Throttle Body Size

[John Hess]

Ah, carburetors do like velocity, don't they.  There are several 
problems that you might see if the throttle body on a fuel injection 
system gets too large.

Off throttle becomes ragged.
Wide open throttle drops the vacuum in the plenum too far and 
adversely effects the MAP sensor.
Cruise becomes a real problem, as you now cruise at a barely open 
throttle position.
Torque suffers due to no velocity at low RPM.
High RPM Horsepower is normally better.

Several other things I don't want to take the time to type in.

Note that matching the carburetor to the desired engine 
characteristics was much more critical than matching the throttle 
body, though.


----------
From:  Doug Robson[SMTP:doug at cia.com.au]
Sent:  Monday, December 30, 1996 2:57 PM
To:  diy_efi at coulomb.eng.ohio-state.edu
Subject:  Re: Throttle Body Size

kleenair at ix.netcom.com wrote:
>
> Jonas Hageberg wrote:
> >
> > Nice formula, but can anyone explain to me why the throttle body
> > size is important in a plenum intake EFI system? In my opinion, it 
just has
> > to be big
> > enough to let the required amount of air in, since you do not need 
a certain
> > air speed to keep the fuel atomized, and the acoustics/tuning is 
maintained by
> > the tubes connecting the plenum and the intake valve.
> > The only problem I can imagine with a too big throttle is unlinear 
throttle
> > response.
> >
> > Can anyone explain this to me?
> >
> > Regards Jonas Hageberg
>
> Hi,
> If the throttle body is too small, obviously you will limit maximum 
air
> flow.  If its too big, at lower engine speeds it becomes difficult 
for
> the driver to control engine output (i.e. barely openning the 
throttle
> results in significant air flow increase).  This is especially a 
problem
> if you are trying to drive a high power, low weight car in rain or 
snow!
>
> One inovative solution I have seen to the low throttle angle flow 
problem
> is a lip incorporated into 1/2 the circumference of the new GM 
pick-up
> engines.  Effectively, for the first 20 degrees of throttle 
openning, it
> limits the air passage from 1/2 of the throttle blade.
>
> Best Regards, Mazda
All I know form this is i took my 200 bhp isuzu 8 vale twin Cam 
engine
from 48mm webers to 4 50mm throttle bodies and lost 30 hp, when i put
45mm throttle bodies i ended with 204, such a small change but a huge
effect. I think Taper was very important

regards
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