273 Compression Test Results

These techie posts are very helpful.

The FSM for my 65 Dart says 120-150 psi, 20 psi variation for 2-barrel (9:1) and 130-165 psi, 25 psi variation 4-barrel (10.3:1 HiPo), ditto zhandfull's post. Of course, the compression is greatly affected by the cam. Even if the factory cam, lobe wear can change the readings.

I have never gotten higher readings by propping the throttle and choke plates (multiple cars, usually when I forgot), but that may make the pressure come up quicker. One rule I have read is that it should reach within ~5 psi of final pressure after the 3rd stroke. If it comes up slowly that may indicate bad rings. Not sure I believe that because the compression gage tube volume probably has a lot to do with that. BTW, if you ever change the Schraeder valve in the tip, don't use a regular auto/bike one (been there). The gage requires a special one with a very weak spring, usually has a white plastic seal.

Readings I got in my 1965 273 2-barrel in 2009:
140, 155, 125, 145, 148, 150, 120, 150 psi
very similar to 65DartGTconv. It took 6-8 strokes to reach max pressure in all cylinders.

I recall the engine was warm. I ran it for 30 sec several times with no radiator. Haven't run it since, since busy de-rusting, painting, and rebuilding other parts. Like 65DartGTconv, I needed to know if I needed to re-ring it early on. The engine sat for years before that I think, so maybe some rings need to free up.

Don't fret over the one 100 psi reading yet. I had a 95 psi reading on #7 initially, but later got 120 psi. I had added a little oil to check the rings (don't add too much), but recall getting the same a few days later after the oil should have drained, so maybe the oil helped free the rings. I decided to leave the heads on and test it again after some driving.

I did the same "apply air to cylinder" trick. We must have the same Harbor Freight compression gage. I had the oil pan, intake, and valve covers off so could tell when each piston was at TDC. I could hear air hissing in all cylinders, which is normal since you always get some ring leakage. Sometimes you can feel it flowing out the breather, intake, or exhaust to pinpoint the culprit. I can tell if the exhaust pipe, but the other places are more tentative.

Perhaps a better test of the valves and rings is to turn the engine over by hand, with a socket wrench. In my newer cars, you can feel each cylinder like a spring, especially the 4 cyl, and they hold pressure ~15 sec. I recall that my slant six cylinders would leak off almost as fast as the piston came up, 1 or 2 sec, even though the engine ran fine. I re-ringed my 2.4L and it uses regular rings with an end-gap (not the special overlapping type), so don't know how it holds the pressure so well (even 70K mi after rebuild). Ditto for my 3.8L.