Because of the precise tuning needed to run high compression on low octane, I don't recommend or build engines that will be on the edge unless I have easy and continuous access to the car after installation. That means just my friends! A sticking mechanical advance mechanism can stab your engine in the back awful quick! And it's also a case of "do as I say, not as I do".

Too much: more than 9.5:1 with iron heads or 10:1 with aluminum heads on carbed engines with mild cams makes the move to mid-grade necessary. Add .5 to those numbers if running EFI. This assumes that the engine is specifically built and tuned for low octane.

Most people don't have the feel or the patience to set up a carb and distributor properly and typically use what they have, and would be totally intimidated by an EFI system, so I tend to advise a conservative approach that will work for the majority. For someone like yourself or a few others out there, I'd definitely build an 11:1 (or maybe higher, what's your altitude?) 400 that would make 450+hp and get 15+mpg on the highway all on 93 octane pump gas - but it wouldn't be cheap!

If you want to get more pump gas engine building tips, start buying Engine Masters and locate as many back issues as you can. And Jim, I apologize for getting my knickers twisted, I took your questions the wrong way! Ask away - but be warned, I tend to be pedantic!