If youve ever wondered what effect a turbo would have on your car, read on how to calculate the resulting horsepower. but before you start whipping out your calculator, you must first understand a couple things. First, you must understand Rankin. Put simply, Ranken is farenheit only zero in Ranken is absolute zero. If you can understand Kelvin, you can understand Ranken. Absolute zero in Farenheit is -460, so zero in Farenheit is the same as 460 in Ranken. You dont need to understand it, its just better to to understand how the following formulas work. If you dont understand why to use 460, dont worry, all you have to do to get the according degree of farenheit in ranken is add 460 to the according degree. So 85 in farenheit is 460+85 in ranken, or 545. Second, you need to understand the variables i will be using in these equations. Heres a list of them:

Pco=pressure out of compressor
B=boost pressure
A=atmospheric pressure
I=pressure across intercooler (assume 1.5, do not use if you dont have an intercooler)
Pr=pressure ratio
Di=density of air post-compressor/intercooler
R=53.3 (a constant from the Ideal Gas law)
Mf=mass flow rate
D=displacement (in cubic inches)
Mp=RPM of match point (peak power)
V=volumetric efficiency (on most 16 valve engines assume 90% or .9, on 8 valve assume 80% or .8)
CMf=corrected mass flow (ratio of atmospheric pressure compared to pressure drop caused by turbulence in the filter)
Ci=compressor inlet temp (temp of the air being taken in by the compressor, or air temperature outside)
Ca=Corrected air pressure (air pressure in turbo after filter but before compression, about 13.95, calculated by Garrett)
Hp=horsepower (duh )
Fa=feul to air ratio
BSFC=brake specific feul consumption, in pounds of feul/hp/hour (assume .5 in most conditions)

*'s will be around #'s you are to square root

now that you have those, lets calculate your horsepower!

for examples i will be using a B18C1 running at 6psi of boost with an intercooler at 1 bar of atmospheric pressure (14.7) on an 85 degree day. Temperature wont affect the HP much, but it can to a certain extent.

1)Calculate Pco

Pco=B+A+I

ex.
Pco=6+14.7-1.5
Pco=19.2

2)Calculate Pr

Pr=Pco/A

ex.
Pr=19.2/14.7
Pr=1.31

3) Calculate density of air post compressor/intercooler
To get this, you must estimate post intercooler air temp (about 130F)
This is where knowing Ranken comes in

Di=(Pco)/(Rx12x(460+130))

remember, R=53.3
12 is used to keep the inches units of this formula

ex.
Di=(19.2)/(53.3x12x(460+130))
Di=19.2/377,364
Di=.0000508

4) next we must calculate the Mf

Mf=(DixDxMp)/(2xV)

ex.
Mf=(.0000508x110x8,000)/(2x.9)
Mf=44.704/1.8
Mf=24.836

5) Calculate CMf

CMf=(MFx*(460+Ci)/545*)/(A/Ca)

ex.
CMf=(24.836*(460+85)/545*)/(14.7/13.95)

in this example the square root will be the square root of one, which is one.

CMf=(24.836)/(1.053)
CMf=23.586

5)Next is the final step, calculating your horsepower. YAY

Hp=(CMfx60)/(FaxBCFC)

60 is to convert minutes to hours
Fa is about 10.5:1 on a turboed car
assume .5 for BSFC

ex.
HP=(23.586x60)/(10.5x.5)
HP=1415.16/5.25
HP= about 269.55


have fun, it takes about 15-20 mins so dont think ur gunna b wasting ur time, its worth it considering its less than half an hour.

 

Wow, I thought I was the only one who actually cared about turbo formulas. Nice post.

 

it is nice to kno that u can actually calculate the amount of power u can expect by boosting ur car though. Very interesting post.

 

glad to see some ppl appriciate it...i spent three hours going through a physics book to find formulas to calculate that it was fun tho

 

was just readin through one of my Honda/Acura performance books...i found that they used the exact same formulas as i did ('they' are pros) suddenly...i feel a lot smarter than normal :P

 

(Youre a good man Charlie Brown.)

[IMG]local://upfiles/2201/1A122725E61546AA86FDA8DB6FA099B4.jpg[/IMG]

 

i found out why the number seemed so high. i added the post intercooler pressure instead of subtracted. so i'll re-crunch the numbers and fix it.

EDIT: k just changed it, it dropped about 20 hp which makes the number seem about right. so if you calculated ur hp w/ a turbo before i changed it to subtraction, your going to have to re calculate it. sry for the problem

 

bump for ppl who joined since and are wondering