Venturi effect with efi throttle bodies

[Tom Cloud]

In a TBI, the petrol is injected in the throttle body, not at the
end of the runners, as in TPI.  Velocity (a.k.a. turbulence) is
needed to properly mix/atomize the fuel/air charge.  Without
some velocity, the gasoline will just "rain" onto the bottom
of the plenum.

I've had the same thought (about partially closed butterfly
valve causing the needed velocity/turbulence).  I don't know
about that.  It must not be as effective as a reasonable
"venturi" throttle body else the manufacturers of TBI's would
make them all real big.  Consider the extreme in which
the throttle bore were made _very_ large with the traditional
butterfly valve throttle.  The air velocity would be extremely
slow, and the gasoline would not be mixed -- in fact, it's
conceivable that part of it could actually vaporize and rise
away from the throttle bore if the air flow were slow enough.

>In an EFI system, if you want velocity, design it into the runners
>not the throttle body. (unless it's individual t-bodies, in which case the 
>t-body is part of the runner)
>
>I don't see how lack of velocity through a t-body on a plenum setup
>would hurt low end. If you want velocity(in the t-body) back off on the
>throttle.
>>
>>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?

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