Vacuum Secondary Conversion

For the AVERAGE guy you only can go by what people are averagely getting away with, in the 80s/90s it seemed like 9:1 was the safe bet on the street, but in the last couple of decades 10:1 is a pretty common occurrence on iron and 10.5:1 with aluminum and a decent cam.

I see no evidence that aluminum is effecting cylinder pressure all that much as you don't generally see a drop in torque per cid in similar built iron vs aluminum headed engine as for less sensitive to detonation it's hard to tell, more iron headed guy's would have to bump up there cr and we'd see, till then for MOST it's just a guess, and the safe bet for most is to not venture far from the generally proven path.