The first motor i built was a 93 stock 5.0 bottom end with a set of hyperutectic slugs. I drilled steam holes in the top of the block and fitted it with a set of 1966 289 heads, had to get a special set of push rods, the slugs were flat top and the block deck was zeroed out - 53cc chambers, i'm guessing it had somewhere along the line of 11.0:1, never had any issues with detonation, and i ran some of the crappiest gas you could find in it.
Now the 331 i just built, this is a lil different, SCAT 1-45255-1 rotating assembly, Canton crank girdle. Lunati 510A8LUN 565/565 cam with a set of 1969 351w heads. All clearances made by me, i started out running 100 oct race fuel and now that i'm starting to get the tune better on it i have started to drop oct to 92, we'll see how it gose, the ultimite goal for me is to end up being able to run e85 in it, from what i understand the ACTUAL octane ratting on e85 is closer to the 97 range.
It's not only mechanical compression ratio that you have to be concerned about, but cylinder pressure. The cam you choose will have a big effect on this.
I had to give Comp Cams all of the engine's specs including the bore & stroke, pistons, rods and heads plus what I was going to use the car for in order for them to factor in their cam suggestion - they asked for a lot of info
The gasoline mixed with it lowers the octane. The octane rating of the gasoline added varies. The refinery we load at mixes it with 83-84 octane gasoline. This is the only octane regular unleaded they make there. To make 87 conventional gas (less than 5% ethanol) they mix it in a 60/40 ratio with 91 octane premuim gas.
