DIY_EFI Digest V4 #713

[Clare Snyder]

> I've read that the purpose of the venturis is to raise the speed of the
> air/fuel mixture to improve atomization and to drop the air pressure to
> help evaporate the gas.
>
> - --
> John Nutter '85 CJ7
> Admin for Jeepoffroad Mailing List
> http://www.pclink.com/jnutter/jpor.htm
>
> President of Trailriders 4x4 club
> http://www.off-road.com/~riders/
>
Actually, the purpose of the venturi is very simple. It serves the same
purpose as the high pressure fuel pump in an EFI system - to provide an
adequate pressure differential across the
"nozzle" to cause fuel to flow into the airstream in an atomized stream. If
the venturi could produce a full vacuum you would have the equivalent to a
14PSI injector.

Now, in order to develop this pressure differential at low engine speeds and
low airflows, a smaller venturi is required. The "injection signal" from a
large venturi on a small motor is not strong enough to adequately "fire" the
nozzle. This causes poor running, and low power, at low speeds. At higher
speeds a larger venturi breathes better, and if properly jetted (right sized
injectors) you get more power at full throttle, at the expense of low speed
performance and economy. Same thing happens with a MAP sensor EFI and an
engine with too big a cam. At low speeds there is not enough MAP signal to
adequately control the fuel mixture.

Also interesting to note - one carb per cyl or a 4:1 manifold requires very
close to the same carb size (aprox 24% of piston area) because the
"injection signal" is close to the same, it just fires 4 times as often on a
manifold.

A different way to look at it, I'll admit - but we used to describe EFI in
carb terms when carbs were common and EFI new - now that EFI is common and
carbs "new" to many, I do it the other way around.