Date: Thu Nov 28 08:34:28 1991
Subject: Need More Fuel
>A friend of mine has a 77 Z with a larger throttle body, mild cam
>and somewhat opened up exhaust (no headers though). When I drive
>it and compare it to mine, there seems to be little difference and
>considering that my car is stock (I had headers for a while, but
>got tired of the noise) it would seem that there should be more
>power from his setup.
>I also seems that perhaps he needs more fuel in the engine. There
>is a larger fuel injector available, I have been told, but I was
>wondering if it would be possible to add some resistance to the
>water temperature sensor loop to make the fuel system think that
>it is colder, and dump in more fuel.
>Has anyone tried this. Any thoughts on whether it might work?
The water sensor is not the place to do it. The optimal way is to
recalibrate the air flow meter. Once the cover is removed by breaking
the silicone seal, there are several adjustments available. Among
spring bias - serves as a span adjustment
wiper offset - serves as a baseline adjustment
segment resistors - calibrates the mixture at a particular
The wiper offset adjustment, accomplished by loosening the screw that
locates the potentionmeter wiper on the flapper shaft, has the most
marked effect on the low end of the range. This is because the
resistance vs flow is very non-linear and the most rapid change is
off-idle. Small changes make BIG mixture changes.
Changing the segment resistors, while a lot of work, is well worth it.
One can then change the mixture at a particular flow that might be
problematic. the segment resistors are those laser trimmed silk screened
resistors that shunt various parts of the main pot. resistance.
The procedure is as follows:
Cut one trace to each resistor as near to the resistor as possible.
Measure the resistance and record it. Select a 10 turn trim-pot
equal to twice the value for each resistance. This means that the
pot will be set at approximately the midpoint for stock conditions.
Mount the pots appropriately. Very small pots can be fit under
the cover. Larger ones can be mounted in a small box with wires
run to the flowmeter. Attach the wires to the main resistance by
carefully scraping the traces to the old resistors clean, superglueing
a wire down to each trace for mechanical strength and then making
the electrical connection with silver-bearing electrically conductive
paint (or epoxee if you have it.) The paint for repairing rear
window defrosters works fine. The wires should connect such that
they connect to the traces upstream of where you cut the original
resistor out of the circuit. It should duplicate the electrical circuit
of original setup.
Calibration is most easily accomplished by setting each pot to the midpoint
by measuring the ohms before glueing them down. After the pots are
bonded in place, connect a voltmeter between the low end of the pot (where
the wiper rests when the engine is off) and the wiper. Run the leads
so that the voltmeter is in the cockpit. Drive the car and note the voltage
at which some problem is observed. Stop and manually position the air flapper
until you get the same voltage reading. Then observe the arm position
and notice which pot spans that location. That pot can then be adjusted.
As you might expect, the adjustments interact so make small ones and
keep careful notes so you can duplicate where you were last when you
screw something up.
An exhaust analyzer makes the task much easier. Inexpensive lambda
sensor-based units are available from MSD and Summit Racing. tuning
the air flow meter combined with adjusting where the WOT contacts
make up on the throttle body (essentially a step change enrichment.)
will yield a very responsive engine that can easily pass an idle
emissions sniff test.