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JD G compression

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Rich

03-23-2001 17:08:06




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What should the compression be on a 1948 JD G be. Not the ratio, the lbs on compression test.
Thanks




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burns

03-26-2001 06:20:43




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 Re: JD G compression in reply to Rich , 03-23-2001 17:08:06  
If you add high compression aluminum pistons with the cyclone head you will end up with a cranking compression of around 130-140. If you add a flat head with the aluminum pistons, you will jump up to around 200+. Not really the answer to your question but just something to chew on.



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clooney

03-24-2001 11:03:43




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 Re: JD G compression in reply to Rich , 03-23-2001 17:08:06  
Rich, the compression should be 60-65 psi. with the engine warm, choke & throttle wide open. If it takes more than a couple of puffs on each cylinder to reach max psi you probably have some ring wear or a plugged air cleaner. If electric start try to check each cylinder with the same number of puffs.



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John T

03-24-2001 10:32:53




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 Re: JD G compression in reply to Rich , 03-23-2001 17:08:06  
Rich, I dont have the factory specs here and, of course, its obviuosly higher wet with the rings oiled up than a dry test. BUT, not knowing the age or condition of the cylinders, rings or valves etc etc, if it reads around 90 psi wet, its in pretty decent shape. They were designed to run on low octane fuels and didnt have real high compression brand new you know, so they wouldnt pre ignite. Its kind of important they also be about equal on both cylinders. If its way down to like 40/50 psi, it may?? go ahead and run, but use oil and be hard to start and low on power. Ive seen em around 60 that ran, but numbers nearer 80, 90 or over a hundred even or better are what youre kind of looking for as a general answer to your question. Ol John T

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G Taylor

03-23-2001 19:16:06




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 Re: JD G compression in reply to Rich , 03-23-2001 17:08:06  
Depends on the cam, intake manifold, aircleaner & exhaust system. These affect the volumetric efficiency of the engine. The cranking RPM affects the gauge pressure due to time for leakdown & the Universal Gas Law's effect on pressure in a closed vessel. A 10 to one compression ratio won't work out as "10 x 14.7 = 147.0 cranking psi"



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