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Old 12-08-2009, 05:09 PM   #1
j.fro's Avatar
Join Date: Feb 2005
Location: Virginia
Posts: 1,355
how lean is lean???

After a dyno test, the dyno operator said my car was running lean. the graph showed the air-fuel ratio half way between 14 and 15:1 throughout the dyno pull. I've read that 14.7:1 is ideal, which is about where my numbers were, but I've also read that under full throttle, it should be around 12 or 13:1.
Am I really that lean??
BTW, it was a Dynojet, and my RW HP was 224.73
2000 Box S with headers, no cats, and an EVO intake system w/ a dryflo air filter.
128,000 miles
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Old 12-08-2009, 05:43 PM   #2
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
14.7:1 is the ratio of complete combustion of gasoline in air in an ideal situation. Meaning that all the fuel is atomized and there's enough time and conditions are perfect when the flame front comes through.

Obviously, nothing's perfect. It takes more fuel than the ideal amount to account for non-atomized fuel and incomplete combustion.

Richer combustion also serves to lower the temperature of combustion products, helping with the reliability of valves and valve seats. At full load, there's alot of energy being released and for the best combustion and the most reliable engine, AFRs of 12:1 are better. Turbos usually run even more rich at full load. Not to mention that spark timing is set to coincide with an expected fuel ratio.

The best ratio depends on the engine as airflow patterns and mixing are always different. That's what dyno runs are for.

At light loads, where there's less stresses on the engine and mixing is better, ratios closer and sometimes even leaner than 14.7:1 are possible.

Running 14.7:1 and higher at full throttle is bad for your engine.

Last edited by blue2000s; 12-08-2009 at 05:53 PM.
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