Re: Corky Bell's "rules of thumb"...

["Graydon D. Stuckey" <graydon@apollo.gmi.edu>]

On Thu, 20 Nov 1997, Jeffrey J. Goggin wrote:

> I've been reading the Bell book and am puzzled at how some of his
> conclusions seem to differ so markedly with those BTDTs posted here.  For
> instance, he says there's no point in having the exhaust system any larger
> than the turbo outlet (in the case of our cars, roughly 2.25") yet those
> running 3" downpipe/exhaust systems claim significant performance
Jeff,

	In case you skipped Thermo (my thermo prof was an oooooollllddd
Italian professor that used to be the head of R&D at Ford - a really neat
guy), the turbo hot side is a heat driven turbine.  Its output is directly
related to the pressure drop across it (and a few other factors, but for
this conversation, pressure drop is the important factor.) If we have
35psi in the exhaust manifold for example, and 2psi of backpressure from a
restrictive exhaust system, we only have a 33psi pressure drop across the
turbine.  If we use nothing on the turbo outlet at all, then the turbo
outlet is at atmospheric pressure, and we have a 35psi pressure drop
across the turbine, therefore we get more power.  I am sure there is much
more to it than that, but that's it in a nutshell.  Experience shows that
the larger the better with turbo exhaust systems I think.  Furthermore, I
think there is some merit to a diverging nozzle exhaust system, where the
system expands gradually all the way to the back of the car.  If you did
this, only then would I accept that a 5" or 6" tip would be beneficial,
but the exhaust system would have to taper down to 2.2" at the turbo, in a
smooth gradual manner. 

Later,
Graydon D. Stuckey 
"The Muffler Man"