Hello,
I recently did a compression test on a newly purchased 1972 corvette SBC, CKX 200hp 8.5:1 engine. The car runs, but doesn't have very much power, which is why I'm trying to diagnose.
From what I'm reading, the reading should be above 150 psi; is that correct for this engine?
Please forgive me, because I haven't done a compression test in a long while.
I'm pretty sure the compression tester tool was screwed all the way into the spark plug hole, but the readings came back very consistent, more consistent than I expected. The tester tool could only really be hand tightened in as well, because of clearance issues, but it also didnt look like the tester tool was intended to have a wrench or pliers to tighten it in.
I also didn't have WOT during the test. How much of a difference would that make?

cylinder 1: 110 psi dry, 135 psi with oil
cylinder 2: 115 psi dry, 140 psi with oil
cylinder 3: 110 psi dry, 135 psi with oil
cylinder 4: 110 psi dry, 140 psi with oil
cylinder 5: 115 psi dry, 135 psi with oil
cylinder 6: 115 psi dry, 140 psi with oil
cylinder 7: 115 psi dry, 140 psi with oil
cylinder 8: 110 psi dry, 135 psi with oil

I would think if there was a ring issue it would have at least one cylinder significantly lower than the rest.
Is the difference in dry and wet measurements concerning?
Does this indicate the engine needs new piston rings?

Has anyone who had piston ring issues that resulted in all cylinders having low readings?

 

it should be at WOT to get a true high reading but the readings you have are even and not far off. I would look elsewhere for the problem plus it is a 72 not sure what your power expectations are? Did you do a basic tuneup first, all filters, cap and rotor, plug wires if high resistance, points and condensor, carburetor adjustment.
Also make sure that when the pedal is held fully to the floor that the carburetor linkage is fully opening the throttles, the cable can be adjusted.

 

Not only the total compression, but how it's gets there. If the first stroke was xx, then does it take 5 of 6 pumps to get to the total? Any 2 adjacent cylinders read EXACTLY the same? Each stroke? Same max total?
Normal rule of thumb is a 10-15% difference max.
A compression test tells a lot, but for accurate diagnosis it needs to be done with a leak down test. How much pressure will that cylinder hold? Is air going by the rings, head gasket, or valves?

 

Hi Mel,
Thanks for your quick reply.
I have been doing all the regular tune up items, cap & rotor was replaced, rebuilt the original rodchester carb (although still working on getting it 100% right), all new fluids.
I just dont want to put in a bunch of work on the engine if it will need to be disassembled and new piston rings.

 

'power' is relative....

Keep in mind that a new Toyota Camry or Honda Accord will out accelerate most older cars including the Corvette.

 

As long as the o-ring on the thread-in compression gauge fitting is in place and in good shape , hand tight is fine. Just make sure you grabbed the hose firmly near the end and twist tight. The numbers are low, just to be sure I would do it again making sure the throttle is wide open.

 

The numbers you have look good for an 8.5:1 engine. Do a tune up and if you still need more HP, you may have to either live with it as is or start with some mods. The 8.5:1 CR is the primary culprit of low power...

 

lots of things can effect your pressure reading. Having a low battery is one. Gauge accuracy is another, engine hot or cold etc.
Looking at your numbers I would say that they are low without the oil. Since the oil brought them up quite a bit the rings may be tired but if it's not burning oil or having running issues I'd leave it, unless you want to do a rebuild.
Rings wearing evenly, such as yours, is a good sign really. It means there is no major problem creating a loss of compression on one two cylinders.

Were these taken with the engine warm or cold?

Cold will typically give lower psi readings than warm.

Hot day, or relatively cool. High altitude where you are at?
All of the plugs removed during the test as well?

I get a 10 psi difference just on a high pressure day vs a low pressure day. For my engine that's about 5%.

 

#s dont look out of line, like said above these are low compression motors that didnt really make any power to begin with.
driving a stock C3 for the first timer is usually disappointing.

Just drive it and slowly put together another with the good stuff.

 

You have 200 HP. What do you want.

 

The same thing we all want.....

