CFM Continued...

[David Sagers]

Thanks to Ken, Todd & Gary for your answers regarding CFM.  Perhaps I didn’t 
ask the question correctly.   A stock TPI runs out of flow about 4800 rpm on 
a stock 350 ci engine.  I’m looking for an EFI intake system for my twin 
turbo 434 ci engine.

The guy putting the turbos together said that the stock TPI is a good choice 
because it will give me something really important for a fun to drive street 
car, great throttle response and low end torque.

The turbo system is designed to start making boost about 2,500 RPM and by 
3,000 RPM the turbos are really coming on strong.   He  explained that about 
the time the NA stock TPI would be running out of flow the turbos will start 
pressurizing the intake system and with 15 lbs of boost building even the 
small runners would flow enough to feed the engine without going to larger 
tube runners.

I realize that if this were a NA engine the stock TPI would be a bad choice, 
I would need something like a TPIS mini ram.  The mini ram flows some good 
numbers but even TPIS admits it will make the throttle response and low end 
torque somewhat soggy.  However, the situation changes when the intake is 
pressurized with a turbo
system instead of running a vacuum in the intake.  Now my question, even 
with 15 lbs of boost it seems to me that the stock TPI is still small for a 
434 performance engine, or will the boost really make up the difference for 
the small tubes?

Finally, no web site yet, but I’ll talk to some computer guys I know and see 
if I can set up a site with some pics.

Thanks for your help.

Date: Tue, 04 May 1999 09:09:44 -0400 From: Ken Kelly <kenkelly at lucent.com>
Subject: Re: CFM Questions

Dave, Flow is measured for a given pressure drop across the runner. I really 
can't imagine that this means a 600cfm flow with a 15 psi pressure drop. Who 
would ever spec a runner with that much drop. What is the pressure 
differential that creates a 600cfm flow through the tube. Then can you live 
with that much pressure drop? Ken

Date: Tue, 4 May 1999 10:14:46 -0400 From: "Gary Derian" <

derian at oh.verio.com> Subject: Re: CFM

Questions Talk of CFM is very confusing. When someone has a 600 CFM Holley, 
it means if flows 600 CFM at 1.5 in. Hg pressure drop. That same carb will 
flow 840 CFM at 3" and 350 CFM at 0.5". There is no absolute flow limit 
until the air reaches the speed of sound. So how much can a TPI runner flow? 
Those are known to be small and limit power above 4000 rpm. Sure you can 
boost the daylights out of it but that creates excess heat and backpressure. 
I think if you had 15 psi blowing through a TPI runner to atmosphere you 
would have really high flow but that is meaningless. You need flow in pulses 
and you need to have very low pressure drop. Most guys that build engines 
don't really know what goes on.

Can't help ya with the CFM Calc Formula, sorry, However, a Stock 426 Hemi 
only flows between 300-400 cfm per runner and it was supposed to outflow 
ANYTHING... Good luck with your project... Do you have a web site which 
details your build up? ALSO, is it a single or dual turbo setup?

David Sagers wrote: > > Anyone know how to calculate CFM rates? I was 
talking to the shop that's > building the turbo set up for my chevy engine. 
He said that at 15 lbs of boost each runner on a stock TPI will flow 600 
cfm. I think the stock runners are 1.25" But 600 cfm sounds really high for 
such a small tube, even at 15 lbs of boost.


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