Throttle Body Size

[John Hess]

----------
From:  Chuck Tomlinson[SMTP:tomlinsc at ix.netcom.com]
Sent:  Monday, December 30, 1996 6:40 PM
To:  diy_efi at coulomb.eng.ohio-state.edu
Subject:  Re: Throttle Body Size

> From: John Hess <JohnH at ixc-comm.net>
>
> 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.

Why?

I could answer that with...Mine is not to reason why; but, ....At the 
low throttle opening positions, there is simply too wide a variance 
with too small a throttle position change.  As a consequence, the 
throttle opens too fast and you find it hard to control low engine 
speeds.

> Wide open throttle drops the vacuum in the plenum too far and
> adversely effects the MAP sensor.

This doesn't make sense.  Zero plenum vacuum at WOT is pretty
desirable, IMHO.

Again, we are not talking about what is desirable IYHO, we are talking 
about what is desirable in the humble opinion of the engine sensors 
and the control unit.  If the plenum vacuum (pressure) drops out of 
the range of the sensors and computer, you go open loop.

> Cruise becomes a real problem, as you now cruise at a barely open
> throttle position.

I don't think this is a big deal, but as Mazda pointed out earlier,
high throttle gain (angle vs. power) can cause serious problems on
slippery roads in powerful cars.

Anybody who doesn't think this is a big deal has never driven a car 
with too much flow for the engine to accommodate.  I have an '86 
'vette.  When I had a 58mm throttle body on the car, it was hard to 
drive just off idle AND cruise at speeds under 55 would surge.  A 52mm 
throttle body fixed this problem and did not seem to effect power 
adversely.

> Torque suffers due to no velocity at low RPM.

This doesn't make sense for a plenum-mounted throttle body.
It may not make sense to you, but try increasing the size of the 
throttle body on a MAP TPI system without changing out the chip.

> High RPM Horsepower is normally better.

Agreed.

Also, if the torque changes dramatically with throttle angle
(which would be true for a huge TB), the TPS resolution may
become too low.

Actually, the runners would have more effect on the torque range than 
the throttle body;  but, on a MAP system, the throttle body could very 
easily "confuse" the computer, causing all kinds of drivability 
problems.  To give one a sense of just how easily the system can be 
mixed up, something as simple as a high flow exhause can cause 
drivability problems.  How?

The higher flow doesn't allow the oxygen sensor to maintain at least a 
600 degree temperature, causing the sensor to misread the exhaust gas 
(toward the lean).  It richens up the mixture, causing the exhaust 
temperatures to further reduce.  This effects the timing of the engine 
as well as causing it to go in and out of closed loop operation as the 
conditions vary back and forth.


Just my 2c, and a few questions...
--
Chuck Tomlinson