[crm318]

I will be doing a compression test soon and I was wondering what type of oil is used to test the rings. Seeing as the cylinders are at a 45 degree angle, how do you get the oil evenly distributed around the pistons diameter?

[68WarDog]

Bring engine to temp. Turn off engine. Open throttle wide. Disengage ignition coil. Do compression test without oil.

[Blued and Painted]

Oil is used when trying to diagnose a low cylinder.
Lets say one cylinder is at 50 psi. Add one or two spoon fulls of engine oil to cylinder then test again. If there is a substantial increase in compression, the piston rings would be suspect. If there is little or no increase, the problem would more likely be with a valve.
Then there are those pesky head gaskets. They can leak into the adjacent cylinder or into the cooling system.

[STEELCITYFIREBIRD]

Regular motor oil, anything you would normally run. An old style pump squirt-er can, aimed appropriately and then rotating the engine a few times to distribute works well.
Compressed air works well for diagnosing major problems. Leakdown tester for smaller leakage, more precise diagnosis.

If the engine is still running, rpm drop by killing one cylinder @ a time, a cylinder balance test as it is called, works well for finding a weak lung/s and will show ignition and air/fuel supply problems too. The compression test will show if it is a base engine problem or ignition and/or air/fuel related.

Where to start the diagnosis depends on the symptom/s.

HTH

[Schurkey]

Originally Posted by 68WarDog

Bring engine to temp. Turn off engine. Open throttle wide. Disengage ignition coil. Do compression test without oil.

Yup. No oil.




If you don't have low cylinder(s), you don't need to add oil to the cylinder.

If you do have low cylinder(s), perform a leakdown test INSTEAD of adding oil to the cylinder.

Adding oil to the cylinder for a compression test is friggin' pointless. Either you add too much oil, which then jacks-up your compression pressure by adding a bunch of CCs of incompressible volume to the cylinder, or you don't add enough, and the leaking rings still don't seal, falsely indicating valve problems.

The leakdown test bypasses all the problems of "wet" compression tests.

[crm318]

Thanks guys. Yes, I meant in the event that there is a low cylinder. The engine runs fine, I'm just doing a check up. I seem to remember that the valves are adjusted to 3/4 turn past 0 lash. In the event that I removed 1/4 turn from that, do I have to go cylinder by cylinder in the firing order or can I just remove 1/4 turn from all cylinders at once?

[STEELCITYFIREBIRD]

You can just remove it from all.

Myself, I would run each to see where I was and then put them where I want to be....That's me.

[crm318]

Well, I got 125 psi on the number one cylinder. I will just wait and do a leak down test. The short block has 2000 miles on it and consumes zero oil. The spark plugs are have a reddish hue, but no oil or carbon. I lapped the valves on these heads and i remember having some thin margins on some of the seats. The motor runs and sounds great. It will obliterate the tires and and get squirrely on the first to second gear shift, but it does seem to lack some top end (could just be me). Im contemplating removing the heads and having a proper valve job done.

[Schurkey]

Compression pressure is affected by so many variables that it's hard to list them all.

125 seems low--what is your altitude? Engine cranking nice 'n' fast? Was the cam degree'd?

[crm318]

Spun nicely. I did not degree the cam. I also backed the valves off 1/4 turn (they were at 3/4) and no variation. I am at 2800 ft.

[Schurkey]

Originally Posted by crm318

Spun nicely.

Good.

Originally Posted by crm318

I did not degree the cam.

Not the end of the world if the engine runs good otherwise. Since you're complaining of poor high-rpm power, it might have been nice. I'd also be looking at fuel pressure.

Originally Posted by crm318

I also backed the valves off 1/4 turn (they were at 3/4) and no variation.

Shouldn't make a bit of difference as long as there's SOME lifter preload. Once you have "some", having more or less changes nothing in the way the engine runs except time between service.

Originally Posted by crm318

I am at 2800 ft.

Yeah, 125 seems low. Gauge accurate? If it's the quick-disconnect style, they use Industrial Interchange (Milton "M") coupler plugs. If you build a test fixture using one or more of the "M" plugs, along with whatever plug fits "your" air system, you can use shop air and the gauge on the regulator to verify your compression tester.

[track73]

I put together a real neat .040 over Chevy 400 with Olds rods. I ran it on my test stand to verify the engine integrity and break in the cam and lifters, I did a compression test with my new Harbor Freight compression guage and got reading of under 100 on all cylinders. I did the oil thing with no gain and was besides myself wondering what I did wrong. Then I spotted my Sun Compression guage and gave it awhirl. Well that was the problem . The HF guage was junk.

[crm318]

I just figured it was lacking on the top end because I assume that's where the cylinder pressure really starts to build. The fuel pressure regulator reads 5.5 and the qjet is filled with Cliff appointed components. It doesn't buck, surge or make any suspicious sounds, it just doesn't pull very hard. I'm also not sure if the valves are one or two piece, so it's probably time to go with some Ferrea's. Like I said, i remember my seats being a bit thin, so maybe it's a "few birds with one stone" situation.

[Will]

You could be starving for fuel on the top end. Is your fuel pressure gauge where you can see it while driving? I had this problem at the track, the car would fall on its face near the top of 2nd gear. Turns out I was losing fuel pressure, which I was able to diagnose by mounting a fuel pressure gauge to the cowl so it stuck out from under the hood at the base of the windshield.

[crm318]

My gauge is under the hood, so I have no line of sight. My drop in power comes from when I shift into second gear (oddly enough, after I catch traction from shifting from first).

[Will]

Originally Posted by crm318

My gauge is under the hood, so I have no line of sight. My drop in power comes from when I shift into second gear (oddly enough, after I catch traction from shifting from first).

Try it on a full tank of gas and see what happens.

I found that my problems occurred with 1/2 tank or less. The fuel pickup was getting uncovered by the fuel sloshing to the back of the tank on hard acceleration.

[kyle_blake]

I always thought if the valve's weren't sealing 100%, than a compression check would show variances enough to warrant a leak down test.

I mean all 8 cylinders were lets say within 5 psi of each other than one would think it was awesome!

If one or more are I dunno 15-35 psi differences than one has to think that the valve sealing ability might be suspect. Of course rings could be suspect too.

So I dunno, I've never built engines before but if I did I'd totally expect each cylinder to be within 5 psi or dead on. I mean how many other builders here do a compression check right after building an engine, probably not many...

Keep me posted!!!

[STEELCITYFIREBIRD]

Originally Posted by Will

Try it on a full tank of gas and see what happens.

I found that my problems occurred with 1/2 tank or less. The fuel pickup was getting uncovered by the fuel sloshing to the back of the tank on hard acceleration.

+1.....and the extra rear weight should help traction on the 1-2 shift!

[Schurkey]

Originally Posted by crm318

My gauge is under the hood, so I have no line of sight.

Now you have another project: Move gauge to where it can be seen from the driver's seat, so that it's actually useful. AFTER you have verified fuel pressure through all your gears, at WFO and max power, you can remove it or put it back under the hood.