SA compression test

Farmall Super A, AV, 1939 - 1954

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SA compression test

Postby Gerry Powell » Sun May 02, 2010 8:08 pm

I did another compression test today and now I'm getting 80-78-85-80 after installing a new head gasket and cleaning the valves. The I&T manual says the compression ratio is 6:1, do you multiply this by 14.7psi (atmospheric pressure) to come up with new compression of 88psi? Another post at a car site has a table that calculates PSI by PSI=(compression ratio-1) X 14.7. Using this method, 6:1 would give you 73.5 psi.
Also, in the I&T manual below the line for compression (standard) is 3 more lines of compression specs, what are these?
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Re: SA compression test

Postby Eugene » Sun May 02, 2010 8:45 pm

The lines following standard compression ratio, 6:1, are for other pistons that were originally available for the tractor. The next lower line in my I&T for the SA is a 5:1 compression ratio followed by two blank lines.

Conduct the compression test dry, then wet (oil added to piston).

Your current figures are a bit on the low side. The C-60, Cub engine has a standard compression ratio of 6.5:1. You would normally see compression reading of 120 psi or a bit better on a freshly rebuilt engine after a couple hours of run time (break in).

Kind of an aside. I have a WC Allis Chalmers with low compression pistons, either 4.2:1 or 4.5:1 compression ratio. This engine has compression readings, hand cranking, very close to your SA readings. The WC parts books lists 9 different compression ratio/piston sets that were available from the factory.

Edit: Wikipedia. Generally, cylinder pressure for common automotive designs should at least equal 10 bar, or, roughly estimated in pounds per square inch (psi) as between 15 and 20 times the compression ratio.
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