Exhaust backpressure

[Dan Bubb <jdbubb@ix.netcom.com>]

Well! This subject has really taken off and there are a lot of really
good thoughts here.

Just thought I'd say a couple things since I asked the original
question.

1. Burned exhaust valves. I think this a legacy of the old carburetor
days. Taking the exhaust pipes off a carb engine without rejetting the
carb will result in burned exhaust valves, but what's really happening
is the carb runs leaner because of the increased airflow.
Leaner=hotter=burned valves. Just look at the Rolls Royce Merlin in a
P51. The thing has exhaust stubs that look about 6" long from valve to
tip and it obviously didn't have a problem running this way. If you make
changes to engines you need to retune, unless, of course, you have CIS
or any FI that measures mass air flow and is self compensating.

2. It's pretty well known that changing the diameter of primary pipes in
a set of headers can change the shape of the torque curve. There are two
mechanisms at work here.
One is the organ pipe analogy. When the exhaust valve opens a positive
pressure pulse is generated that travels to the end of the pipe. When it
hits the end of the pipe it is released to atmosphere and a negative
pressure pulse is generated that travels back up the pipe to the exhaust
valve. At certain engine speeds for a given pipe length the negative
pulse will arrive right when the exhaust valve is closing assisting in
evacuating the chamber. Bigger pipe=lower pulse pressure=less assist.
The second mechanism is the scavenging effect. Here the pulse from one
cylinder causes a negative suction effect on adjacent pipes as it passes
through the header collector. This can help to suck the exhaust out of
the adjacent pipes. Bigger pipe=weaker pulse=less suction in adjacent
pipes.
So, larger pipes (and also short pipes) generate weaker pressure waves
at low to mid RPM which can result in less efficient exhaust scavenging
and less torque. But, the bigger pipe also flows better at high RPM and
will have higher power. (Side thought. What would happen to power by
putting a really hugh downpipe and system behind the 84 GTI toilet bowl
manifold?? I bet it wouldn't hurt midrange!) I don't think the issue
here is backpressure as much as the strength of the pressure pulses.
Also of note is the relatively recent addition of steps in the exhaust
primaries. Again, these can have an effect on exhaust flow and
scavenging by generating pressure pulses of opposite sign traveling in
the opposite direction. The step wouldn't generate as strong a pulse as
the end of the pipe, but would generate a weaker pulse that would arrive
back up at the exhaust valve sooner than the primary pulse. Beneficial
if tuned properly.

3. Backpressure in the entire exhaust system. Personally, I think once
you get past the catalytic converter, pressure pulses are pretty weak
and so broken up that they make no difference to tuning. Here
backpressure is just something the piston has to work against to flow
the exhaust out of the cylinder.

So, my summary is that backpressure is not beneficial in any way, but
the presence of pressure pulses, and how they are tuned and interact
with other cylinders, can make a huge difference in engine performance
or the shape of its power curve. Going too big on pipe size isn't bad
because it reduces backpressure. It's bad because it reduces the benefit
of pressure pulses. And this only applies to exhaust primaries, not the
tailpipe!

Just my .02 as usual
Dan

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