Always wondered about this Tim.
Traditionally you suffered a throttled response in exchange for, obviously the engine speed you desire and a far better fuel draw because venturi only restriction gives you that.
The point of highest speed and lowest pressure is always at the jet.
But with a diffuser placed well before the jet ( as would be the case with gauze stretched over a bell mouth), the incoming air is slowed prematurely thus losing draw at the jet.
I get that any intake restriction would throttle an engine but fuel draw must suffer somewhat.
As a fine tuning aid I like the idea but really it should be no sustitute for a correct sized venturi.
OY! What happens to the fuel draw if you restrict it completely, like, say, with your finger? In that case essentially no air flows through the venturi, but it sucks fuel like gangbusters, in fact, it might suck up to about .40 cubic inches (or whatever the swept displacement of the engine might be), depending on the port timing.
In the typical venturi, the draw is a function of both the Bernoulli flow effect, and the upstream restriction. The restriction generally *helps* with overall fuel draw, otherwise choking it wouldn't
work. It does reduce the power, but power is irrelevant as long as you can use any sized engine you want, like a piped 60 in an airplane that we once would have flown with an ST46.
Moreover, given that power is not an issue, the diffuser diffuses the flow so it is much less sensitive than an "ideal" venturi to variations to the inlet air direction, which seems to be a much more important problem than loss of power. So-called "True Venturis" optimized to the extent possible for flow efficiency, tend to have *much* more variation in flight from, apparently varying angles of attack, and the blockiest and turbulence- and restriction- generating configurations seem to be the least prone to in-flight variations.
Brett