Dynamic Effective Compression Ratio Re-Visited
Dynamic Effective Compression ratio is a better guide for fuel selection than static compress ratio. Our effective compression ratio calculator will give you a ratio for your engine considering piston, cam, and rod ratio. Unfortunately, engines do not care as much about compression ratio as they do to changes in compression pressure. Compression pressure can be measured at cranking speed with a simple compression tester. If you get 240 PSI you can probably forget plans for using a big carburetor and pump gas. If your class allows high compression and requires a small restrictor plate, 240 PSI may be what you need. REASON: The small restrictor allows only a small volume of air into the cylinder at mid and high RPM, so your 12:1 engine thinks it is running 9:1. The main point is that Dynamic Effecitive Compression ratio is the real compression ratio figure from intake-closing-time to top-dead-center, and pump gas requires 8:1 or less at sea level. A supercharged engine's intake charge being compressed the same 8:1 will probably have problems because it may be compressed at 2:1 before the piston gets its turn to do its 8:1 for an effective ratio of 16:1 - time for alcohol. When using the effective compression ratio calculator be sure to add 15 degrees to the @.050" intake closing spec. to establish the real closing time. You don't really make compression until the intake closes.