Size of Air Filter for Cubic feet of air (Under Boost)
 

Thanx for looking-
The turbo aspect throws me -
Engine - Twinn cylinder, water cooled - 98 x 70 mm , Turbocharged 1056cc , 4-stroke.
Boost is around 20 lbs max . 9000 RPM
I have no idea how to figure it out when compensating for the boost.
The question is - what size air filter to use for CFM drawn or pushed?
Thanx!

Trick question. Run no air filter;)
V twin? sounds hand built. I would run any cone filter that can fit directly to the inlet of the turbo.

Hmmmm I probably put too many sig figs for the application, but it should get you into a ball park.

Try
([(64.441 in^3) x (9000 RPM) x (1.360 atm)]/3456) = 230 cfm rounded up

1.360 atm = (20psi/1)/(1 atm/14.7psi) at sea level.

Did a guick google search. Does this help?

Turbocharger Compressor Calculations

Use the biggest air filter you can. Less restriction is better.