Brad,
Gas flow velocity is important for a carburetor to atomize gas into droplets
and to prevent those droplets from falling to the bottom of manifold.
Velocity is also necessary to blow around the cylinder, pushing out the last
of the exhaust gases before the exhaust valve closes. It promotes movement
inside the cylinder as the charge ignites, ensuring that the entire charge
burns completely, and not just that part near the plug. Various methods of
mechanically increasing flow velocity (at lower revs, there is not problem
at higher rev/higher flow) have been tried over the last 40 years or so. All
of them (including the one you mentioned) do so at the cost of reducing flow
at higher revs by restricting high volume flow.
In other words, it works, provided what you want is increased performance
and efficiency at the low end of the rev range by reducing efficiency at the
high end.
It should be said that there are better ways of doing this, not least of
which is to use the correct size carbs and manifolds in the first place.
However the example you mention (Harley's) are a collection of compromises
between looks, noise, and nostalgia without consideration of performance or
efficiency anyway, so it probably wouldn't matter. (You said you wanted to
start a discussion, that ought to do it).
Steve Jones
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