Dynamic Compression Ratio

The dynamic compression ratio is calculated based on the intake valve closing point. I use 0.020 tappet height as the closing point. Some people use 0.000, 0.001, 0.005, 0.010, 0.050, or 0.053 for Harley-Davidson camshafts. As long as you are consistent and become familiar with the results, I do not think the exact height matters.

The dynamic compression ratio is calculated the same way as the static compression ratio. The volume in the cylinder when the intake valve closes is used instead of the full volume of the cylinder when the piston is at bottom dead center (BDC) is the difference. To find the location of the piston in the cylinder when the intake valve closes will take some involved math if done by hand. Like everything else done today that involves calculations, software and a computer is used. The location of the piston is determined by the stroke of the crankshaft and the connecting rod length. A profile report of the camshaft or a degree wheel and dial indicator will be necessary to find the degree of crankshaft rotation for the particular tappet height you have chosen. A program on the internet can probably be found to do the calculations for you.

Once you have the piston location (actually it will be the center of the small end of the connecting rod) the volume of the cylinder can be calculated. The compression height of the piston will need to be added in for the actual top of the piston location. Any dish or humps on the piston top will also need to be added in just like static compression ratio calculations. The dynamic compression ratio will obviously be less than the static ratio. How much less will have a large impact on the performance of your engine.

The dynamic compression ratio will be a more realistic number to use when choosing a camshaft. The importance of the intake valve closing point can now be seen. Use this next time you are changing camshafts. It could make a big difference in a good way.

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