Compression test ?'s
Compression test ?'s
Postmaster
Joined: Mar 2001
Posts: 4,968
Likes: 3
From: So Cal
I have not heard of a conversion that is accurate to figure out your CR from pressure, but I have noticed a few Hot Rod articles where they talked about CR and anything over about 12:1 would give pressures around 170psi. From my experience most stock engines with about 8 or 9:1 usually had around 150 psi, but like I said I don't think there is an exact relationship between psi and CR(would be if things were perfect). I could be wrong, since I never worried that much about it. I only worry about psi or CR when the psi drops a lot or is uneven, or when the engine knocks because the CR is too high or something else is causing it.
A wild guess estimate would be the CR times sea level air pressure should equal cylinder pressure. As an example, sea level is about 14.7psi. A 10:1 CR should give about 147psi. But like I said nothing is perfect and engines have leakage, carbon deposits etc and air pressure varies all day with temperature and altitude and weather, so I would expect things to be lower but not necessarily. My old 74 Datsun had something like 9:1 but read a nice steady 150psi for many miles. If I used my wild guess formula I should have only gotten 9x14.5= 130.5psi. So my formula doesn't take into account the many variables.
Probably with your engine if we knew what engine and what year, the guys could give a better answer. The rest of my answer is more conversational speculation.
Good Luck,
Jim Henderson
A wild guess estimate would be the CR times sea level air pressure should equal cylinder pressure. As an example, sea level is about 14.7psi. A 10:1 CR should give about 147psi. But like I said nothing is perfect and engines have leakage, carbon deposits etc and air pressure varies all day with temperature and altitude and weather, so I would expect things to be lower but not necessarily. My old 74 Datsun had something like 9:1 but read a nice steady 150psi for many miles. If I used my wild guess formula I should have only gotten 9x14.5= 130.5psi. So my formula doesn't take into account the many variables.
Probably with your engine if we knew what engine and what year, the guys could give a better answer. The rest of my answer is more conversational speculation.
Good Luck,
Jim Henderson
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
I can promise you that the compression psi numbers will not convert to cr, ever. Some of the newer marine efi motors with 8.5 or 9:1 cr's are throwing 165psi numbers and the manuals tell you they should. My old, 1969, 428SCJ at 10.5:1 tossed our 210psi when still stock, after cc'ing the heads to get 10.7:1 was up to 220psi. You can also change the psi numbers by spinning the starter faster or slow, seen it myself so know it's true.
