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

uiuc240 · 11-26-2002, 09:12 AM

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (Kreator @ Nov. 26 2002,09:57)</td></tr><tr><td id="QUOTE">Uhh, lemme bring this up back from the dead <img src="#/f/iB_html/non-cgi/emoticons/crazy.gif" border="0" valign="absmiddle" alt=':crazy:'>

I guess this question will go all the way back to the CFM vs PSI part. How come the formula for calculating CFM (CFM = V*RPM*VE/3456) doesnt include boost? Does that mean that at 25psi i will need only as much flow as on a n/a engine? Or is it just cuz CFM is this imaginative figure which is used to calculate the compressor mappings, and if you raise boost u stay at the same CFM and just move up along the y axxis(pressure ratio)?

Basicly trying to figure what'll happen if i decide to run 8psi</td></tr></table><span id='postcolor'>
All you're figuring out in that first calculation is how much airflow comes OUT of your motor.  Then, the turbo, in all its super-efficient glory, turns that in to MORE airflow by spinning the compressor.  So yes, in your baseline calc, it's the same if you were N/A.

Once again, PSI really is only part of the issue.  You can use it to reference with a gauge, and you can use it for a RRFPR.  But that's just about it.  The biggest thing for making power is the total CFM that the turbo is producing at the PSI that you select to open the wastegate.  If you choose 6 psi, and that's 300 cfm (at 7000 rpm on the engine) on turbo A and 250 cfm on turbo B, turbo A will make more power.  Easy, right?  

So ya see, PSI is related, but you can't convert between the two.

We took this sooooo far from BOVs...awesome.

Eric

11-26-2002, 09:12 AM	#26
uiuc240 Nissanaholic! Join Date: Apr 2002 Location: Naperville, IL Age: 42 Posts: 2,093 Trader Rating: (0)	</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (Kreator @ Nov. 26 2002,09:57)</td></tr><tr><td id="QUOTE">Uhh, lemme bring this up back from the dead <img src="#/f/iB_html/non-cgi/emoticons/crazy.gif" border="0" valign="absmiddle" alt=':crazy:'> I guess this question will go all the way back to the CFM vs PSI part. How come the formula for calculating CFM (CFM = VRPMVE/3456) doesnt include boost? Does that mean that at 25psi i will need only as much flow as on a n/a engine? Or is it just cuz CFM is this imaginative figure which is used to calculate the compressor mappings, and if you raise boost u stay at the same CFM and just move up along the y axxis(pressure ratio)? Basicly trying to figure what'll happen if i decide to run 8psi</td></tr></table><span id='postcolor'> All you're figuring out in that first calculation is how much airflow comes OUT of your motor.  Then, the turbo, in all its super-efficient glory, turns that in to MORE airflow by spinning the compressor.  So yes, in your baseline calc, it's the same if you were N/A. Once again, PSI really is only part of the issue.  You can use it to reference with a gauge, and you can use it for a RRFPR.  But that's just about it.  The biggest thing for making power is the total CFM that the turbo is producing at the PSI that you select to open the wastegate.  If you choose 6 psi, and that's 300 cfm (at 7000 rpm on the engine) on turbo A and 250 cfm on turbo B, turbo A will make more power.  Easy, right?   So ya see, PSI is related, but you can't convert between the two. We took this sooooo far from BOVs...awesome. Eric __________________ 2002 Mazda Protege5 1989 S13 w/SR20DET (sold)