Stock 302 compression pressures
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Senior User
Joined: Jul 2003
Posts: 135
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From: Bandera, Texas
Stock 302 compression pressures
Help please. I have a 78 F-150 Custom w/ a 302. Had the heads reworked about 6 months ago and truck runs great. In the process of selling it and the guy did a compression check. All the cylinders came in at 73-78 psi. He initially had a problem with his tester so I don't know if those readings are right. The engine has about 55000 on it. What should the pressure range be. The book says it is a 8.2:1 ratio. Is there an engine problem or more likely a gauge problem. I haven't added any oil to the cylinders yet because they are all within 5 psi of each other. Any help would be great. Thanks ahead of time!
Elder User
Joined: Jan 2004
Posts: 561
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From: houston
I agree with MUSTANG that the PSI sounds low. However many people lose sight of what a compression test is telling you. The fact that the readings were consistant say more about the engine than the psi reading. Sometimes, if the reading was taken with the throttle door closed, they will be lower because the engine couldn't draw in enough air. And I also suggest that the engine be turned over the same number of times for each cylinder instead of cranking until you get the highest reading. The idea being to keep the test the same for all cylinders. As an FYI, with an otherwise sound engine, the cam has a great influence on the PSI reading. You can have a very high C/R(like 13:1) and very low PSI(like 75-80) due to the valve over-lap created by the cam. True, this would be for high performance engines, but you would be surprised what people will put into an engine. Even a daily driver. Good luck.
Post Fiend
Joined: Jun 2001
Posts: 5,228
Likes: 5
From: Running Springs CA
It cant be that low and run well. The gauge has to be inaccurate. I compression test a warm engine with the trottle open all the way. Ford just specifies that they all should be within 25 percent of each other, not an actual number.
Jim
Jim
Post Fiend
Joined: Nov 2001
Posts: 14,947
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From: Topsfield, MA
Originally Posted by GoldenDually
Thanks for all ya'lls help. It was a bad gauge. It is pumping out 158 psi per cylinder. We scraped the old gauge. Thanks again.
