Throttle Body Size

[Chuck Tomlinson]

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

At the risk of repeating Scot Sealander's comments, zero vacuum is well
within the range of the TPI MAP sensor, and TPI goes open-loop at WOT
anyway.

> 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.

Sorry, sounds like a calibration problem to me.  As Scot said, the 
accel enrichment (vs TPS rate) tables need to be updated for a
substantially larger TB.

> > > 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.

Once again, AE vs TPS rate changes.  Runner velocity at low rpm 
is *not* dependent on the size of the TB.

> 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.  

All of which are unrelated to air velocity.

> 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.

There are lots of MAP cars running around with high-flow exhausts and
without the drivability problems you describe.  I still don't see how 
increasing gas flow rate could reduce the temperature of EGO sensor.
--
Chuck Tomlinson