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Stuart Stu
07-21-2001, 10:18 AM
The broker on a 1974 36' we're looking at tells us both its US Marine 454's (not original) were refurbished ~120 hours ago. Compression readings were as follows:
Port engine
1 120 2 125
3 120 4 125
5 120 6 125
7 120 8 125

Starboard engine
1 125 2 125
3 105 4 125
5 120 6 112
7 118 8 125

A shot of oil in the Starboard #3 cylinder did not change its reading.

Should I expect better readings from an engine with such low hours? I would appreciate opinions on these readings.

Greg Kelly
07-21-2001, 09:06 PM
" Compression readings in general are looked at as a basic guide to engine condition, not soem empirical test of engine health. There are too many variables. In General, you look for a difference od more than 15% to 25% between the high and the low as the breakpoint between acceptable readings or a problem cylinder. Gauges vary in readings as much as they vary in quality and manufacturer. (How many times they have been dropped counts, too.)

Compression tests are usually set up with a completely open throttle, fully charged batteries and approximately the same test rpm throughout.

Personally, I would not worry about the 120 psi being the high. If you are, then buy a proffessional grade gauge and retest. I would also rerun the tests, recharging the batteries between tests of engines.
Greg Kelly, mechanic for over 32 years. "

Gil Plumb
07-23-2001, 08:02 PM
" Good compression readings on a 454 should be 150 lbs at regular cranking speed. Run the engine before taking the compression readings.

Regards,
Gil "