340 compression variance

[frozenfish]

Long time since I've been on here. Spent the last 9 years building my house so no time or money to work on my car until now.
I plan on pulling the engine out and freshening it up. The car sat for 16 years before I bought it in 2005. The previous owner told me the engine was built by a local machine shop for Friday night drags. They are now out of business.
Anyways, before I yard it out, I did a dry compression check on all cylinders warm, and have variances. The lower ones I checked again with oil:
1. 180. 2. 170
3. 180 4. 170
5. 160. 6. 150
7. 170. 8. 170

#5 was 180 with oil, and #6 was 160 with oil.
So the question I ask is, what do these numbers tell you? High compression engine but #5&6 rings getting worn?
I never had blue smoke with this engine but would sometimes get oil out of dipstick tube at high RPM highway speeds. It has a PCV valve and breather on opposite valve covers. In the past I had a hard time dialing in the timing and carb, I believe due to a vacuum leak. Vacuum was erratic before and never higher than 14". Smokes gray a lot, runs like crap. I managed to limp it into the shop tonight for winter rebuild. The block is a 73. Sorry for long post but would appreciate some input. Thanks in advance.

 

[krazykuda]

Long time since I've been on here. Spent the last 9 years building my house so no time or money to work on my car until now.
I plan on pulling the engine out and freshening it up. The car sat for 16 years before I bought it in 2005. The previous owner told me the engine was built by a local machine shop for Friday night drags. They are now out of business.
Anyways, before I yard it out, I did a dry compression check on all cylinders warm, and have variances. The lower ones I checked again with oil:
1. 180. 2. 170
3. 180 4. 170
5. 160. 6. 150
7. 170. 8. 170

#5 was 180 with oil, and #6 was 160 with oil.
So the question I ask is, what do these numbers tell you? High compression engine but #5&6 rings getting worn?
I never had blue smoke with this engine but would sometimes get oil out of dipstick tube at high RPM highway speeds. It has a PCV valve and breather on opposite valve covers. In the past I had a hard time dialing in the timing and carb, I believe due to a vacuum leak. Vacuum was erratic before and never higher than 14". Smokes gray a lot, runs like crap. I managed to limp it into the shop tonight for winter rebuild. The block is a 73. Sorry for long post but would appreciate some input. Thanks in advance.

If you get improved compression numbers wet vs dry, that indicates some leaking through the rings...

If you have some low numbers and they don't improve with wet compression test, then look into the valves...

Have you tried doing a leak down on those cylinders??? That will also help determine if the rings are leaking....

 

[frozenfish]

If you get improved compression numbers wet vs dry, that indicates some leaking through the rings...

If you have some low numbers and they don't improve with wet compression test, then look into the valves...

Have you tried doing a leak down on those cylinders??? That will also help determine if the rings are leaking....

Thanks, I was thinking maybe#6 could be a valve issue. I haven't done a leak down test yet. Have to scrounge up the right adapters.

 

[RustyRatRod]

Sat for 16 years. Rust on the cylinders. Bust it off and run it. Compression tests are technically supposed to be done hot. I bet it gets better after it runs.

 

[krazykuda]

Sat for 16 years. Rust on the cylinders. Bust it off and run it. Compression tests are technically supposed to be done hot. I bet it gets better after it runs.

I didn't catch that it sat for 16 years...

Good point about rust on the cylinders...

 

[furrystump]

Sticky rings? Bores out of round?

 

[frozenfish]

Sat for 16 years. Rust on the cylinders. Bust it off and run it. Compression tests are technically supposed to be done hot. I bet it gets better after it runs.

I tested the engine warm and put about 3000 miles on the engine since I bought it so it is what it is. Are the variances considered bad? My initial thoughts are break it down, inspect it, have the heads inspected, then go from there.

 

[MOPAROFFICIAL]

Long time since I've been on here. Spent the last 9 years building my house so no time or money to work on my car until now.
I plan on pulling the engine out and freshening it up. The car sat for 16 years before I bought it in 2005. The previous owner told me the engine was built by a local machine shop for Friday night drags. They are now out of business.
Anyways, before I yard it out, I did a dry compression check on all cylinders warm, and have variances. The lower ones I checked again with oil:
1. 180. 2. 170
3. 180 4. 170
5. 160. 6. 150
7. 170. 8. 170

#5 was 180 with oil, and #6 was 160 with oil.
So the question I ask is, what do these numbers tell you? High compression engine but #5&6 rings getting worn?
I never had blue smoke with this engine but would sometimes get oil out of dipstick tube at high RPM highway speeds. It has a PCV valve and breather on opposite valve covers. In the past I had a hard time dialing in the timing and carb, I believe due to a vacuum leak. Vacuum was erratic before and never higher than 14". Smokes gray a lot, runs like crap. I managed to limp it into the shop tonight for winter rebuild. The block is a 73. Sorry for long post but would appreciate some input. Thanks in advance.

Okay.

You either have a couple sunk exhaust valves.. or there was some rust on the cylinder walls and those aren't sealing great anymore.
I think the no.5 and 6 see leaner mixtures than the rest and I believe it is a symptom of a dual plane intake... but regardless they are hotter being in the middle of the rest...It's just what I've seen ime.

 

[ir3333]

many engines that have sat improve drastically with use.I've seen really bad "smokers" clear right up in time.

 

[MOPAROFFICIAL]

I tested the engine warm and put about 3000 miles on the engine since I bought it so it is what it is. Are the variances considered bad? My initial thoughts are break it down, inspect it, have the heads inspected, then go from there.

Run some stones through them and new rings.

 

[frozenfish]

Thanks for the input guys. I'll probably pull the engine in October once we got snow on the ground and all the outside stuff is done. I'm really curious as to what kind of internals it has.

 

[67Dart273]

If you get improved compression numbers wet vs dry, that indicates some leaking through the rings... ...

You have to be careful with this.......if you get a little too much oil in there, the oil alone will raise the compression ratio and screw your numbers

 

[RustyRatRod]

I tested the engine warm and put about 3000 miles on the engine since I bought it so it is what it is. Are the variances considered bad? My initial thoughts are break it down, inspect it, have the heads inspected, then go from there.

Meh, I think they would be normal considering that it had a long dormant period. That doesn't do valve springs any good, either. Some open. For 16 years? Even with hydraulic lifters that bleed down, some will still be open and that will weaken them. You might try dumping a can of Sea Foam in the crankcase and in the gas tank too. Run it for an entire oil change in the crankcase. Couldn't hurt.