CFM Continued...

[Gary Derian]

A single runner flows for only 1/3 to 1/4 of an engine cycle.

Gary Derian <gderian at oh.verio.com>


> With what you say in mind, then WHY does a STOCK 426 Hemi SONGLE intake
> runner flow about 300 cfm each, from the factory??
>
> Just wonderin...
>
> I have heard that you want your cfm to be about 30% MORE than what is
> theoretically required, due to our nonprefect world operating
> environment and all...
>
> Thanks for your formula's and input in this subject, it's
> enlightening...
>
> Todd....!!
>
> ---------
>
>
>
> Greg Hermann wrote:
> >
> > >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
>