Air-Fuel Ratio
The ratio of air to fuel in the combustion chamber. Lower number = richer (more fuel). Higher number = leaner (less fuel).
Why it matters
Too lean under load and the engine runs dangerously hot. This is how pistons melt and head gaskets fail. Too rich wastes fuel and can foul spark plugs. At wide-open throttle, your engine needs to run richer than normal to keep combustion temps safe.
What to look for
At WOT on gasoline, target AFR is usually 11.5-12.0. On E85, target is around 8.2-8.8. A deviation of more than 0.8 AFR leaner than target during WOT is critical.
Related terms
Another way to measure air-fuel ratio, expressed as a ratio where 1.0 = perfect stoichiometric combustion. Below 1.0 = rich. Above 1.0 = lean.
The ECU's real-time, moment-to-moment adjustment to fueling. Think of it as the ECU saying "right now I need to add a little more fuel" or "right now I need to pull back a little."
The ECU's permanent, learned correction to fueling. Unlike STFT which changes moment-to-moment, LTFT is the ECU saying "over time, I've learned I consistently need to add or remove this much fuel."
Pressurizes fuel to ~2,900+ PSI for direct injection. When it can't keep up with demand, fuel pressure drops and the engine runs lean.
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