Compression - How many Lbs.

[plym_46]

I believe my manual states that 110 is optimum.  I got 112 on my fresh rebuild with a dry test.

The thing with compression on old motors is consistancy.  Your shows diminished but farily even comp across the cylinders.  How was your test done, wet or dry, hot engine or cold, plugs in or plugs out?  You might want to do a comparison test and do the dry wet prcedure.

Pull all the plugs, and test the cylinders recording the readings for each cylinder, then repeat the test after putting two teaspoons of clean oil into the cylinder before attaching the instrument.  Record these readings.  If there is significant improvement with the wet test, this indicates your compression loss from factory is due to worn/broken rings, or cylinder bore wear. The cylinders that do not improve indicate valve problems, as the oil will temproarily seal rings but will have no effect of stuck. burnt, or worn valves and seats.

It is not uncommon for these engines to run well with 70 lbs of compression.  A common problem seems to be broken top compression rings.  The engine I rebuilt had the top rings broken in two or more pieces, flopping around in beat up lans.  but the engine ran fine and didn't smoke. 

[Bill James]

For what it's worth, my Motor's manual shows 105 min compression at cranking speed, for all models 1949 through 1954. In my opinion your's is a little low, but if it's running good, I wouldn't worry about it. The motor's manual I'm looking at is the 20th edition, covering 1949 through 1957. There is a very interesting test using an old spark plug with an air fitting brazed to it, and compressed air. these books are found quite often on e-bay and are reasonably priced. I've always liked the motor's books, now if I could just find a 1st edition that I could afford ::)

[roynan]

Hi, My 1950 Business coupe is my first mopar product in many years and my very first flathead {0r OHV] six cylinder ever as I always thought if it wasn't a V-8 forget it. I have changed my mind and really love my little Plymouth and as long as it goes down the road I find I dont care what kind of motor it has.My question is I cant seem to find a compression value expessed in PSI in any of my books. The motor runs fine, starts easy and holds a vacuum at 19-20 lbs.,and compression runs a spread of 81 to 90 lbs. I would like some info as to what compression should read and whether or not mine is low as it seems to me. Thanks, Roy