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

uiuc240 · 11-18-2002, 10:28 AM

This is becoming our own personal thread.  Hehehe.

CFM drops on a compressor when you raise the pressure level because the pump cannot keep up.  Think about it like this.  A spray gun needs a lot of air at a fairly high pressure to atomize paint...$$$ compressor.  A nail gun needs high pressure, but low flow...$ compressor.  Make sense?  As for how this relates to turbos:

The turbo has an efficiency range.  It is mapped for a given lbs/min and a given pressure ratio.  I figured out a bunch of that stuff just the other night for my car.  Strangely enough, piping diameter never really figures in to the calculation.  I think it basically has nothing to do with the flow potential of the compressor.

Here are my conclusions.

1.  CFM is CFM and PSI is PSI.  The two are *not* directly related.  The reason we use PSI is that it is easily measurable with a boost gauge...a CFM meter is basically the same as reading the voltage on your MAFS (like with an S-AFC).  

2.  Piping diameter will only affect performance if it's TOO small.  Keep in mind that as the air flows through the pipes and the intercooler, it will shrink (thermal contraction).  So, I think that even if a 2" pipe at the beginning causes a bottleneck, it'll average out by the time the air gets to the throttle body.

3.  However, as always, bigger is better.  If you have the room and the money to use big intake piping, it can't hurt.  I think the reason the stock SR piping is 2" is just because that's the size of the turbo outlet, and that's really more than you need to flow the CFM of a T25 set at 7psi.

Eric

BTW, what psi do you want to run?  I could size you a turbo right now.

11-18-2002, 10:28 AM	#8
uiuc240 Nissanaholic! Join Date: Apr 2002 Location: Naperville, IL Age: 42 Posts: 2,093 Trader Rating: (0)	This is becoming our own personal thread.  Hehehe. CFM drops on a compressor when you raise the pressure level because the pump cannot keep up.  Think about it like this.  A spray gun needs a lot of air at a fairly high pressure to atomize paint...$$$ compressor.  A nail gun needs high pressure, but low flow...$ compressor.  Make sense?  As for how this relates to turbos: The turbo has an efficiency range.  It is mapped for a given lbs/min and a given pressure ratio.  I figured out a bunch of that stuff just the other night for my car.  Strangely enough, piping diameter never really figures in to the calculation.  I think it basically has nothing to do with the flow potential of the compressor. Here are my conclusions. 1.  CFM is CFM and PSI is PSI.  The two are not directly related.  The reason we use PSI is that it is easily measurable with a boost gauge...a CFM meter is basically the same as reading the voltage on your MAFS (like with an S-AFC).   2.  Piping diameter will only affect performance if it's TOO small.  Keep in mind that as the air flows through the pipes and the intercooler, it will shrink (thermal contraction).  So, I think that even if a 2" pipe at the beginning causes a bottleneck, it'll average out by the time the air gets to the throttle body. 3.  However, as always, bigger is better.  If you have the room and the money to use big intake piping, it can't hurt.  I think the reason the stock SR piping is 2" is just because that's the size of the turbo outlet, and that's really more than you need to flow the CFM of a T25 set at 7psi. Eric BTW, what psi do you want to run?  I could size you a turbo right now. __________________ 2002 Mazda Protege5 1989 S13 w/SR20DET (sold)