I did a compression test an unknown 76 block 455 with J heads. I dont have any info on the engine other than what the serial numbers showed and the fact that it had an Edelbrock performer intake an HEI distributor on it. I thought it would be a low compression engine. I ran a compression test on all the cylinders today with the spark plugs out, engine cold and the throttle closed and my results were:

Cylinder 1: 180psi
Cylinder 2: 160psi
Cylinder 3: 170psi
Cylinder 4: 160psi
Cylinder 5: 160psi
Cylinder 6: 155psi
Cylinder 7: 152psi
Cylinder 8: 160psi

They are all within 80% of the lowest reading. Does this indicate that work has been done to the engine? I was told a 76 block with J heads would be low compression 8.5:1 and would read around 130psi. My 69 Cutlass with a 455 with C heads got 150psi the last time I tested it so that is why I am a bit confused.

 

My '71 455's (8.1:1 or 8.5:1 depending on where you read) compression was around 160psi, cold engine, all plugs out, carb open. The PO put in a new timing chain & gear, but I'm suspicious that its retarded. So that could have a small effect on my readings.
So I would say your's is in the ballpark for a stock engine.

 

Camshaft specs(and timing chain slack) have a huge influence on compression. If anything on the engine has been changed, the factory spec of compression is useless. All that matters is the readings are fairly even and the highest and lowest are within 80%.

It’s best to remove the plugs, disable the ignition, and mechanically hold the throttle at WOT. That ensures the engine speed is highest, and eases the load on the starter. Hopefully the battery is good enough to keep cranking speed constant.

 

I would bet it has some better pistons in it, maybe 9.5:1 I would run 93 octane in it to be careful

 

I like to ensure the battery is fully charged before I start compression testing and conduct the test with the charger hooked up. This ensures the cranking speed remains constant from the first cylinder to the last cylinder.

 

I used a new battery and each cylinder took about the same time of cranking to max out. I can try again when I get a chance. Does leaving the throttle wide open make a big difference to the reading? I tested my 69 the same way mentioned above when I got the 150psi reading across.

 

Opening the throttle may allow more air to enter the combustion chamber. I guess the important thing is the throttle setting was the same through out the test. Your numbers look good, but higher than a usual engine with is about 125 PSI.

 

I’m thinking those compression test figures put that engine squarely in low 8’s:1 for CR.

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A leakdown test is much more informative, but more of a hassle to do. As I said earlier, if anything has been changed in the engine, the actual compression readings are useless. All that really matters (within reason) are how consistent the numbers are.

If your really worried, or just want to satisfy your curiosity, do a leakdown test. If the numbers are acceptable, then the next time you do a compression test if the numbers are different you know there is a problem.

 

Unfortunately a compression test will only tell you the basic condition of the motor when it is being cranked over. It's a diagnostic tool. It will not tell you what your CR is or cam specs. Your readings seem normal with nothing out of the ordinary. It's best to do with all plugs out and WOT. (Make sure you ignition is disabled).

 

Plenty of solid information already provided. I'll simply add I perform a compression test both dry & wet - which often times can present more informative information helping to determine if a leakdown test is a valid more informative direction to pursue; yet, this is really to assist in determining a possible/perceived "issue".

 

Just for clarification. Dry means just put the gauge in and crank. Wet means squirt some motor oil (maybe a teaspoon full) into the cylinder and then put the gauge in and crank. If there is significant difference in pressure between the two methods, then the rings are suspect.

 

I've fired up the engine and it didnt smoke. I'm just trying to figure out if it was modified or completely stock (pistons and heads) but I guess that is not achieved with a compression test. I initially thought a low compression 8.5:1 would read 120-130psi but based on some of the comments here thats not the case as this block has a higher reading that my 69 block with C heads.

 

It's the combination of the condition of the rings, sealing of the valves, compression ratio and valve overlap of the cam. So if you have a freshly worn in motor with 10.5:1 compression, good valves and rings and an RV cam, the cranking pressure will be fairly high. If you have an old worn out motor that burns oil, has leaky rings, 8:1 compression ratio further reduced by a fat Felpro head gasket, leaky valves, with a monster cam with tons of overlap then you'd expect the cranking pressure will be quite low.

Now of course if you don't know what pistons were used, or what cam was installed it's just too many variables, and too hard to tell.

 

Let’s put this compression discussion another way.

Let’s say you have a really well sealed up completely stock engine. You run a compression test, every cylinder is 150 psi. Now, put in a huge, bottom of the page camshaft that shakes the fenders off the car type of cam, the compression test might show 120. Nothing changed as far as engine condition, the much larger cam bleeds off cranking pressure. Now advance the cam timing, suddenly the compression readings are much higher than with the stock cam timing. Retard the cam timing, compression reading will be lower. Nothing changed with how well the rings or valves sealed, it’s all in the cam timing.

As someone mentioned, compression testing is a diagnostic tool. As long as the actual numbers are fairly even, and the the testing method is the same every time, don’t get too hung up on the actual number.

 

That makes sense. Thanks for writing that up and for everyone who replied. Continuing to learn as I go.