Aajaynefour@cs.com wrote:
> I believe this topic was discussed about a week ago, but the results seemed
> inconclusive. The IAT, as I understand it, is a variable resistor that
> responds to temperature changes in the intake air stream and sends a voltage
> to the PCM that is used to select the "optimal" air fuel ratio.
It is used as a variable in a formula to find the mass of the air entering the
engine. MAF-based systems attempt to measure the amount of air actually entering
the engine, MAP-based systems attempt to calculate the amount of air entering the
engine. One of the sensors used during the calculating process is the IAT. Once
the calculating is complete, the PCM uses the calculated mass of air to determine
what the fueling requirements of the engine are at that particular time and
schedules an injector pulsewidth accordingly.
The calculating process is only occurring during open-loop mode (WOT and
transients).
> So far so
> good. Bernd has developed a simple but effective way of sending a "false"
> voltage to the PCM making it think that the air is cooler than it actually is
> and thereby selecting a different (richer ? - more performance biased) fuel
> curve.
Only under WOT or transients (open-loop mode) can it "select" a different fuel
curve - and then it is only shifting it (rich or lean) by a finite amount.
> Still good. My questions are as follows: Does the IAT in the stock
> location react to air temperature changes to maintain a stoichiometric air
> fuel ratio?
No. Closed-Loop mode (where you spend most of the time) is an O2 feedback
control loop which attempts to maintain a stoichiometric (14.7 to 1) air fuel
ratio. This is done primarily for emissions control. The best
power/torque/horsepower is said to occur around 12-12.5 to 1 ratios.
> If so, then basically Bernds IAT adjuster causes the PCM to
> deviate from stoichiometric in the direction of a richer mixture as the air
> temperature and therefore density stays the same.
The PCM will never deviate from stoichiometric (unless you modify the O2 sensor
signal) as emissions control (not performance) is its main goal. This only
applies to closed-loop control mode. In open-loop mode it can only guess at the
fueling requirements. Bernd's device modifys this guess.
Before attempting to modify the PCM's fueling strategies - I would recommend an
A/F gauge as an insurance policy. Most people are already too rich (during
open-loop). Most people relocate their IAT into a cooler location. Problem with
that is - richer isn't going to help - instead it hurts. During WOT or
transients (passing, pedal stabs, decel, etc) on the other hand, can provide a
perceived improvement. At the dragstrip or on the dyno (both WOT) it may show a
slight improvement. But in daily stree-driven usage - it only makes for a sooty
tailpipe and poor performance.
My $0.02,
Shane
-- '96 IndyRam-HisIndy-MPI/TB/Pulleys/AccelCoil/MPComp/HookerSuperComps/CompTAs '96 IndyRam-HerIndy-numbered(#142)"Track Truck" '74 Triple-Black Dodge Challenger Rallye 360 home-brew EFI R&D vehicle '68 Black Corvette Convertible 427 (For Sale)
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