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Cams And Compression


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Low compression pressure:

The higher the pressure within the combustion chamber when the air/fuel mixture is ignited, everything else being equal, the more power your engine produces and more efficiently it runs. However, if the pressure it too high, detonation (pinging) may occur which can destroy an engine.

Each combustion chamber design has an upper pressure limit above which serious, damaging detonation is likely. With modern American 92 Octane lead-free gasoline, a reasonable upper pressure limit is 180 psi for the Evo Big Twin and 190 psi for the Twin Cam. A well-tuned motor should not suffer detonation with these pressures.

The standard method for determining the compression or cranking pressure of an engine is to remove the spark plugs, install a standard compression gauge into one of the spark plug holes and, with the throttle full-open, crank the engine over with the starter motor until the pressure gauge needle stops rising. This usually takes 4 - 8 compression strokes. Both cylinders should be tested.

Stock Evo and Twin Cam motors develop cranking pressures in the 150 psi range. If a late-closing cam is installed, with no other changes, the cranking pressure will go down. The reason high compression ratio pistons and racing cams are so often associated is because the higher compression ratio pistons (and/or milled heads) are needed to regain even the normal moderate cranking pressures, let alone raise them for more power and efficiency.

Low cranking pressures (because of late closing cams and stock pistons) can significantly reduce performance in the mid-rpm range.