Compression

[mountain33]

So I'm curious about compression. My 71 383 is all original. Has 104k miles on it. When I bought car the motor was seized. I put ever lube know to man into the cylinders. Working the crank for months till it broke free. I had the heads redone. New lifters. Car runs great. My compression is only 110 on all cylinders. The question I have is. Shouldn't the car smoke? Is it possible to have low compression and have no smoke coming from exhaust. I would assume that the rings that are allowing compression to pass would allow oil. Just curious. Probably will rebuild motor soon anyway

 

[CarolinaCharger]

Oil is much more dense than air, so it's very likely that your rings are allowing compressed air to pass by, but not allowing any oil to get by.

 

[Cranky]

Warm up the engine then pull the oil cap off the valve cover and see how much blow by is coming out of it. Might even plug the PCV vac port to get a better read. All engines will have some but an engine that's worn will have more.

 

[mountain33]

Ok will check it out. I had always thought that if rings were shot and you had low compression. The engine would smoke. I was just trying to clear my mind about why no smoke. Thanks for responding

 

[67 GTX]

Motor could also be in good shape. Was it seized due to rust in the cylinders?

 

[mountain33]

Motor could also be in good shape. Was it seized due to rust in the cylinders?

I assumed that was the problem. i really didnt know. I basically googled and checked on this site what to do for seized engine. followed recommendations did the heads rebuilt carb and started it up. Been running with the 110 compression for 3yrs now. I never thought it would last this long before it would need rebuilding. guess you can tell newbie at this. Motorcycle NP car motor never touched one until this car.

 

[1967coronet]

Good battery with a charger hooked up.
Engine warmed up, all plugs pulled.
Fuel line unhooked and plugged. Coil wire pulled.
Carb wired wide open
engine spun over several rotations while checking each cyl.
If it were mine I would repeat the test like above with a different gauge.

 

[khryslerkid]

I assumed that was the problem. i really didnt know. I basically googled and checked on this site what to do for seized engine. followed recommendations did the heads rebuilt carb and started it up. Been running with the 110 compression for 3yrs now. I never thought it would last this long before it would need rebuilding. guess you can tell newbie at this. Motorcycle NP car motor never touched one until this car.

If you didn't hold/wire the throttle open during the compression test, your reading will be low. The cylinder needs air to compress and with the carb closed its just a vacuum.

 

[Kern Dog]

The goal is even numbers across all cylinders. The numbers you listed do seem low but as stated, I suspect that it is due to the procedure and the gauge.
I spin the engine over several times for each cylinder too. You’ll see the gauge needle bounce on each compression pulse. It gets to a point where the number peaks.

 

[BB BELLA]

doing wet and dry compression tests will tell you if the rings are the issue! could be a number of things causing low compression, burnt/carboned valves/worn cam/stretched timing chain will give low c. numbers with no smoke! if your numbers are the same across all eight,i'd definately try another gauge!
wet/dry test and then leakdown might point to the area where the problem is but if it's running okay you can drive it until you decide to put more life into it,but driving a motor that was previously seized might cause more expense to fix if you wait!

 

[Curiousyellow71]

A 71 383 should only be a 8.5:1 motor. 110 is a little low, but freshened up its likely only around 125 give or take.
Most common mistakes doing compression tests are not cranking the engine enough and not holding the butterflies open.

 

[451Mopar]

With many of the old Mopars I have had, smoke from burning oil is usually the valve guides and seals. Many times the seals dry up, and crack off.
Cylinder wear usually not as big a problem if the engine was maintained and does not have a ton of miles on it.
110 dosen't sound that bad to me, but at this altitude all engines have less cranking pressure.

 

[AR67GTX]

I have experienced a big difference in results using two different gage sets about a month apart on the same engine. Not sure why but trying a different gage set as suggested by Kern Dog might be worth a try. But as noted it’s the consistency across cylinders that’s more important than the numbers.

 

[Hey-O]

To me, different readings in each cylinder is a bigger fear than all reading the same. Time for a try with a different gauge.

 

[BB BELLA]

With many of the old Mopars I have had, smoke from burning oil is usually the valve guides and seals. Many times the seals dry up, and crack off.
Cylinder wear usually not as big a problem if the engine was maintained and does not have a ton of miles on it.
110 dosen't sound that bad to me, but at this altitude all engines have less cranking pressure.

seals/guides is usually pretty easy to diagnose when the smoke get heavy as soon as you back out of the throttle!

 

[mountain33]

Thanks to all for responding. Didn't know about holding open butterfly. Makes sense. I definitely should replace timing chain. Little nervous about screwing it up. Gotta learn sometime. Thanks again