Originally Posted by Bruce H.

The reason tuning that rich is old school is because the fuel supply and monitoring has changed so much in the newer cars to make more safe power, improve fuel economy, and reduce emissions. The addition of a second oxygen sensor after the catalytic converters on new cars allows the ECU to constantly monitor and finely calibrate air fuel ratios, with the ECU constantly trying to lean it out, and also advance timing as much as possible. Older cars ECU couldn't monitor or control fuel as carefully, and had to run them richer to allow a greater safety margin from pre-detonation.

Your RX7 probably runs upwards of 19 psi of boost, Albert, perhaps double what the factory boost level was. Max is adding less than 3 psi with his pulley, and maybe no more than 1 psi in the mid-range where the torque limitters are reducing his power (compare to Matt's with XKR-S ECU and tranny). Where your RX7 was pushed hard, Max's isn't. The large safety margin you tuned into yours is not needed with his, and where you likely had no ECU anti-destruction knock response system, he has a sophisticated ECU with many sensors monitoring every aspect of operation, with the ECU ready to put itself into "limp mode" at the first sign of trouble. In addition to fuel mapping according to RPM and boost, it will additionally be mapped at least according to engine load.

But the tuning of the fuel map is just one part of a comprehensive tune to improve safe levels of power and performance. The other is the ignition map, with more advanced timing making more power, and bringing you closer to the threshold of detonation. Remember, the ECU will normally adjust operation so that the engine is running on the threshold of detonation, not a long ways away from it. The tuning procedure that is used is to adjust the fuel map first, and then the ignition map if necessary. AFR's would normally drop from lean at low rpm and engine loads to richer from peak torque to redline.

As Max mentioned, Jerry worked for hours on his AFR, and on Matt's I would assume as well. Where Jerry indicates he didn't want to risk running lean at higher rpm, the AFR plot indicates fuel running so rich it falls right off the scale at high rpm. You'll notice the same on Matt's plot, but in that case it appears a horizontal scale was set at ~10.5 AFR, and all readings below that disappear onto that line. Where you tuned yours to 10.5:1perhaps from mid-range to redline under full load and boost, these plots show what's called "fuel dumping". The stock ECU would normally do that only in extreme conditions, and if we were to see a dyno run with less than full throttle we would likely see an AFR plot closer to 13.0:1 I expect. What I would be asking is "why is the ECU fuel dumping under full throttle.

I has seen hundreds of dynos on different engines that have been tuned and they always are tuned to a pretty consistent AFR out to redline, and never richer than 11.0:1. Perhaps it's a limitation of the ECU, or perhaps the advance in the ignition map needed to be retarded to compensate for a better AFR, or perhaps the tuner just didn't try to optimize it. many tuners are reluctant to adjust the ignition map unless they are able to monitor the engine's knock sensor. That would likely have been impossible for Jerry to do when he was tuning from another location, and would have required a lot more work than just adjusting the AFR.

Why should anyone care if the AFR goes crazy rich if the customer is happy with his power gain? That depends on why it's going crazy rich on the dyno. Is it just something the ECU likes to do when it sees you going full throttle, is it a reaction to higher intake air temps caused by the intercooler's inability to cool the air charge at higher load and boost levels, is it because of higher coolant temps on the dyno, because you've been diddling with its fuel maps, removed a speed limitter that it didn't want you to, etc, etc. Should you be concerned in normal use with a high grade of fuel (unlikely), or worried under sustained full throttle and high load use like on a race track?

If you think it's a reasonable question to ask why the ECU is fuel dumping at high rpm, and I most definitely do, then you should look into it carefully. It might be easily explained, and be quite an acceptable operating condition, but at this point it looks more like a lack of tuning rather than an optimizing of one. And I'd be delighted to be wrong.

Bruce