Good Leak Down but Bad Compression after rebuild
 

After having the top end of my 3.2L Carrera rebuilt with used 3.3L Mahle cylinders and pistons, the leak down numbers are coming back OK but the compression numbers are really low (average 80 PSI per cylinder). Cylinders were re-ringed. Heads received a valve job and were twin plugged. Moved to 964/993 twin plug dizzy and added an Andial splitter. What do you suspect the problem(s) could be?

The compression test was performed multiple times by a pro and yielded the same results. Thanks for your help!

Has the motor been run in yet?

Good question. I've got about 5K miles since the rebuild.

What was the advertised CR of the 3.3 P&C set?

8.0:1. Valve reliefs were already cut into the pistons when I received them.

Try a different compression gauge?

Whats your cam timing/overlap?

It was intended to be stock. Would the car run OK if the cam timing were off? The car seems to run smoothly. Could the wrong cam timing cause the compression to be around 80 PSI in each cylinder?

The more overlap the less static compression. It can make a huge difference but that's not how you should be setting it.

Are there any other symptoms of bad cam timing? I'd imagine it that fuel economy would go to the crapper?

Overlap is built into the grind of the cam; it's the separation between the intake and exhaust lobe of any given cylinder. The timing of the cam can't be out by much or you would get some piston to valve contact. The more overlap you have the shorter the time is that both intake and exhaust valves are closed on the compression stroke which is where you build cylinder pressure. With 8:1 pistons and relieved valve pockets you could easily be at 7.5:1 or less. What cams are you using?

How is the shop determining 80psi. Do they do 5 cranks or do they just turn it until it levels off?

What should it be with 8:1 and 3.2 cams? If the CR is really 8:1, then I'm calculating it should be right about 100 psi. (14.69 x 8) - 14.69 = 102.8 psig. This is purely a static calculation. If you have significant overlap, then that may explain why you're at 80. Also, how sure are you about stated CR, i.e. did you measure?

Stock 3.2 Carrera cams

I'm not sure. The test was performed by a very reputable mechanic with 30+ years experience. He said he ran the test multiple times because the results were so negative.


I thought compression ratio had no impact on the amount of pressure that a cylinder could hold?

No measurement was performed.

Engine needs to be warm to seal, and ring end gap will cause less pressure, as will cam timing. The real thing you look for in a compression test is that all cylinders have the same compression.

By hearing your responses, it sounds like bad cam timing can only explain *some* of the low compression. Is that right? Again, we're seeing an average of 80 PSI per cylinder, and the leak down numbers look OK.

In layman's terms, leak down tests determine how much air/fuel can escape from the combustion chamber. And compression tests determine how much pressure can be held in each chamber. Is that right?

So, it sounds like the results must be explained by some combination of these 3 points:

1. False negative on the Compression Test
2. False positive on the Leak Down Test
3. Bad Cam Timing

Am I missing anything else?

What is the difference in head/combustion chamber volume between the 930 and 3.2 and could this be part of the issue?

I don't think cam timing has much of anything to do with your low compression numbers. I think the piston / head combination you have is the culprit.

I hadnt thought about that. In fact, I thought it was commonly understood that 930 cylinders/pistons were slip fit for otherwise stock 3.2L Carrera engines. Perhaps other modifications are required to fit 930 p&c's?

Valve timing is not part of the static compression calculation.
Valve timing affects dynamic compression. The higher the overlap the lower the dynamic compression.

3.3 production 930 pistons have an advertized compression of 7:1.
If they have valve pockets that number goes down. By simply changing the deck height to .060" the compression ratio goes down. .020 deck change will reduce the compression by .24:1.

Henry, do you think valve timing alone could cause the compression to drop to 80 psi? Any known issue with installing 3.3 l pistons and cylinders on a stock 3.2?