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

[Luis Marques <marques@rtis.ray.com>]

I think I can chip in a little in here.  Although I have close to zero
practical experience with turbo exhaust systems, some years ago I had the gas
radio controlled car bug.  These are the cars that produce over two hp out of
a 0.21 cid single piston two stroke engine at 40,000 rpm using a mixture of
nitromethane, methanol, and oil, and exhaust was critical in achieving those results.

What you wanted was a header/pipe with progressively larger diameter from the
engine exhaust port to the exhaust outlet, but right before the outlet, you
would constrict the exhaust tip to about the same size as the exhaust port
outlet (about 1/4 the diameter of the pipe at this point).  If I remember
correctly, the reasoning was that the expanding gasses (due to the increasing
diameter of the exhaust) would tend to "keep on moving" creating some kind of
a vacuum that served to "pull" or scavenge the following exhaust pulse.  The
kink at the end was intended to limit flow so that the preceding exhaust pulse
would not exit before pulling the next one.  It all had to be tuned to the
engine based on size and port modifications and was most easily accomplished
by varying the length of the exhaust.

Now, I'm not saying that all turbo exhausts should be done this was, but here
is food for thought for all the thermodynamics experts here on the list.