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This engine rebuild was designed with a 0 deck, and the final compression ratio poured out to be 9.49:1, approximately 1 point higher than stock. A higher compression ratio is required for throttle response as well as thermal efficiency. During the '70s, the mindset was to lower the compression ratio to limit the oxides of nitrogen (NOx) emissions. NOx is formed as a result of pressure, heat, and exposure time. It's created when the leading-edge, flame-front temperature reaches 2,500 degrees F. Even though the SD-455 left the factory with an EGR valve, the logic then was to lower the cylinder pressure to limit the one mechanism in the NOx equation.