CFM Continued...

Greg Hermann bearbvd at sni.net
Fri May 7 04:07:50 GMT 1999


>Thanks to Ken, Todd & Gary for your answers regarding CFM.  Perhaps I didnít
>ask the question correctly.   A stock TPI runs out of flow about 4800 rpm on
>a stock 350 ci engine.  Iím 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.
>

How fast are you planning to turn the engine? Say for a 434, 5400 will be it?

Then, the engine will be trying to breathe in 434 x 5400 x 0.5 x 1/1728 ,
or 678 cfm at the highest speed you will be turning it. The runners to each
cylinder will be wanting to flow 1/8 of this much, or about 85 cfm each.

The trick is to size everything in the flow path so that you do not have
excessive pressure drop at any one point in the path at the amount of flow
the engine wants to draw in. What the guy talking to you is missing is that
with a pressurized intake--the pressure drop in the runners is proportional
to the density of the air flowing through them!

Of course--you will also have proportionally more pressure you can afford
to lose with the turbo motor, So whatever will work well with the same size
and speed range  NA motor is pretty close for a turbo motor. Going maybe a
fuzz BIGGER than what you would use for a similar size/speed NA motor will
tend to lower backpressure from the turbo on the motor, and lower EGT's
some.

Going any smaller than what you would run on an NA 434 would hurt
performance some, and durability more.

Flow on the inlet side of the turbos will be the amount of air the engine
breathes times the manifold density ratio--a LOT more cfm. Everything on
the inlet side of the turbos should be sized accordingly, and also to have
VERY low pressure losses at this design flow The amount of back pressure
which the turbo(s) will put on the engine to make a given amount of boost
is EXTREMELY sensitive to losses in the inlet tract to the turbos!.

Regards, Greg





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