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Why 3rd gear for WOT datalogs?

On many engines the K20Z3 included, the AFR will change with gear. This is a result of cylinder, and manifold temperatures as well as RPM delta. The RPM "rate of climb" or" change per second", changes with gear, but yet many other factors like ECU speed (cycles per Revolution) or closed loop operation do not change with gear. When closed loop is disabled at WOT the result of those factors is a degree of richness the taller the gear is. If we tune 4th gear for 13.5 AFR (borderline lean), we may easily find 3rd gear running 13.7 (lean) AFR and 2nd gear running 13.8-13.9 AFR (very lean).

3rd gear allows for safe speeds even at redline (in most setups), and provides a midway between 2nd and 4th gear WOT runs. 1st gear WOT lasts (2 seconds)… and 5th gear WOT is hardly ever seen, and even when it is, it will just run slightly richer (safer) then 4th gear. This makes 3rd gear the ideal choice for measuring fueling and knock values… unlike a dyno graph where the optimum gear for accuracy is the one that provides the closest ration of 1:1 of wheel speed vs Engine RPM. (IE 6000 RPM at the crank should equal 6000 RPM at the wheel).

Remember to allow at least 1 minute between WOT datalog runs. The more cooling time the better, lower gears are more forgiving for primary O2 sensor accuracy vs taller ones.

Here is a 2nd, 3rd and 4th gear run, looking at AFR’s: 

Second gear was tuned for 13.1 but yet by 4th the AFR reads 12.7ish on the LC-1 WB02 due to above described affect