Whats the difference between the two?. Im guessing using context clues that staic would be the compression before boost?. And dynamic would be at xpsi of boost?. Am I right?

 

Static would be the calculated value based on bore and stroke deck height, gasket etc.. Dynamic is the value that figures in valve events and boost on the compression ratio.

 

The way most people use the term, static CR is simply the relationship between the volume above the piston at BDC over the volume at TDC. Dynamic CR refers to the ratio of the volumes above the piston at BDC over the volume at the point of intake valve closing (when compression actually starts). Dynamic CR is always lower than static. With a big (long duration) cam, it can be a lot less. I had an NA street motor once with a 12:1 static CR but a pump gas friendly 8:1 DCR due to the big cam.

Boost plays into it as a contributor to volumetric efficiency. In a running motor, the greater the amount of air and fuel taken in during the intake stroke, the greater the cylinder pressure will be. As VE increases, the cylinder pressure rises. A blower increases VE and thus cylinder pressure, as does mods like better flowing heads, a bigger cam (in the right rpm range), etc. This is not usually expressed as a ratio. There are calculators that purport to calculate "boost comporession ratio", but they really make very little sense, since the VE varies so much with rpm.

Rich Krause