Quote:
Originally Posted by jwd
Having the readings being all close to one another is more important than the reading itself. Cam timing will dictate the overall reading. I've seen motors with a 13:1 CR read 140 and ones with 9:1 read 180.
Jim
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Exactly, cam timing/overlap probably has as much to do with cranking compression as anything.......
My 10.5 to 1, 351-W has about 130 lbs. cranking compression, but my camshaft, not being a monster cam does have a lot of overlap, thus bleeding off compression at low cranking rpms.
By contrast, I've seen a bone stock 351-W with 160 lbs. cranking compression, most stock cams have little or no overlap thus giving a higher cranking compression reading.......
What you need to be concerned about is you don't want more than 10% difference from the low to the highest reading on you cylinders.
After doing a cranking compression test, one at a time, squirt a couple of squirts of motor
oil in each cylinder and re-test, if you reading goes up, your rings are leaking, if the reading stays the same, it's your valves leaking, assuming you have a low reading to start with...
David