Back in the olden days (mid-fifties) I worked at a wind tunnel facility
associated with the U. of Minnesota. My job was to design nozzle blocks to
produce a specific mach number in the test section.
Jim Hayes' analysis is essentially correct and illustrates that is tough to
fault basic Alfa engineering. Now the oil pressure driven belt tensioner on a
GTB6 is another matter.
Dave Mericle the Elder
Leucadia, CA BEACH-N-RIDES
Jim Hayes wrote:
> 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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