Throttle Body Size

[Scot A. Sealander]

John Hess wrote:


>> > 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.

This answer is in error on several counts.  There is no check for too little 
vacuum at WOT. ;-)  And you are already in open loop with the foot to the 
floor in this application ('86 Vette).  Being out of range of the sensors is 
not what makes it drop out of closed loop once you go WOT.  From the answer, I 
don't think you know what closed/open loop is.  I may be wrong. :-)

So I agree, low manifold vacuum at WOT is a good thing..... Oh, and the MAP 
ECM fuel tables from the MAP equipped TPI's are perfectly happy with a 100kPa 
MAP sensor.

[mismatch of engine/TB/intended use text deleted]

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

Resolution may be the problem, but more likely it is a mismatch of AE and TPS 
delta.

 
> 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).

This presupposes that the airflow is different at idle with a high flow and 
stock flow exhaust system.  Did you see any different MAF flow numbers with 
this system?  I do not think there is a significant exhaust restriction at 
idle, even with a stock system, otherwise the engine would really suffer at 
higher RPM.  This sounds more like an O2 sensor that is moved from the stock 
location.  It is easily fixed with a heated O2 sensor anyway.


[rest deleted]

Scot Sealander   Sealand at clarityconnect.com