Originally Posted by pasadenahotrod
The weight of the car you've got that engine in makes a differnce too. My old T bucket roadster had a PowerPack 283 punched to 292 with a solid lifter cam and 12.5:1 compression. I always used straight regular cheap gas and never had any problems with heating or detonation or dieseling, but that same engine put in a 57 Chevy wouldn't perform well without high-test.
Are you sure you didnt miscalculated your compression ratio? If not, Im guessing your cam probably had enough overlap to bleed off the compression that made it somewhat workable, still 85 octane and 12.5:1 is hard to believe. I have 10.68:1 with aluminum heads and right around 2000 lbs and have heard pinging at times.