John:

** I doubt that the stronger spark causes a power loss.**

It *could*, depending on the timing.  A stronger spark has the effect of advancing the timing. That could increase or decrease power based on the thetaPP at the normal timing and power setting.  If the power is reduced, the stress on the engine will go UP with the movement of thetaPP toward TDC.

I constantly remind myself that when I was dinking around with the timing on my 57 Chevy back in High School, I'd never heard of internal cylinder pressure or thetaPP and I was doing so on a water-cooled engine with an IRON block and heads.  That's probably why I didn't destroy it.

Walter