What should the compression readings be?

[Fred K-OR]

Tried to search this site but did not find this information. I have a std engine with no high compression head, etc. So the question I have is what should the compression readings be on this kind of engine? I measured #1 64 psi, #2 59.5 psi, #3 59.5 psi, # 4 64.5 psi. Are these readings in the about what they should be?

Thanks for the info.

[Mike V. Florida]

More important then the actual number is the same number across all cylinders +- a pound or two.

I thought the stock head was about 62lbs so I'll leave the analysis of your numbers to the engine re-builders on the forum, but I need to ask, what made you check them, something not right? What procedure did you use to get the measurements?

[Fred K-OR]

Originally Posted by Mike V. Florida

More important then the actual number is the same number across all cylinders +- a pound or two.

I thought the stock head was about 62lbs so I'll leave the analysis of your numbers to the engine re-builders on the forum, but I need to ask, what made you check them, something not right? What procedure did you use to get the measurements?

Mike, I just wanted to check the compression on this engine I installed in my coupe. I purchased one of the fittings from one of the vendors to adapt to a modern tester and used a modern digital compression tester borrowed from a friend.

Other that I just wanted to check out things, after adjusting the adjustable valves, the engine runs great. Have a bit of problem with the carb when approaching a stop sign-it dies. So still have some more work to do on the carb.

[MikeK]

#1 - Does it run well? If it ain't broke, don't fix it. Those readings are fine for a stock A that's been run a few seasons.

Have you torqued the head recently? Do so and then try the test, both cold and hot. Also try it after squirting some oil into the cylinder with the piston down, so it get on the walls. A cylinder leakdown test (uses compressed air) will give you a more complete health report. The compressed air will also allow you to listen to the carb and exhaust for leaking valves, listen in the oil filler for how much blowby hiss, and also look in the coolant for bubbling from a gasket leak.

With that said, if it were mine with those readings, I'd go back to #1 above.

[Al 29Tudor]

Fred - try this great site by Tom Endy.

http://modelabasics.com/Carb/ZenithC...l2011-2007.pdf

I had the same problem and the info here said to lower the float level, I did and it seems good now.
Al Leach

[V4F]

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55# all the way thru is good for stock ..........

[Fred K-OR]

Thanks for the info on the compression test. It looks like the results I got may be what a person should expect. Like MikeK suggests, it may be interesting to go back and do a little more testing.

I have to go back and review the stuff that Tom E has put out on a carb rebuild and see what I can do with the carb. I thought I had a couple of extra carbs in my parts stash that I could have swapped out but looks like my parts inventory records are a bit messed up. Could not find the extra carbs yet-still searching.

[Special Coupe Frank]

Fred,

Sounds pretty darn good. The old (1940's-'50s) MoToR's shop manuals used to recommend (beyond the ideal factory compression ratings) that so long as all cylinders read within 10 lbs of each other, that was acceptable.

The second test with some oil shot into the cylinders is a quick and reliable check on the rings /valves: no compression increase with oil = leaking valves / gasket / other.

Good luck !

SC Frank

[ctlikon0712]

Also, the # can be different when you check if the engine is cold vs. hot. Mine is all stock and runs between 53-59 lbs. for all 4 cylinders. I hold the carb open with the throttle rod set to full down, take out all 4 plugs and turn the engine over 5 revolutions for each cylinders reading.