I decided it was time to do a major tune up on the 340hp 62 and thought I would run a compression test on the engine while I had everything easily accessible. I was shocked when I found the cylinder pressure ranged from 235 psi to 250 psi. My guess is that whoever built the engine last used high compression domed pistons but used a standard cam. I can’t think of anything else that would give you this much compression. I have not pulled the heads but they appear correct and it does have the correct exhaust manifolds. Any thoughts?

 

decked block, milled heads, thin head gasket, carbon buildup; to name a few.....
Bill

 

And..............................Maybe a much shorter duration cam to boot.

Hot crank problems?

Did you check your guage against one of known accuracy?

 

With my engine rebuild many years ago, I used the higher compression domed pistons. The machine work had no decking of the block or shaving of the heads, and I think a standard head gaskets. I can't remember what the cam was but it was not stock by any means. My compression is at 180 pounds per cylinder. I'm thinking decking and shaving is a strong possibility with your engine.
Lowell

 

Something is a little strange here. The stock 340 motor has a compression of 11:1. If someone has changed the cam, GM used a lot of exhaust duration to bleed compression at low rpms, then that might explain the problem.

In order to increase compression by cutting the heads, you need to remove 10 CCs from the head to increase compression by 1 point. That amounts to a cut of .060.

I would need to puzzle out how much you would need to cut the deck, but by the time you did all this the manifold would start to fit baddly.

So, the cam and the pistons would be my things to look at.

If you pull a head, you can CC the combustion chamber to see if they are close.

Under 200 cranking compression would be my target.

 

That would be my guess.......... otherwise your up to about 14:1

 

I did immediately check my compression gage I connected it to my air compressor and verified that the compression gage read the same as the tank pressure. The two gages were within a couple of pounds from 10 – 120 psi

 

Also note that if the block was bored, and then pistons with the same dome volume were used, compression ratio will also rise.

But have to agree that a short duration cam, along with a high static compression ratio is probably the cause.

Any problems with knock?

Plasticman

 

NO problems with knock. When I first got the car 2 years ago the timing was advanced to about 17 degrees and it did knock. I set the timing back to 10 degrees and the knock went away. It does run a little hot (about 200) in the 95 degree Texas summer and sometimes it can have a bit of a hot start problem.

Mike

 

If the block was ever rebuilt in the past it's likely that the block was decked to raise the compression to the factory book specification. This was common for people looking to have the rebuilt engine match the book specification (match the advertised compression from GM, rather than the usual factory result). This would put you close to the 230psi numbers with a mild cam, or with a modern short duration high intensity cam with similar 0.050 duration numbers as the original 097' cam.

The loss of stamp pad numbers and the finish on the pad are tell-tale indicators of the block being decked. If the VIN numbers are gone and the pad reflects circular-arc machine marks, it's been decked.

If the engine did not ping with the 10-degrees initial and the installed distributor curve, it's not likely to ping after a good tune up.

A good tune-up test for performance however would be to read the total advance at 3500 rpm with the vacuum advance plugged. The factory centrifugal advance curve would limit the total advance to about 36-38 degrees in this configuration. There is a timing help paper authored by Lars that will guide you through getting the most performance out of the timing without detonation (and remember to reconnect and test the vacuum advance when nearly complete).