I know this may be a noob question, but how exactly do you calculate CR? I have rebuilt my motor, but unsure of the CR. It's an 87 350, 60 over, ZZ4 cam, aluminum heads (113 casting), flat top hypereutectic pistons, and full double roller timing set.

I am curious if I will have to buy the higher octane gas.

Thanks a bunch!

Pat

 

Easiest way is with any of the dozens of HTML calculators you can find on the Internet; or write your own Excel spreadsheet; or like we all used to do before those things were invented, we did the math!!! {gasp}

You need several pieces of information:

Your pistons' dish or dome volume (flat tops usually have valve reliefs with around 6cc of "dish")
Your pistons' "deck clearance" to YOUR block (how far "down in the hole" they are at TDC; about .025" below the deck in a stock motor, about .045" in a typical "rebuilt" motor, ideally .000" but can only be achieved by specific machine work)
Your heads' combustion chamber volume
Your head gasket thickness, and for greatest accuracy, its bore diameter (usually larger than the cylinder bore)
Your engine bore and stroke, or displacement

Without that information, ACCURATELY determined, NO calculator will give you an accurate result!!! No guessing allowed on any of it.

You see alot of stuff like "9.73:1" that people quote. In reality, there's WAY more uncertainty in it; this is one of those "measure with micrometer, mark with chalk, cut with axe) kind of things. For example, FEW blocks are straight, such that all the decks are the same height, unless you paid to have that done; NO heads have all 8 chambers the same size; you DON'T know the chamber size unless you measure it (typically, those "9.73:1" types are using the cc's they found on Mortec.... which have production variations of around 3-4 ccs ON THE SAME HEAD, let alone on 2 different heads); and so on.

Google "compression calculator". Or, visit the Keith Black site www.kb-silvolite.com or any of the other piston mfrs' sites (JE, SRP, Wiseco, etc.)

If all you want to know is whether that combo will need premium, then it's safe to say though, yes it will.

 

Thanks for the reply and of course, all the information! I didn't even think to google for it, duh?!!! Ha!.

Thanks again, and I will definitely look this up and see what I come up with.

Pat