Venturi effect with efi throttle bodies

Johnny allnight at everett.net
Sat Mar 29 05:45:42 GMT 1997


Tom Cloud wrote:
> 
> 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.
>

Is that what he asked? I missed it too. I couldn't understand what the
venturi had to do with the manifold velocity.

You bring up a great point about TBI in general; The intake manifold
design is still critical in that fuel puddling and loading up can occur,
while with port injection, you are only passing air through the
manifold, thus the shapes aren't so critical, just mostly the volumes
and lengths.

Now that I am re-reading the original question, I think that we are
still both missing it. He wanted to know how TBI worked without a "real"
venturi like a carb has, and how does it have any throttle response this
way. The answer (for real this time I promise), is; that's the beauty of
TBI or any injection. The fuel is sprayed out through a nozzle (or
spraybar or what have you) under pressure via a pump, and at more
pressure that you will get with atmosperic differential in a carb. With
a carb, the nozzle is more or less just an open tube, so atomization is
a result of the fuel more or less falling out into a fast moving air
stream. With injection (of any sort), the fuel exits through a spray
nozzle of some sort (many sorts), and doesn't rely on a venturi to speed
up the air flow, although most TBI's that I have seen still use some
venturi effect and the nozzles exit right at the venturi point.

Now, to add to the confusion, my Accel air door has a small amount of
venturi shape to it. While having nothing to do with the fuel spray
(it's port injection) I can only guess that it has more to do with
directing the air past the throttle plates, and increasing the velocity
at the throttle plate to allow for more precise throttling. Of course
that is all just theory on my part.

-j-



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