Compression question
 

Ok, Just did a compression test on my 1968 289.
this is what I got from facing the engine from the front:

Rear
136- -135
133- -131
160- -135
160- -125

Front

All seemed pretty fair up untill I was threw off by the front two on the left side...can anyone explain or give a reason why those two would give a psi of about 30 over what the others gave??

I did the compression check because I am about to do a top end rebuild and what too make sure it was work it and the internals on the engine were still good. Please give me some advice or answers on this so I can make my next move. Thanks

Welcome.

The 160 cylinders are where you want to be, but it's just as important that they are all close to the same value. In other words if they all read 125 that would be low but acceptable.
You could squirt oil into the low cylinders and see if the readings come up which would indicate worn rings. If the readings stayed the same the leakage is in the valves or valvetrain.
There's other ways to determine where the loss of compression is but oil is the easiest way to at least get an idea.
How much of an upper end rebuild were you planning? If it's the heads and you're going to do the heads you'll be all set. How fat is your checkbook? What do you want the car to do? It'll run with uneven compression but it'll never be right.

Im about to do a check again on the two 160 cylinders to make sure we read it right, but Im pretty damn sure we did. If they do come back at 160 again and I put oil in the weaker cylinders how much should I put in there to perform the method you described?

Right now if I can I only want to get the heads redone and put all new gaskets and other things like water pump, new ignition system etc. Im not trying to get to far inside the motor unless absolutley necessary. Thanks

You don't need a lot of oil. I's like 1/2 tsp? Couple squirts from an oil can. Also, pay attention to what it blows on the first stroke and like the fourth. Not sure what the top end will get you cuz it sounds like you may have worn rings. Old motor?

yep, make sure to only measure with the throttle wide open.
re-do the test and for every cylinder try to remember at what stroke it starts climbing good and where it peaked. like

first cylinder
2nd:90
3rd:120
4th:120

second cylinder:
2nd:110
3rd:160
4th:160

like that
once you are done, redo them but squirt a bit of oil into the cylinder. as said, maybe a teaspoon. 2 squirts is fine. If that brings up compression by a lot I'd say the piston rings are done.

Kalli

You also need to check the compression with the engine at running temp or your results will be off. Compression tests are good, but a leak down test will tell you more about where your problem is and how bad it is.

+1 on that. forgot to mention

So far he has only done a cranking compression test, maybe with a cold engine, maybe with the throttle plates closed so you can't make that assessment. Warm engine, throttle open, cranking compression test, then if cylinders are low, you can do the oil in the cylinders to SEE if it is possibly rings. However, a leakdown test will tell for sure what the problem is. Low compression could also be from valves not sealing and a top end would correct that. Also, the high cylinders could have excessive carbon buildup giving the high reading.

As for low compression, it is an indicator, but let me give an example from aviation. An aviation engine has leakdown coompression checks with inlet pressure at 80 psi. A drop of about 10 to 20 psi is considered "normal, but I have been present for engine test runs where the leakdown test showed leak rates of about 50 psi and you know what. When that engine was run in a test cell it produced the engines full rated horsepower. Static leakdown tests or low speed cranking compression is drastically different that the dynamic compression an engine produces when running.

If it is a stock engine it is probably around 8:1 or 9:1 compression and 125 psi wouldn't concern me all that much. Yes it is a little low, but as I said we have a compression check right now that was maybe done cold, and maybe with the throttle plates closed. Even so, personally, if it were my engine, if it isn't using excessive amounts of oil, and I had even readings (done correctly) around 125 I'd leave it alone.

BTW, an old trick for glazed cylinders is to sprinkle Bon-Ami into the carburetor throats with the engine running. The bon-ami will break the glaze and sometimes allow the rings to re-seat. However, this is a last ditch effort on an engine with even lower readings than what we have here. I did this exact trick years ago on a Chevy 307 in a Chevelle I had and it worked. After the treatment and running for a week or so, a new check showed higher readings good enough to save having to re-build the engine on what was then just an old car and not a classic yet.

I am going off the information given. It doesn't much matter if the throttle plates were open or not. If all cylinders were tested under similar circumstances, the readings should be similar. I would expect to see something around 160 with a new or fairly new motor. He has that in 2 cyl's. (so, I wouldn't expect carbon build up) Also, I wouldn't really worry about readings around 125-130....that is to be expected (in an older motor). However, when you see the low side of 125 and the high at 160, clearly, there is some kind of issue. Since 6 out of 8 are on the low side, I would say it is either an old motor, someone drove it thru some excessive pinging and broke a few rings, or, you have 6 cylinders with worn valves....that's why I figure rings.

Granted, a leak down test is recommended as the best way to find out exactly what is going on but, not eveyone has that ability. The oil test is the next best way as it will not raise compression if it is a valve issue.

Ok, I warmed it up for a good 30-45 minutes in the driveway and tested it while keeping the throttle open this time. The 160 cylinders came out at around 152 this time and the others including the one that was at 125 came out at around 142-148. So I would think its not in that bad of shape now. Sorry I couldnt give more information on the motor, but the lack of information is one of the reasons I was doing the compression test to begin with. I bought the car off of a women that never done anything with it once she got it and she didnt get any information on what had been done to it by previous owners. I dont know if the interals are stock or if its been rebuilt or what, I can tell you it has a edelbrock intake and 4bbl carb, but everything else on the outside of the motor appears stock. Once I pull the heads in the next couple of days we can tell if its been bored over any.The other reason I wanted to know if it had good compression is because I am about to pull the heads and have them redone and replace other various parts like the intake and water pump, distributor and so on. So based off of the better results I got today I am going to go ahead and say the block is good and put some money into it unless someone gives me a good reason otherwise. Thanks for all your input guys any more would be just as usefull. I like to learn as much as I can from places like this were people have first hande knowledge to share.

Oh yeah, I also wanted some input for this as well. Ive been told I should go ahead and change out the cam since I am pulling everything around that area anyways. So how hard will it be to pull the cam out? I figuire I have to pull the radiaor atleast for clearance. Also, What Cam should I go with??? It has a edelbrock 4bbl car with a edelbrock performer intake, I am getting the heads that I assume are stock rebuilt. I am looking for something with good idle quility and idle to midrange gains. I wasnt sure on what cam would be ok to run with this setup. thanks again