Ron got it right. Resonances, standing waves, etc vary according to
diameter, length and the particular kinks of the specific exhaust system. And
all
these conditions change dynamically based on RPM and valve timing. Getting
all these variables harmonized and optimized is what puts the "tuned" in tuned
headers. E.g. the infamous "Bundles of snakes" on Formula cars.
Still, there's more than a grain of truth in the "backpressure" argument.
After all, an internal combustion engine is basically an "air pump". The more
air you can cram through it, mix properly with fuel and ignite -- the more
horses you make. So in theory, any part of the path that restricts airflow is
the limiting factor to making ultimate HP. Thus there are different exhaust
design philosophies for Formula cars vs. floppers.
The amusing thing is how much effort the automakers put into optimizing the
intake design, and how comparatively little they normally put into exhaust
design. I guess partly because it's underneath where it's dirty and hot, and
the average buyer won't stick their snoot down under there.
As an old two-smoker race bike rider I've developed a passing acquaintance
with these issues over the years, because exhaust tuning is critical for those
beasts ;-)
Cheers,
Mark L.
94 atx 135k
>>>>>>>>
Everyone needs to keep in mind that the term "backpressure" is a misnomer. I
get
the impression that when folks think of "backpressure", they envision a
potato
in the exhaust pipe.....or something along those lines.
Probably without using the exact engineering terms that I don't feel like
looking up at this time of the night, the actual effect are the exhaust
pulses
that produce the return wave up the pipe, which helps to further extract
exhaust
gasses. The extraction happens at a lower rpm in a small diameter pipe, and
at
higher rpm in a bigger pipe.
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