Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum - View Single Post - Turbo Sizing Tips and Compressor Map How-To's

Kreator · 11-18-2002, 10:03 AM

Ok i kinda get it, but this question has been buggin me since i bought my compressor but never had a chance to ask. Why if i raise the pressure, the CFM drops? From what i was thinking more pressure => air coming through faster => more output per minute. So why does CFM go down as pressure goes up?

Now lemme get this straight:
CFM - cubic feet per minute - volume of air the turbo flows per minute right?
PSI - Pressure per square inch - pressure of the air inside the pipes(between the turbo and throttle body) correct?

Now say i want to flow 300cfm at 4000rpm. If I use 2.5" piping i get about 19.6si area. If i go 2.0" i get 12.5si area. But since i still want 300cfm flow, won't the turbo have to spin faster so it could push the air faster and thus raise psi?

Oh, and i'm considering a T3 turbo with .60/.63 i/e housing a/r ratio (if i luck out) or .42/.48 in the worst case. I'm going to reread The Book tomorrow (when my friend gives it back to me) so i'll compute the cfm and all the other stuff from there...

11-18-2002, 10:03 AM	#7
Kreator Nissanaholic! Join Date: Mar 2002 Location: MD Age: 37 Posts: 1,975 Trader Rating: (0)	Ok i kinda get it, but this question has been buggin me since i bought my compressor but never had a chance to ask. Why if i raise the pressure, the CFM drops? From what i was thinking more pressure => air coming through faster => more output per minute. So why does CFM go down as pressure goes up? Now lemme get this straight: CFM - cubic feet per minute - volume of air the turbo flows per minute right? PSI - Pressure per square inch - pressure of the air inside the pipes(between the turbo and throttle body) correct? Now say i want to flow 300cfm at 4000rpm. If I use 2.5" piping i get about 19.6si area. If i go 2.0" i get 12.5si area. But since i still want 300cfm flow, won't the turbo have to spin faster so it could push the air faster and thus raise psi? Oh, and i'm considering a T3 turbo with .60/.63 i/e housing a/r ratio (if i luck out) or .42/.48 in the worst case. I'm going to reread The Book tomorrow (when my friend gives it back to me) so i'll compute the cfm and all the other stuff from there... __________________ кто болеет за Зенит, у того всегда стоит!