Skip:

**Can you explain why spark advance is a good idea in auto engines and a bad one in airplanes?**

It's not necessarily.   It all depends on the power and fuel being used.

You can actually gain power in an aircraft engine at takeoff power by RETARDING the timing from the historically chosen timing.  That has several advantages.

OTOH, you can gain power in cruise in most situations (NA) by advancing the timing during that power demand.

If the thetaPP is 10 like it is at takeoff, retarding the timing will make more HP.  If it is 25, like it can be in cruise, advancing the timing will increase output.   IOW, if the thetaPP is 16 and you advance OR retard the timing, the power output goes down.

The objective is to have a thetaPP of 16dATDC in all conditions.  That results in the most mechanical advantage for the fuel charge being burned.  It improves the BSFC.

Water-cooled, iron-head engines have a wider detonation margin and withstand detonation a lot better than our aircraft engines, so we need to pay closer attention to these issues.

Walter