Hey all! lookin for some help! 72 f100 with the 360....what should be the compression be I'm getting between 90 and 100psi.
Thanks in advance!!!

 

i work in a wrecking yard an i do all of the run testing of the engines . anywhere from 125 to 150 compression is good . the last 390 i did had 140 to150 compression an 50 psi for oil . hope that helps.

 

Just remember that actual pressure readings that you get during a compression test will vary with a few factors, such as how fast you get the motor spinning during the test. If you spin the motor over with a fully charged battery and a good strong starter, you'll get higher readings. Also, did you run the test with all of the plugs out, except for the gauge in the one cylinder being tested? If you spun the engine with the other plugs in, that will slow it down and reduce the average readings. Also, the gauges on many of the compression testers aren't really that accurate!

My point is that, if the readings are fairly even across all of the cylinders, I wouldn't worry too much about whether you're reading 90 psi or 150 psi. Anywhere in that range tells you that the cylinders are basically sealing up. The purpose of the compression test is to quickly identify dead cylinders, ones with leaky burned valves. A dead cylinder will read below 50 psi. Don't try to read too much precision into compression tests from one engine to another, unless you're really equalizing all of the conditions.

If you want a more precise test of how well the rings and valves are sealing up, then you do what's called a cylinder leakdown test. That's a more involved test, but it will tell you within a few percentage points how tightly each cylinder is sealing up.

 

well i use a fully charge battery and i run the engine to bring it up to operating temp then pull all the plugs and preform the compression test . if the reading are say 140 to 150 thats good if i get one cylinder thats 110 then its considered a running core at the yard where i work . something that i run tested say has 90to 100 across the board would also be a running core .that the way my boss wants thing done .

 

Yeah, that's what I meant by equalizing the test conditions. You've got a standard procedure set up for how you run the test in your yard. So, comparing the pressure readings from one engine to another does have some value, as long as you're doing the test on both engines using the same procedure.

I'm just saying that you've got to be careful comparing the numbers you're getting with the numbers someone else is getting, because of all the variables in the test.

For example, I remember running compression tests using the car's on-board battery, and seeing the pressure readings gradually go down by 20 psi between the 1st and the 8th cylinders. It was caused by the lowering voltage of the battery and the heat buildup in the starter, as I went through the test on each cylinder. And I remember re-doing the test on the same cylinder using two different gauges and seeing 40 psi difference between them!

 

i do agree plus on the fuel injected engine disablement of the injectors is a must or you will wash the cylinders an get ultra low readings

 

Awsome info! I did have all of the plugs out and it was a "cold" test. I had one cyl at 30psi and one at 15, the 30psi was a stuck ring and the 15 had damage to the piston.....