Quote Originally Posted by DennyW View Post
The compression can be changed by simply picking the correct cam to run on at 91 octane fuel...With what you have...and getting the distributor advance in order, along with timing, and the curve settings...
Just so we don't confuse some of the members here who are learning, STATIC COMPRESSION RATIO can only be changed by changing the cylinder volume, piston deck height, piston crown design and/or combustion chamber volume.

A fellow can, though, change the CYLINDER PRESSURE (the pressure you see on a gauge when you do a compression test of the motor) by altering the cam timing as Denny suggests. Compression in the cylinder does not begin until the intake valve has closed to seal up the cylinder, so if you alter the intake valve closing point on the cam lobe, to closing earlier or closing later, you can vary the cylinder pressure up or down. This is where most newbies make their mistake with choosing a cam for their motor. They might have a low compression motor, something on the order of ~8.5:1 for instance, but they will, out of ignorance, choose a cam that would work well with an 11.0:1 static compression ratio motor. Their thinking is that the cam is a stand-alone part that will make more horsepower in the motor regardless of the accompanying parts in the motor. Nothing could be farther from the truth. The cam is dependent on ALL the other parts in the motor to work in concert toward a certain goal. The result of using a long cam without support from other parts in the motor is that there is little cylinder pressure made until the motor "gets up on the cam" in the higher rev ranges. They are left with a motor that will pull hard from, for instance, 4500 rpm's and upward, but has little cylinder pressure and little horsepower down lower in the rpm range where they operate the motor every day.

Here is a tutorial that I wrote that explains it pretty well....
http://www.crankshaftcoalition.com/w..._compatibility

.