPloss 13.7 = 14.7 - 1


Where:

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

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

P2c / P1c = Pressure ratio 45.25/13.7 = 3.30


Pretty simple huh? Now just look up the compressor maps and find out exactly which ones you need. I also figured 1200 fwhp would equal 1000 rwhp in most drivetrains. Again, you will be able to find out what size turbo you would need in a single turbo configuration by not dividing Ma (Airflow PPM) by 2.

 

What answer did you come up with on your set-up?

 

i tried to do that but i got lost.
so a single turbo for 1100 fly wheel hp, im confused

 

88 mm Turbo moves roughly 120 lb per min. 120ppm=1200 rwhp, it'll make that power on a 2l engine or on an 8l engine. Compressor side is easy, its figuring out the exhaust side that's tricky...

 

It's so painful to still see people trying to plug away at the old school equations ...

Check out my Turbo Calculator

It can take you from 0 to a list 10 proper turbos matching your needs in under 5 minutes.

 

Or you could do the simple math yourself and save the money. I'd rather do it myself than put my trust into somebody's calculator. Sure, there's a lot of steps but none of them are difficult.

 

Sorry if there is confusion...this is just the math that people can use when they want to learn for themselves how to read compressor maps on there own, and NOT use just a "calculator". Every tool serves it's purpose, but the knowledge behind the tooling (math)...to make your own tools is what inspires true thought.

 

Great post, very informative. So what is the secret to hot side sizing? That's what I want to know.

 

I'd love to know this secret as well. Unfortunately all I know is of what has "worked" for others so its easy to just use what others have had success with. That being said I'm assuming flow area of the turbine exducer, CI of the engine, engine rpm and compression ratio and even fuel used are factors involved. Again the compressor side is fairly easy, the compressor moves a certain amount of air and that equals a certain amount of HP. So you can have two compressors (I.e 76mm both capable of 700+ hp) one for a 2.0l four cylinder and one on a 6.0 v8. Same compressors but will require totally different turbine wheels. Am I way off on this?

 

Dead on, although I'm in the same boat on the turbine side. I'm guessing there has to be something out there that sheds some light on it though, even if it just uses displacement and rpm as variables to help pick a wheel and a/r.

 

Hot sides I have never seen any real "flow data" or mandatory size needed. The reasoning is that hotsides are sensitive to downpipe size and every car has different hot side setups. Honestly I have tried two different sizes and stuck with the one that made the most power. Everyones setups will be different.

 

If you are looking at a consumption chart and it says 44lbs/minute at a given rpm does that mean that you divide that value by two?

So 22 for each turbo?

 

Honestly...hot sides have flow data as well. Most turbo manufacturers advertise hot side flow data when they sell a turbo. But, in general terms, you want the LARGEST hot side you can live with. What I mean by live with is the larger the hot side...the laggier the turbo, but the more peak HP it will make. For an LS, I would run a .63 on a twin setup up to 800 rwhp, anything more I'd go with some .82's. Single turbo...you just gotta find the right one for you. No other way to put it.

 

I did the math on this thread 3 times and something doesn't add up. For a twin turbo 427 big block, and wanting 1000 flywheel HP. This is what i came up with. 75LB/MIN and .92 pressure ratio. Something is wrong with this picture. I have never been able to get a pressure ratio over 1.88 with any formula's i have found. And yes i am good at math. Any suggestions would be greatly appreciated. Thanks in advance.

 

Type out the math you did. You might find a mistake when you do Don't know if anyone has used this math and tested their setups, but it is very much an ideal situation. There are many real world losses that factor into the end horsepower result.

 

Jake, the problem is the numbers that i have to guess at. BSFC and AFR. this engine isn't built yet. just starting. i did find that i had switched the BSFC and the VE. i corrected the mistake and the pressure ratio was worse. .20 from a .92. it's the values i'm putting into the formula's. i'll have to wait until it's on the dyno and i can get some accurate numbers. until then no turbo math!!LOL thanks

 

If the math isn't working out for whatever reason, you can use the free calculator at this site:

http://www.squirrelpf.com/turbocalc/

It is quick and dirty and only takes the compressor into consideration, but I have found it useful on many occasions. It also has a decent list of compressor maps to choose from.

 

thanks jake. i'm currently reading all the posts here under "forced induction". new to the turbo stuff so any information i get will be a big help. thanks for the link.

 

Don't forget to plot lower engine speeds ie. 3000, 4000, 5000, to watch out for the surge line.
If you plot 4 or 5 spots on the map it will give you a better understanding of how the turbo is working down low.

 

thanks 99. the link smokeshow gave me does a pretty good job. i think it maps 4 different RPM points. i know this isn't carved in stone. but it is interesting to change the AFR and watch the LB/MIN and the Pressure Ratio change. Again i will be consulting some professionals on twin turbo setups just to make sure i don't waste alot of money. I will do it on my own in the beginning just to see how close i get without any help! Thanks for the tip.