Thread: Compression Ratio

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With a stock rebuild, 9.0 is the generally accepted limit for iron heads and 10.0 with aluminum heads using pump gas.

By building the block with zero deck and 0.035" to 0.040" squish and using a cam with an advanced intake closing point that is compatible with the static compression ratio, you can run up to 11.0 with iron heads and good gas, assuming reasonable ignition lead. You'd want to eliminate any sharp corners in the combustion chamber and on the piston top and spark plug electrode. You'd also want to use a gasket with sufficient bore diameter to prevent overhang into the cylinder bore.

However, this is a double edged sword. If you increase the compression and use a cam with advanced intake closing point, the dictates that the duration of the cam will be increased over the duration of a cam that you would use with a lower compression ratio, so with the "larger" cam, the window of operation will be moved up in the rpm range of the motor, away from low-rpm efficiency and moving toward mid-range efficiency. The larger cam may require a looser torque converter and stiffer gears in the rearend.

So, you see, 9.0 with iron heads isn't all bad for a street motor.