VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Compression in 50 to 60 range

I presume I need to do some engine work. The folowing are my compression test results. 1st reading is after one cycle, second is after three cycles.
50-60
50-50
60-62
50-52
60-61
50-55
Of course, my compression guage could be bad.
What are the thoughts.

And, where in Los Angeles could I get it rebuilt? 1929 194 engine.

Jim

Jim,
Did you do the test dry? If you did, you should also add some oil to help seal the rings to see if it increases. If it does, it's a ring issue. If it doesn't change much, it's a valve issue. I recently did my Olds (very low compression ratio of 5.6 to 1) and while it ran very well, it blew some smoke at high throttle. #5&6 were down at 50lbs. All the other 4 were at 70-80. Your whole engine is pretty much even with not much over 10lbs difference among the cylinders. I would assume a ring/bore issue as 50's is on the low side I believe.

i did one back in 2014 on my 1929, here is the posting
Maybe someone else can chime in and confirm that these numbers are right or off, but no one barked about them in the discussion.

The fact that all the numbers are fairly close together indicates nothing too serious is wrong.
The is probably a little going past the compression rings which would be expected in an old engine. Also the compression gauge reading low or cranking speed below normal.
If the engine runs OK, idles smoooth, etc. I wouldn't worry about it.

Was the throttle full open when you did the test?
Tino

As soon as my son gets here, will redo the test. Yes, it was a dry test.

New test results:

The folowing are my compression test results. 1st reading is after one cycle, second is after three cycles, third is after squirting oil in the cylinder.
50-60-60
50-50-58
60-62-62
50-52-55
60-61-61
50-55-61

Those values show a tired but not dead engine. It should run okay for many years. If you intend to drive it a lot then a rebuild will improve performance. I have driven cars from that era with similar wear. Back in the day it indicated a Bon-Ami treatment. I never tried it but heard it improved compression.

I agree, just what I said yeaterday.
Bon-Ami use was suggested when new rings were thought to not seat and the Bon_Ami roughened up the cylinder walls.
If anything I would remove the spark plugs and pour an ounce or two of Marvel Mystery oil in each cylinder and let soak for a day.....or use ATF.

I am curious about the statement "tired but not dead engine". I just did a compression test on my 31, and I got 60-61 for dry and 60-63 for wet. Very consistent across the board but certainly not the 75 that BearsFan got. However my car is at high-altitude with an atmospheric pressure of only ~12 PSI. So I am still getting right around a 5:1 to compression ratio which is actually similar to a 75 PSI reading at sea-level. Am I correct in assuming that a 62-63 PSI reading at 6000 feet in elevation is all that can ever be expected of this engine?

Good question. I have never seen altitude mentioned in repair manuals regarding cylinder compression.

You could go to the top of Mount Evans and do a compresson test Just couldn't pass that one up.
but I do understand your concern.

Yes ~60 psi is what you would expect.

The compression ratio is the increase in pressure from the piston squeezing the air in the cylinder from the bottom to top of the stroke. In the case of a '31 Chevy with ~ 5:1 compression ratio at sea level with ~14.7 psi atmospheric pressure ~73.5 psi would be the expected value. When you are at altitude with a ~12 psi atmospheric pressure then ~60 psi would be expected.

At low altitudes above the sea level, the pressure decreases by about 1.2 kPa for every 100 meters. For higher altitudes within the troposphere, there is an equation (the barometric formula) relating atmospheric pressure p to altitude h.

Go to https://en.wikipedia.org/wiki/Atmospheric_pressure
for the equation.

or http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html for a more understandable chart.

Extrapolation from the values on the chard would give ~ 8.7 psi at the summit of Mount Evans

Does that mean that tire pressure changes at high altitude, temperature not withstanding?

http://www.carsdirect.com/car-repair/how-temperature-and-altitude-affect-tire-pressure

I'm from New England and a Patriots fan. Sea level and temperature can have a lot to do with fluctuating air pressure!

Tire pressure increases with altitude and decreases with temperature. Temperature also decreases with altitude so the sum of the change might not be a much as expected.

I know heat increases tire pressure but never even though of altitude.

Thanks for confirming my intuition with respect to expected cylinder pressure.

As far as driving up Mount Evans I might wait on that one as I just don't know if I trust the guy that fixed the brakes :)