Quote:
Originally Posted by maverick9611
plugs don't lie.
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Problem is it's difficult to determine:
- how much time was spent at each throttle-opening (10%, 20%, 30%, 40%, etc.)
- how much time was spent at each RPM-range (1000-rpms, 2000-rpms, 3000-rpms, etc.)
- how much time was spent at each 3D point on fueling map (throttle X RPM)
You've got an
accumulated and weighted average that's speaking in generalized terms. You're looking at changing main-jets, but it's very likely those plugs are only reflecting 10% of main-jet usage. While majority of operating-time was using idle & low-speed circuits.
Here's an example of lean low-end and rich top-end:
Note the mid-range rich dip that robs some torque. Leaning out the mixture across-the-board regains some of that mid-range and boosts top-end power. But... it also leaned out already lean low-end even more. To flatten out this fuel-mixture curve we'd need to adjust pilot-screws to richen low-end and perhaps CNC custom needle to remove mid-range dip. Then adjust top-end with main-jets.
Which is where EFI comes in as you can tune extremely precisely in 500rpm increments or less. And very quickly with just a couple mouse-clicks. This tune was done in less than 30-minutes. NA engines make max-power around 13.0-13.5:1 AFR.
Seeing that both carbed and EFI versions have lean low-end with rich top-end would seem to indicate this was done intentionally. Lean low-end most likely for emissions testing and EPA MPG test-cycle. Top-end richness for safety against detonation and melted pistons; albeit sacrificing power as well.