Timing vs. AFR

[ErnieJones]

I came across something the other day about timing vs AFR but can't seem to find it.

If I go leaner in the table should I increase the timing or decrease it? Also the opposite (going richer - increase or decrease timing).

What's the general rule of thumb?

[winstonusmc]

http://www.innovatemotorsports.com/resources/myths.php

This might be what you are looking for.

Not a simple answer unfortunately.

[subwoofer]

Gasoline burns the fastest at ~13 AFR, burn speed decreases either side of this point. Since we want maximum cylinder pressure at a particular point in the cycle (15-20°ATDC, slightly dependent on engine geometry), burn has to be initiated at different times for different AFRs. Both leaning out and richening up the mixture from the max power mixture will require more advance for best results. But remember to err on the safe side, you want as little advance as you can get away with to avoid detonation and and the associated problems that follow. The turbo guys often run richer than max power mixtures to slow combustion and increase the margin to detonation.

[dontz125]

The turbo guys often run richer than max power mixtures to slow combustion and increase the margin to detonation.

They're also trying to cool the mix and the crown (shades of carb'd 2-strokes!).