Brian Evans wrote:
>
> I thought it was the other way round - short intake path (and headers, for
> that matter) tend to increase high rpm HP at the expense of low, and vise
> versa.
As I remember it, that is correct - shorter pipes for higher RPM, just
like an organ (shorter = higher frequency). It's a matter of tuning the
frequency of the compression/rarefaction pulses caused by the intake
stroke. You want the intake to open on a compression pulse to pump in
more air/fuel mixture. And the length must be measured from the valve
head to the end of the pipe, not the port on the head!
Flow is another matter. A bigger intake may not flow as well as a
smaller one, due to the velocity of the mixture being lower. In the Alfa
GTAs, Don Black was able to get higher HP by actually sleeving the
intakes to get a smaller diameter. When we had Mike Sperry port my head
a few years ago, we started with an earlier Alfa head with smaller ports
to get the best flow, confirmed on a flow bench.
Other factors are important: Shimy polished intakes may not flow well
due to aerodynamic drag caused by the smooth surface - a slightly rough
surface may be better. The angle of the intake to the valve is very
important, but not easily changed. And, of course, the angle of the
valve seat is another factor.
Only a flow bench will tell you what works.
Back in the mid-60s, I found some formulae on how to calculate these
things, used some NSF-funded computer time and calculated intake and
exhaust lengths for an Alfa engine. The exhaust tuned lengths turned out
to be almost exactly the same as the Alfa Veloce headers!
Jim
--
Jim Hayes Fotec/Cable U
hayes@mediaone.net http://www.CableU.net
jeh@fotec.com http://www.fotec.com/
All generalizations, with the possible exception of this one, are false!
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