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

uiuc240 · 11-16-2002, 09:38 PM

What you really need to be looking at is CFM or lbs/min, not simply PSI.  For instance, imagine a small turbo that puts out something like 400 cfm at max flow.  This is all it will flow.  Period.  If you put bigger pipes on, the velocity of the air will change, the temp will change, and the pressure drop will change...things like that.  But PSI is fairly irrelevant in the equation.  This is one of the reasons that after a certain level, turning the boost up on a small turbo does not give any more power.  The turbo simply cannot produce usable airflow (CFM) at that pressure level.

It's probably easiest to talk about the type of air compressors you find at Sears.  They have a rating that goes something like this:

6.9 CFM @ 40 psi
4.3 CFM @ 90 psi
2.7 CFM @ 120 psi

Those are made up numbers, but you get the point.  This is showing you that if you need something that uses high CFM levels (spray gun, for example) then you need to carefully consider the PSI level that tool requires.

A turbo works the same way.  

All I'm saying is that it's not going to change your PSI by going from 2.0" to 2.5" piping.  The only thing that *might* change (if there was a substantial restriction) is intake temp and pressure drop (both will decrease).

You need to consider the CFM of your turbo at a given PSI (you mentioned 4 psi...which is low).  I would imagine you'd only be flowing somewhere in the 300 CFM range with a T3/T4 at that psi.  But that's a total guess.

But if you know what the CFM is at your psi level, then you can figure out what pipe size you need.  Get it?

Eric

11-16-2002, 09:38 PM	#6
uiuc240 Nissanaholic! Join Date: Apr 2002 Location: Naperville, IL Age: 42 Posts: 2,093 Trader Rating: (0)	What you really need to be looking at is CFM or lbs/min, not simply PSI.  For instance, imagine a small turbo that puts out something like 400 cfm at max flow.  This is all it will flow.  Period.  If you put bigger pipes on, the velocity of the air will change, the temp will change, and the pressure drop will change...things like that.  But PSI is fairly irrelevant in the equation.  This is one of the reasons that after a certain level, turning the boost up on a small turbo does not give any more power.  The turbo simply cannot produce usable airflow (CFM) at that pressure level. It's probably easiest to talk about the type of air compressors you find at Sears.  They have a rating that goes something like this: 6.9 CFM @ 40 psi 4.3 CFM @ 90 psi 2.7 CFM @ 120 psi Those are made up numbers, but you get the point.  This is showing you that if you need something that uses high CFM levels (spray gun, for example) then you need to carefully consider the PSI level that tool requires. A turbo works the same way.   All I'm saying is that it's not going to change your PSI by going from 2.0" to 2.5" piping.  The only thing that might change (if there was a substantial restriction) is intake temp and pressure drop (both will decrease). You need to consider the CFM of your turbo at a given PSI (you mentioned 4 psi...which is low).  I would imagine you'd only be flowing somewhere in the 300 CFM range with a T3/T4 at that psi.  But that's a total guess. But if you know what the CFM is at your psi level, then you can figure out what pipe size you need.  Get it? Eric __________________ 2002 Mazda Protege5 1989 S13 w/SR20DET (sold)