68 340 compression problem

[coupe32]

I think I might have a problem. My 340 was completely gone through and rebuilt by a reputable machine shop 10 years ago. It was bored .040 The engine is in a street rod and only has 9000 miles on it. I had the car painted last summer and had the engine out so I had the heads freshened up with new valve guides and the 3 angle valve job. I just checked the compression and it is 145 lb in 7 cylinders and 140 in one with the engine warm. Before the valve job it was about the same. I thought it was leaking valves but now I'm thinking rings. This engine should have more than 145 lbs of compression shouldn't it? I believe the engine specs say it should have 180lb plus or minus 20lb. The car does not smoke. Do you think Ive got some weak sealing rings going on here or what? Any suggestions? Thanks

[70AARCuda]

What are the specs on the camshaft as it has an effect on cranking compression, also was the carb held at WOT during the cranking.

I personally dont see anything wrong with it. All the cylinders are close to each other.

[racintracy]

Originally Posted by 70AARCuda

What are the specs on the camshaft as it has an effect on cranking compression, also was the carb held at WOT during the cranking.

I personally dont see anything wrong with it. All the cylinders are close to each other.

My thoughts exactly. 5 lbs. difference is no problem. And a hotter cam than stock can have lower cranking compression. Also if the cam isn't degreed in the cranking compression could show lower than expected.

[dave571]

That variation is well within any real specifications.

Usually the worst only needs to be within 80 % of the best for most manufacturers. In your case, that would leave the lowest at only 116, with the highest at 145. The car would still be considered tunable with that dreadfull of a number, so what you have is fabulous.

Lots of variables as to the cranking pressure.
-altitude
-cam specs
-wide open throttle
-good battery for cranking
-good starter
-all the plugs out while cranking
-engine up to operating temp

yadda yadda yadda

[dwc43]

I agree with these guys too. It's good to go. BEt you put a big cam in it didn't ya ??

[coupe32]

The cam is a hughes 1423 flat tappet cam. I believe it has 213 223 duration and a 480 504 lift. I believe the centerline is 108. The cam was degreed in. Yes I had had the throttle wide open when I was cranking the engine over. I read the stock specs and when I saw 180 I thought something must be wrong..

[23T]

Did you put in oil in the clys before the check?

[coupe32]

No I did not put oil in the cylinders. I just warmed the engine up to operating temperature and then removed the spark plugs and checked the compression. I had the wife in the car to do the cranking and holding the throttle open while I checked each cylinder. All cylinders were the same 145 lb except one that was 140.