Question - Compression Test Results

[73Runner]

I'm pretty much a novice when it comes to car engines, and I'm using my 1973 318cid to learn as I go. So I did my first compression test last night, and I'm wondering if someone more experienced than me could interpret the results...

Cylinder / Pressure - Initial Test / Pressure - 2nd Test with oil added
1 120 150-155
3 120 135
5 120 135-140
7 120 130

2 130 140
4 115 120
6 125 125
8 105 110-115

My Haynes manual says each cylinder should be above 100psi, with no two cylinders differing more than 40psi. So the initial results above would suggest that the engine is perfectly fine. BUT the 8 cylinder kind of worried me because most all of the cylinders jumped up to around 90psi on the first compression cycle except for #8, which took about two cycles to break 90psi. The manual says a cylinder that slowly builds compression may be indicative of worn piston rings, and that one way to check this is to add some oil to each cylinder and retest (thus the second set of results) - if the pressure in the cylinder is "significantly" higher after adding the oil, the rings are most likely worn (I apologize if I'm telling everyone what they already know... just trying to be thorough).

So, most of the cylinders jumped 0-20psi after adding the oil except for #1, which jumped ~30psi. As a result, this puts 1 and 8 right on the borderline of being too far apart (~40psi).

There's no guidance in the manual as to how much is too much. Does a jump of 20+psi in the second test mean that my rings are severely worn or just normal for a 30+ year-old engine? Is a 40psi difference between cylinders bordering on serious problems or nothing to be concerned about? Are the results from the first test (which might suggest everything is fine) more accurate than the oil-added numbers?

Any help would be appreciated.

[maxwedge]

A couple of issues, the throttle must be held wide open during this test, second a lot of oil falsly raises the static compression, so only you know how much you put in the cylinder. Cranking speed is another issue a slow cranking engine will sometimes show low compression, # 8 is a little low, but not enough to pull the heads off. A better way to determine engine condition is a leak down test, more specialized and can pinpoint a bad valve, head gasket or ring blowby.

[MagicRat]

Generally, you do not have to rebuild an engine until it is showing obvious and serious signs of wear, such as excessive oil burning, smoke, lack of power, bad noises that the radio will not drown out, etc.

Unless its doing this, leave it alone. For a 34 year old non - high performance engine, the results you got are fine. It has lots of useful life left.

[73Runner]

Thanks for the replies.

All tests were done with the throttle all the way to the floor and the engine warm. I added three squirts of oil to each cylinder with an oil can prior to the second test. Everything was done per the Haynes manual.

MagicRat - that's pretty much what I figured. I've really had no problems with this engine thus far. I just thought I'd try to get a general idea of what condition it was in. I'd like to learn as much as I can about these old V8's using this 318, with the goal of replacing it with a 360 or 440 in a few years.