ksith

ok so i got my valves relashed and the ticking has gone away. now when im driving and going up a hill or just under a load it sounds like im getting predetonation. i filled my car up with 87 but i dont know what my compression ratio is. is there a way i figure this out with out tearing down the engine and figuring out things likt gasket thickness and bore and all that stuff. i know my cylinder swept volume and my cc for my heads (72) but i dont know if my piston dish volume or the gasket volume. im going to fill up with midgrade and see if that helps next time got half a tank to go.

AlkyIROC

Octane requirements can vary from engine to engine. Typically a low grade octane fuel is for low compression engine however depending on what components are used in the build, 10:1 or even 12:1 could survive on 87 pump gas.

You might only need something like a distributor recurve. Maybe the ignition is too far advanced under load. If you're going to increase the octane level, you only need as much to get rid of the preignition.

The basics to know compression ratio are bore, stroke, piston top displacement (dish, dome, valve eyebrows), head gasket area and combustion chamber size.

It can get a lot trickier when you start figuring out the area on the side of the piston down the the top ring etc. Even knowing how far the piston is in the cylinder at TDC is another calculation. Most factory engines have the piston down about .030". A good rebuild tends to have the block zero decked. It's even possible to have the piston extend past the deck slightly since it would extend into the head gasket area and as long as it stays low enough for the rings to stay in the cylinders.

Without a tear down to get precise measurements, you'll only get a ballpark guess.