I found this information on how to get a pretty good estimate on calculating airflow:
(CID x RPM) / 1728 (use 3456 for a 4-stroke)
So a Polaris 800 has 795 CC of displacement which equals 48.5138766 cubic inches.
Lets use a Max RPM of 8400
(48.513 x 8400) = 407509.2
407509.2/1728 = 235.827
CFM (cubic feet per minute)
That is based on 100% VE (Volume efficiency)
It is possible to have 100% efficiency if you have good porting done on the motor. I'll figure 85% on a stock non ported motor.
235.827 (.85) = 200.53 CFM (corrected)
This is an estimate and usually holds pretty true but obviously if the motor breaths better or worse, it can change. It is a good starting point to select the proper turbo.
Source and even more info:
http://www.theturboforums.com/turbotech/main.htm
