Quoting John M Bittner <jmb@xxxxxxxxxxxxx>:
> As the pulse of gas passes down the pipe it creates
> a flow which in turn creates a vacuum in the pipe
> after it passes. This vacuum helps to pull the
> next load of exhaust gas from the next cylinder that
> fires after it. You can probably start to realize
> that the length of the pipe and the timing or RPM
> of this flow process is critical. The H pipe
> helps to propagate this flow pattern at low RPM's
> by alternating flow paths down two separate pipes.
This is a good visualization. However, there is no vacuum (the pressure is
always a bit above atmospheric at full throttle). Just low pressure, or lower
than average. These are pressure waves that travel with the speed of sound.
These waves create peaks and valleys. If at one instant, a valley of the right
side is connected by the H-pipe with a peak of the left side, this will help
the flow of the left side (by momentarily reducing back pressure), without
hurting the flow on the right side. And vice versa. Part of the reason of
these pressure waves is the fact that each bank of a V8 is not even firing,
while the overall engine fires evenly (every 90 degrees).
Also, these pressure waves can be tuned for different rpm as said before.
That's why the effect is usually at low rpm's rather than high (no effect in
power). Modern cars with fuel injection do the same in the intake system (you
must have noticed how weird the intake manifolds of modern cars are, with lots
of pipes going around, all of even length). The effects in improving
volumetric efficiency are more pronounced with intake tuning. This is exactly
what the cross ram 413 was doing in the early 60's. This is a (rare) example
of intake tuning with carburetors...
D^2
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