re: Strange Suds (manifold question)

[bearbvd@domain.elided (Greg Hermann)]

>In AD v7n665, Dave wrote:
>
><snip>
><<The boxer engine layout resulted in very long runners from the
>centrally located carb to each cylinder bank, presumably increasing the torque
>and reducing top-end power versus the Ti, which had two twin Webers mounted
>directly to the intake ports with essentially no manifold.>>
><snip>
>
>Hmmm, educate me, please. How does the length of the intake manifold affect
>the torque and power? I can understand if there is significant variation
>between one cylinder and the next, but length?
>
>Bends, twists, turns, all that I can understand, too. It affects flow. But
>length?
>
>I'm at a loss on this one (and no expert, either). Anyone?

It sure does, Tom, and very strongly. Has to do with sonic waves and such
bouncing back and forth along the length of the intake runners, as well as
with the momentum of the gas in the runners. Longer runners = lower rpm
torque peak. Smaller diameter runners usually goes together with longer--I
have long felt that runner volume should bear some definite relationship to
individual cylinder displacement. Same stuff applies to exhaust header tube
diameter and length, and with pretty much the same ralationships holding
true--longer and smaller for a lower torque peak.

The math to explain it all gets VERY complex, very quickly, and there are a
variety of fluid dynamic effects involved--the Kendenacy effect is one name
that comes to mind. Lots of experimental research as well as mathematical
theroizing has been done on this stuff, but nobody has ever come up with
any REALLY totally definitive, quantitative relationships--there are just
TOO many variables involved.

Regards, Greg
>
>ciao
>
>Tom
>'63 Spider
>Seattle

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End of alfa-digest V7 #669
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