Optimum AFR for MFI

[bliebler]

Is there a Optimum AFR for power and performance for MFI? I don't care about fuel economy. At 13.1 AFR my car runs good, the plugs and tail pipe look clean, however, in the 12.7 range the car seems to pull better in the 3 to 4000 RPM range. At 12.7 the plugs get a little sooty and it shows a little in my tail pipe even if I use a hotter plug. Am I getting more power in the 12.7 AFR or just smoother transition in that 4000 RPM range? Also, does the car run cooler with a richer mixture?

[304065]

Remember 3 4 5 6 as in 3 to 4 % co idle, 5 to 6 % co for part-load. Those are WAY rich mixtures. 5% CO is 12.64 to 1.

The engine will not run cooler, it will probably run hotter, once you pass the point of diminishing returns on excess fuel.

[orange911]

13.1 would be as low as I would go. Spec is in the mid 13's for part load and idle. I don't believe the car will run well in the long run with sub 13 AFR with the carbon buildup on the valves and rings that develop over time. I set mine between 13.5 and 13.9 for both settings and it was a little too lean, so I adjusted 2 clicks on the PL setting and it is dialed in.

The following is related as a measurement check, although not answering your specific question.
Make sure the car is warmed up enough so that if the heat from the exhaust is not sufficient, the hot oil has time to warm the pump up to temp. I made the mistake of getting my readings after the oil got up to temp, but the pump was not fully warmed up. Thus the readings were ideal until the pump was hot and then it was too lean. Found that the exhaust heat was barely flowing and it would take 5 miles of highway to get the thermostat to full travel. It ran great till that point and then it would pop through the intake as I was cruising up inclines.

So, before you take your next readings, make sure the pump is hot to the touch. Because if you are reading the 12.7 on a warm pump, then you might actually be ideal (13-13.5) when it gets really hot and the thermostat discs fully expand and leans it out. The sooty pipe may get that way from warmup when it is too rich. Make sure you look at everything in the CMA before making AFR adjustments. Timing and thermostat are the 2 I would definitely double check. When checking the timing, I always remove the vacuum and remove one variable from the equation, which results in 5 degrees advance at idle. If you are not getting enough vacuum, or the pod is not working, you could be off 10 degrees and this would affect the low end until you get past 2000 when the curve is more forgiving.

[Mr9146]

The engine will tell you where it wants to run...

Keep in mind that the books were written when the fuel was better.

[bliebler]

In 1975 I owned a 72T with MFI. I had the local Porsche dealer tune it up. When I went to pick the car up the head mechanic ,who was an ex Factory worker, was tuning my MFI. He had a CO meter up my tail pipe and when he was finished he took me along for the test ride. The car hesitated around 3/4000 RPM so when we returned he made an adjustment to the part load. From that adjustment the car ran and idled fine. We talked as the car idled and he opened the rear engine lid and burped the throttle. A little puff of smoke was apparent and he stated that's how he likes to set them up and that the tail pipe should show a little black. He also told me not only do they run better rich, but, they also run cooler. At the time the car had 25,000 miles and I owned it untill 66,000 when I sold it to a friend who sold it at 98,000. The pipe always had a little soot and the plugs showed some also, but it ran great!