re: mixture tweaking, AFM adjusting, O2 sensor resisting, etc.

["Todd Kenyon" <todd.kenyon@xxxxxxxxxx>]

I agree that the only thing that MIGHT help you make your car run
consistently richer is to trick or adjust the ECU to run at optimal mix
for power, like around 13 air/fuel instead of 14.7. If you could modify
the O2 sensor output so that it produces .45 volts at 13 instead of
14.7, you MIGHT accomplish this (I still do not believe a simple
resistor will do this). If you disconnect your O2 sensor, and move your
AFM two teeth richer for example, the car will drive richer in open loop
(only below about 3500 rpms and then the AFM is irrelevant), which it
tends to do anyway. What this will do to your cat is another issue. 

But the best advice is to know what you are doing, and to make any
changes while analyzing mixture, not arbitrarily. The old saw about
these engines leaning out with age is very popular, but I have never
seen any proof. If you discover through analyzing the mixture that you
are lean in open loop, then maybe you need some adjustment, or maybe you
have an air leak etc. All of this tends to be moot in closed loop below
3500 rpm, except that a properly tuned engine should run at about the
same mixture in either open or closed loop, so it may be useful to tune
the AFM for this purpose so the ECU is not constantly adjusting to bring
the mixture near stoich. These cars run around 13 in WOT anyway, so you
won't help that. Another thing is that the percentage power gain at 13
vs. 14.7 is VERY SMALL. Probably not very noticeable. 
 
See
http://www.buckeyetriumphs.org/technical/Carbs/AFMonitor/AFMonitor.htm
which shows power vs. mixture curves (notice the small gain at 13 vs.
14.7). This page also shows you how to make a very nice mixture meter. I
have built one of these and it works great. One of the advantages it has
over many store-bought meters is that it uses 2 chips (all these meters
use this same chip), which allows for a nearly linear output of mixture
vs. LED position. Because O2 sensor output is VERY nonlinear (see output
curve on the same page), most store bought meters just swing from end to
end and are very hard to get any useful information from. This meter
assigns the entire midrange of voltages to 2 leds, so that it is very
insensitive in the midrange but very sensitive at the ends. This is the
opposite of the O2 sensor output, so you get a nearly linear response.
Drive around with this meter in open loop and you will get a very good
picture of what your mixture is doing. I even added a switch to mine
which lets me switch the car from open to closed loop at will to compare
readings.
 
Todd Kenyon 88 M5

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