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

[audistuff@juno.com (Paul R Anderson)]

>
>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.
>


The purpose of the sharp reduction at the end of the pipe is
to keep the exhaust pulse from heading back up the pipe and 
pushing exhaust back into the combustion chamber.  I believe the
correct term for this is a half-baffle and cuts the outlet area to half
it's original amount.  This cancels the returning wave, resulting in
zero pressure in the exhaust system.

When the exhaust pressure pulse reaches the end of a regular pipe, It 
sends a pulse back up the pipe.  If this happens to be timed such that
amplitudes of the incoming and outgoing pulses add up, you can
get twice the backpressure that you would get with no exhaust pipe
at all.  This is all dependent on engine rpm.  The waves travel at
the speed of sound (in exhaust gases), not at the speed of the
exhaust gases (if my thinking is on).  Therefore you can tune the
exhaust such that, at certain rpms, the exhaust pulses will indeed
pull the gases out.  This is why the zoomies on a drag racer are
a certain length (as well as the velocity stacks).  I believe this is
also
why the velocity stacks on old Indy engines were not all the same
lengths. 
This allowed for different cylinders having better flow characteristics
at
different rpms, thereby eliminating huge dead spots in the rpm range.

Unfortunately, we are all required to run exhaust systems including
mufflers and, for most, catalytic converters.  This blow the
whole plan for a truly tuned exhaust system.  On the other hand, the
discussions that have started over exhaust gas velocity and, in
essence, sucking the exhaust out of the engine, may indeed by
applicable.
I am reminded of the old race cars (before my time) which I think were
built by Jim Hall (or maybe not) which had a snowmobile engine, a large
blower, and flexible side skirts to literally suck the car down to the
ground.
This acheived a much greater pressure differential than the rest of the
cars
had and due to that and safety concerns, they were soon ruled out. 
Basically
he took the simple idea of pressure differential and ground effects and
went crazy with it.

I hope these comments spark intelligent discussions and nothing more.


Paul Anderson, Cheyenne, WY  Private email:AndersonPaul@juno.com