Scott "snausages ;-)" Fisher wrote:
>Obviously, the shorter the length of the primaries, the lower in
>the RPM band this negative pulse will be generated.
Obviously?
umm...I think its the other way 'round. Imagine a pipe organ. The
short pipes make the high notes and the really long ones make the
low notes. It has to do with the wavelength of the fundamental
frequency. In a header (and this works on intakes, too), the longer
a branch is, the lower the rpm it is tuned for. If you want to do the
math, you need to know the speed of sound in the exhaust. The
length of the header _plus the distance from the header to the
combustion chamber_ is the length of the standing wave of the
fundamental frequency. Given the desired rpm for maximum
scavenging, it's a simple high school physics problem. I have long
since left high school, however, and no longer know the proper
equation to use.
The reason I mentioned that this works on intakes too, is to note that
some race cars actually use variable-length intake runners - the
length is changed as the engine speed changes to eliminate the
peakiness that comes from fixed length runners.
I also remember reading somewhere that the scavenging does not
work on 6-cylinder cars due to timing of cylinders. I've never done
the math on it, so I can't verify that. But any header that will reduce
backpressure is a Good Thing(tm).
Dave Williamson (silikal@aol.com)
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