the compression ratio is not a ratio of pressure. it is a volume ratio;specifically its the cylinder+combustion chamber volume at BDC/combustion chanber+cylinder volume at TDC. The compression gauge pressure for typical gas engines on tractors is ~ C x pambient x[(compression ratio)^1.4-1] wwhere C is between 0.65 and 0.75. ).67 seems to work well on the Ferguson Continentals and the N seried Fords The compression gauge pressure should always be measured with the throttle plate fully open.
So for your 7:1 compression ratio, if the ambient pressure is 14.7 psia , your compression pressure should be ~ 140 psi.
Having said that if the manifold pressure is 20 inches Hg vacuum, that"s about 4.9 psia so the compression pressure at idle would be ~ 48 psi.
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Today's Featured Article - Talk of the Town: The Saga of Grandpa's Tractor - by The following saga is from the Tractor Talk Discussion Forum. Someone. The saga starts with the following message: Hey guys I have a decision to make. I know what you all will probably suggest and it will probably agree with me way down inside, but here it is. I have a picture blown up and framed in my "tractor room" of a Farmall M. It was my Grandpa's tractor, of which whom I never got to meet. He froze to death getting this tractor out of the barn to pull a truck out of the ditch before I was born. Anyway my dad and aunt had to sell it at the auction,
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