Paul,

Do discontinuities in the intake runner affect the acoustic tuning
severely? I was kind of wondering how much difference in efficiency you
would see between a slide throttle with round hole in the Teflon plate (thus
a continuous round hole from port to bell mouth at WOT)  compared to the
teardrop shaped Teflon throttle plate, which would create a pocket in the
wall of the runner at WOT.

I would guess the effect should be less severe than having throttle
butterflies in there, but then again it would be a sharp edged
discontinuity. Would be pretty easy to test for someone with the slide
throttle equipped engine, just swapping out the throttle plates. Would also
be interesting to see the effect of the teardrop on throttle response, as I
understand round hole throttle plates can be horribly non-linear.

You used butterfly's on the Dyno at Racing beat didn't you? Does anyone have
an engine with your compact slide throttle design running yet? I would like
to include it in my firewall forward.

Mark W.

Yes we ran it on the dyno but never got the mixture tuned
with Tracy's EFI to the point we could get a good HP pull.

Mark also ran it with a prop load before he noticed
the water leak on his Mustang II. Engine ran fine.
No tear drop of course.

The volume of the tear drop is too small to act as a Helmholtz
resonator to affect the tuning of the runner.

Many modern production engines use Helmholtz
resonators to change the intake properties or noise.
My Honda Odyssey has one. I suspect the noise only
is the reason. The Odyssey engine has a noticeable change
in sound at WOT. I'll take some pictures in a day or so.

Here is a Helmholtz resonator on a Porsche engine
intake manifold I saw yesterday. This I suspect does
tune the intake. They seem to be in fashion now a days :)

You could always radius the offending edge if you wanted the last
pony :)

Paul Lamar

Thanks for the answer. Did Mark report any issues with the non linearity of
the throttle response. You read bad things about slide throttles, but they
are probably from car guys. They probably wouldn't be too thrilled about the
response of an O-320 either.

The round throttle slide holes will be easier to make.

Mark W.

I don't recall. I think Dave Lemon at Mazdtrix mentioned it.
But then I think he got used to it. Most slide throttles
historically have had round apertures.

BTW a good example of a Helmholtz resonator is blowing
across the opening of a Coke bottle.