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Corky Bell's "rules of thumb"...exhaust size

A bigger exhaust system is only beneficial to the point of no
backpressure as Graydon mentions.  What he didn't mention
was that exhaust gases cool as they travel through the
pipe.  It may be beneficial to have a 2.5" or 3" exhaust system halfway
back, maybe to the cat, but by that time, a lot of heat has been
lost which results in a lower volume of exhaust gases traveling
through the pipe.  After this point, a smaller, maybe even smaller than
turbo outlet, pipe can be used.  I can't see any reason for a 6" tip

I read in an OLD hot rod magazine (or Motor Trend or one of those) about
a device.  I can't remember it's name, although I'm sure I could check
with my Dad.
It uses a magaphone on the end of the pipe with a cone inside it.  The
purpose of this, I believe, is to increase the velocity of the gases and
draw the exhaust out.  Anyone know what I'm talking about?

>> than the turbo outlet (in the case of our cars, roughly 2.25") yet 
>> running 3" downpipe/exhaust systems claim significant performance
>	In case you skipped Thermo (my thermo prof was an 
>Italian professor that used to be the head of R&D at Ford - a really 
>guy), the turbo hot side is a heat driven turbine.  Its output is 
>related to the pressure drop across it (and a few other factors, but 
>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 
>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 
>more to it than that, but that's it in a nutshell.  Experience shows 
>the larger the better with turbo exhaust systems I think.  
>Furthermore, I
>think there is some merit to a diverging nozzle exhaust system, where 
>system expands gradually all the way to the back of the car.  If you 
>this, only then would I accept that a 5" or 6" tip would be 
>but the exhaust system would have to taper down to 2.2" at the turbo, 
>in a
>smooth gradual manner. 
>Graydon D. Stuckey 
>"The Muffler Man"