Venturi effect with efi throttle bodies

[Tom Cloud]

I guess, now that I'm thinking more clearly (blurry eyes can't
see the screen), the original question I responded to .... I
read it wrong.  He wanted to know why you can't just squirt
the gas into the plenum, basically.  The small throat/venturi
is required to get enough velocity/turbulence to mix the gas.
Once it leaves the TBI throat, it sorta "stagnates" as the flow
velocity slows down and often falls ("rains") out onto the
floor of the manifold, hence the heated crossover.

>> >I was wondering about needing a venturi in a TBI system to aid in
>> >increasing air speed through a manifold, as this is used to suck air/fuel
>> >from a carb into the airstream in a carbed system. Is this unnecessary in
>> >EFI? wouldn't low-end torque suffer due to a lack of air speed at low revs?
>> 
>> I think I have the same mental definition as you -- venturi
>> is the little pipe stuck into the air flow that the fuel is
>> sucked out of ... right?  But, I think I'm wrong.  The bigger
>> picture is that a venturi is a narrowing in a flow path
>> causing a higher velocity in the flow.  If that's right, then
>> the carb or TBI throat is a venturi.  Is that right?
>
>On a Holley (used as an example because we all know what one looks like)
>The main venturis are the large passages as you look down inside the
>carb. The other venturi that sticks out is called the booster venturi.
>Think of it as a venturi inside a venturi. As with all venturis, the
>airspeed is increased so the pressure is reduced, causing a low pressure
>area inside the venturi. This causes the outside higher ambient
>atmospheric pressure to PUSH the fuel out of the fuel bowl and out the
>little tube that resides inside the venturi. The booster venturi creates
>a small passage where pressure differentials are going to be greater
>which aids in a better metering signal for getting the fuel in there,
>while the large main venturi is big enough to flow the amount of air you
>need. Both venturis have fuel ports in them.
>
>To answer the first first question;
>Once the air is past the venturi, the speed is reduced and the pressure
>increased, so the venturi doesn't have really any effect as far as the
>manifold goes. That is a function of the intake manifold plenum volume
>and runner volume and length, and head intake port runner volume and
>length.

Tom Cloud <cloud at peaches.ph.utexas.edu>