 

Yep, 1000 horsepower for $150.00

 

Ok so it looks like most here think 110psi to 115 psi is normal for that engine. For reference here are the numbers from my 1977 L-48. 180 HP stock motor 8.5:1 CR claimed by GM. It was taken at 3500 feet on an 82* day. Cold engine, no oil in the cylinders and 67000 miles on the engine.

1.)158 psi 2.)152 psi

3.)152psi 4.)147 psi

5.)150 psi 6.)145 psi

7.)147 psi 8.)142 psi


These represent a normal PSI reading from an engine such as yours. As I said I believe either the testing was at fault or the rings are worn. The pressure coming up from the oil would suggest the latter.

Still not a big issue if it's running right. Just thought you might want to see some real life numbers from an engine just like yours.

 

The readings were taken after the car ran for about 30 minutes. When I was starting the compression test, I used an IR temperature gun to check the temp where the radiator hose goes to the block. It started at 190 degrees, but by the time i was done it measured 120 degrees.

Not much difference in values though.

 

Doing the test on a warm engine is good. All of your cylinders were pretty close together, you didn't have one that was down below 100, like at 80. If you did and the oil raised the compression back up to like the 135, I would say a ring problem is strongly suspected. Putting an ounce of oil in the cylinder will always increase the pressure, but I am thinking severely worn rings would have shown more pressure than what you recorded. Also, when you did the test it's important to watch that first pump because it's an indicator of the condition of the rings. Yes, the engine is somewhat tired, but I would keep driving it and start formulating a plan. When I first got my C3 (L48), it was not very impressive at all. With it's current 383, it moves rather well.

 

Check your timing if it's too retarded it will run like a dog

 

I put a 502 in my car. That problem was solved.

 

The most common cause of a C3 having inadequate power is that the carb is not opening fully at WOT. Folks think that exercising the linkage by hand [under the hood] is a good way to check that. NOT SO! Someone needs to depress the accelerator pedal to the floor while you are watching the linkage to see if the carb secondaries open fully.

This is a common problem with the Q-Jet carb. The secodaries don't open at all until the last 1/2" of throttle cable travel. There is also a secondary lockout lever on the right side of the carb that can PREVENT them from opening if the choke is not functioning properly.

Do a thorough assessment of all the things that could keep the carb from getting to WOT (via your foot!) before you start throwing money at "go faster" parts.

 

Thanks 7T1,
I am still working on the carb tuning, and will definitely look at the throttle cable linkage.

Keeping with the original topic, though, no matter how tuned I can get the carb, I will want to add more power to the engine. With it being the matching numbers engine, I'd like to figure out if I should learn to rebuild the engine with new piston rings before i put some other improvements on. One of the most popular things I've seen people do is put aluminum vortec heads on with smaller 64cc combustion chambers. This would increase compression ratio, but with my reduced cylinder sealing capacity, would this reduce the benefit of this improvement, or worse, stress the engine outside of its original design parameters and cause the rings to completely fail?

On the topic of carbs:
I bought a premium rebuild kit from Cliff Ruggles, and have been trying the different primary springs. I don't ever really floor it. I just want it to be able to go over reasonable hills and not slow the speed of traffic, which the vette is not really capable of doing unless really mashing the throttle.
I have pretty much a bone stock engine, and tried the orange and light blue springs (the leanest from what i understand). I started with the orange spring (smallest, leanest spring) but I thought I was running a bit lean, with the pop happening, especially with a partially warm engine. So, I switched to the light blue spring, but it seems to run slightly worse. The rods are 43B primary, 74 primary jet, DA secondary rods, and they are kind of weathered, and could be original as the carb. I may try to get one size leaner primary rod.
Does this sound like a good idea?
Would a tired engine suck in a little less air, thus needing slightly leaner rods?

I think you may be right; I think the throttle cable has stretched over the years and doesn't open the carb all the way. There is definitely some cable slack by the pedal, and it appears to be the original cable. From what i can tell, there is no cable adjustment on the stock throttle cable, but I have read some articles saying they put some zip ties or some kind of spacer by the pedal to resolve the issue.
Please correct me if I am wrong on this!

 

mongoose87 -

Just out of curiosity, what is the engine manifold vacuum at?