Compression readings - What do they mean?

[Jack O.]

As a result of my popping sound I did a compression check on the driver side where I hear it. Here's my results (cyl #/reading): 1/125, 3/137, 5/130, 7/100. This would indicate there's an issue #7, correct? Valve or cam related I suppose since I wouldn't expect rings becuase the plugs all look good and the car doesn't smoke at all. In fact, other than a rather rough idle (which might be expected givene tne cam) the car runs really well. Is there anything else that can be concluded from my compression readings? I suspect they are really low since the enigine was rebuilt by a previous owner to be ran with a supercharger.

BTW, I adjusted my mixture, needed to be rricher, and not only did it help (but not eliminate) the popping but overall the car just runs better. Thanks everyone for the help and suggestions!
Jack

[Anthony P.]

Re: Compression readings - What do they mean?

Jack I would suggest getting readings on all cyls. and compare from all of them, mechanical cam or hyd? could have issues related to valve timing such as worn lobe, worn rocker arm, broken valve spring etc. just something else to keep in mind. good luck keep us informed.

[Joe C.]

Re: Compression readings - What do they mean?

All of those readings seem low. Many average in the 170-180 range with 200 being "near maximum accepted" for street driving. This is probably "bunk" because mine gets near 230 across the board.

Did you run the test with a warm engine?
Did you have a strong battery and removed all 8 plugs for the test?
Did you let each cylinder hit its compression stroke at least 3 times before checking the reading?
Did you block the carb throats wide open before cranking?
Are you certain that you had a good seal at the spark plug hole if you used a tester with a rubber seal/tip?

If you didn't inject a couple squirts of oil into each cylinder before the test, then do so after you do a baseline test again, as above, and report the pressure rise, if any.

[George W.]

Re: Compression readings - What do they mean?

With the poping sound and the descriptions I have been reading; I suspect a worn cam lobe. Remove the valve covers and crank the engine and look for one or more rockers not mashing the valve down as much as others. If this is the case...and we hope it is not...it is tear down time with all new valve train.

[Dick W.]

Re: Compression readings - What do they mean?

Next step is to add motor oil to the cylinders. See where the compression goes. Oil will somewhat seal leaking piston rings. If the weak cylinder comes up to about the same as the rest, it would more than likely be rings.

If it were my car, I would do a cylinder leak down test. You can tell if it is rings or valves that way.

As mentioned, it is possible that you have a bad lobe on the cam. Before I started tearing the engine down, do the leak down test

[Jack O.]

Re: Compression readings - What do they mean?

I had to use my old cheaper tester which may affected my readings. the other side came up 2/120, 4/135, 6/135, 8/135. Once I get the adapter for my better tester I'll recheck some of the cylinders - meanwhile I plan on invetigating #7.
Thanks everyone.

[William C.]

Re: Compression readings - What do they mean?

Leakdown test is an easy sure answer before you do a lot of work.

[Chuck G.]

Re: Compression readings - What do they mean?

They all seem low to me. I seem to recall in another thread that this engine was built as a "blower motor", and thus would have less compression. I believe you removed the blower, and put a carb on it.

Recently, I helped a friend remove a 331CI Cadillac motor from his 1957 Caddy. Motor had 60K +/- miles on it. It still had 185# plus in each cylinder, without oil added to the cylinders.

Chuck

[William C.]

Re: Compression readings - What do they mean?

Some "blower motors" are built with as low a 7.0 compression so it makes the unblown version less powerful than the worst "smog motor". You really need to get the true specs as to what the engine was built to before setting expectations, or just start a rebuild with "known quantanty" parts

[Joe C.]

Re: Compression readings - What do they mean?

Some "blower motors" are built with as low a 7.0 compression so it makes the unblown version less powerful than the worst "smog motor". You really need to get the true specs as to what the engine was built to before setting expectations, or just start a rebuild with "known quantanty" parts

The possibility of lowered SCR exists, and all things he said in his prior thread complaining of gas fumes taken into account (as was done prior to my post #3 above), engines using boost also generally employ non overlap cams with very modest durations (early closing intake valves) which would still not account for such low pressure readings.

[Terry M.]

Re: Compression readings - What do they mean?

I am used to seeing compression test readings with two values. One is supposed to note the pressure at the first revolution and the maximum reading after three revolutions.

Other conditions as noted above (throttle full open, all spark plugs out, fully charged battery, etc) are important and will alter the readings as well. The lack of data and answers to the standard test conditions leave us in the dark.

I did say on the other thread I much prefer a cylinder leak down test, but it does require more specialized tools and a supply of shop air.