How to figure out which turbo and to expect

Because of all the quesions as to which size turbo should I use heres a little info I found on how to get a rough idea as to which size turbo to use and how much power you can expect to get from it. All you need is a little basic info and the compressor maps (any reputible manufacturer will supply them). Its a lot of calculating but it works.



1. Calculate the airflow for the engine in naturally aspirated
form. Use this formula for standard atmospheric pressure:
(CID x RPM x 0.5 x VE) / 1728. The airflow rates will be in CFM
VE stands for volumetric efficiency usually 0.8-0.9


2. Knowing the desired boost level, calculate airflow rate under
boost by multiplying the pressure ratio by the airflow rate
(NA-CFM). To figure out the pressure ratio take (14.7 + boost)
divided by 14.7.


3. If you are runnning twin turbo divide the total cfm under boost
by 2.


4. To convert CFM to lbs/min, use (CFM x 0.076 = lbs/min).


5. Use compressor maps to find the turbo best suited to the
airflow rate and pressure ratios you have obtained.


Heres a low and a high range for horsepower expectations

Low: 0.052 x CID x (psi boost + 14.7) = bhp

High 0.077 x CID x (psi boost + 14.7) = bhp

All information was obtained from HKS USA