Rodger,
You may wish to look at these sites. Some fairly good information on Avgas.
Avgas
Avgas2
There are major differences in Avgas and autogas. Vapor pressures, specific gravity, etc.
If it were my car I would go to the race fuel folks and mix if I was having a problem with detonation based on low octane ratings.
I share your problem. My 928 does not like the current offerings in California. The engine management system is constantly backing off the timing and going rich to avoid detonation. It loses in the area of 75 hp. with the current available fuel over 25% race fuel/ 91 Shell mixture.
(This on a chassis dyno.)