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Old 10-29-2008, 01:01 AM   #21
williaty
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You should post a this on RomRaider's forum as well for peer review. You got a couple of details wrong but those guys would do a MUCH better job of explaining all of it that I can.

One thing I will touch on, though, is your section on AFRs. Stoich is not ideal for anything other than emissions. Stoich produces the lowest sum of hydrocarbons and oxides of nitrous. It's not the best for fuel economy (leaner than stoich is) and it's not the best for power (richer than stoich is). The reason that richer than stoich is best for power is that, due to real-world imperfections in charge mixture, running an engine at stoich results in both excess oxygen and fuel in the exhaust stream. This is due to some of the oxygen and some of the fuel not being able to find each other in the combustion chamber. Oxygen is the thing it's hard to get into an engine so we want to be damned sure that we burn every last one of the buggers. To ensure this, we add extra fuel to make sure that none of the oxygen escapes alive. For "normal" engines, this results in Mean Best Torque being somewhere in the ballpark of 13 to 13.5 AFR. Now, many engines, especially forced-induction engines, will have problems with knock at that AFR. By further enriching the mixture, the efficiency of the combustion can be reduced to the point that knock is easier to control. Make sure you read that carefully, though. The increased resistance to knock is a result of reduced efficiency. So over-enrichment (richer than 12.5 or so) costs both fuel efficiency and power. However, it allows for higher boost and/or more timing. It's a trade off and different tuners will prioritize different aspects of that trade off. In the end, most theories on that compromise will end up making roughly the same power, just in different ways.
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