Re: Elastic air and inertia

[Simon Favre <simon@domain.elided>]

Actually, some standoff will occur on any carbureted motor. They all
have SOME overlap. Even on old, wheezing American pig iron, if you
pull the traditional round top off the air cleaner, you will see some
gas residue on the underside, whether it's a fire breathing V8 with
dual quads, or a wimpy inline 6 with a 1-bbl. I've had both.

As to wild cams, here's a data point (non-Alfa, but Italian). The Fiat
1100 motor in my Formula Junior race car has 80 degrees of overlap,
300 degrees of duration, and about 10.7mm lift after the rocker arms.
It will pull strongly WELL beyond the safe limit of the bottom end
hardware (AMHIK). ;=( Stand-off? You betcha. It has a beautifully
crafted Aluminum air box with ~2" brass air horns inside, and a large
K&N ducted from inside the cockpit (cool air, but no ram effect). I 
lined the inside of the air box lid opposite the air horns with cork 
to seal it and reduce the howl it makes, and the cork always has a 
little coloration on it from the purple race gas I use. The cork layer
is roughly 5-6" from the venturi throats!

Godfrey DiGiorgi wrote:

>I don't know that there is an easily seperable distinction between 
>resonance effect and valve overlap... However, if you're getting 
>blow-back from the exhaust side through past the carburetor bridge on 
>overlap, you have one radical cam in there. Even with a very mild, 
>tractor like cam (like in my Moto Guzzi 850 engine, 20/52/52/20 timing 
>that's only 40 degrees of overlap), you still have a certain amount of 
>standoff upwind of the jet bridge due to intake tract resonance. 
>
>Godfrey
>
>>BTW are you sure the "standoff" you refer to  was caused by resonance
>>rather than valve overlap? it seems surprising that a wave effect could
>>actually reverse the flow of air and fuel droplets back through the
>>carburetor.

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