Please School Me On Testing PSI Cylinder Compression

1971 Chevy 307. We Are Checking The Cylinder Compression by doing a Dry Test.We are just Cranking The Engine continuosly, not Fully Starting The Engine. The Fuel Line is Not Hooked up, So We Can't Fully Run the engine. We are getting a reading of 120 psi for cylinders 1-7 and 60 psi for # 8. I understand that since this motor was sitting for a year or so a valve might be hung up or maybe a ring, causing low #8 psi. We did a wet and #8 went up to 80 psi. What are We doing Right? What Are We doing wrong? And how do these Psi readings Fare? Thanks.

P.S. Motor was allegedly Rebuilt a couple years or so ago

 

120 is a low reading,60 is bad and so is 80.

when doing a comprerssion test you need to have the car at W.O.T and the battery needs to be fully charged as slow cranking will effect readings.


there should not be anymore then a 10% difference in readings between cylinders.


you should do a do a "dry test" then a "wet test" a wet test is done by squirting a bit of engine oil into the cylinders.

the readings you got for cylinder 8 at 60-then 80 indicate a possible ring issue.if the readings did not go up it would indicate a valve/head issue.

at this point you will most likey be pulling at least the heads to see whats going on but you could do a "leak down test"
basicly you are pressurizing each cylinder with regulated air with the valves colsed and listening to hear were the air is escaping.

is you hear hissing in the intake or exhaust you have a head/valve issue,if you hear it coming from the crank case you have a ring.bottom end issue.

hope this helps.

 

have you run this engine? if not i would get it running first. then do as vandenplas wrote.

 

Yes, definitely get the engine running and then test again. I would hate to tear down an engine due to a sticky valve or piece of carbon that a little running would clear. And depending on your compression ratio, RPM the engine is cranking, accuracy of your gauge, etc, etc, 120psi may not be low - the critical part is the 10% difference.

 

How long has the engine not run. If the engine sat along time with coolant or water in it the head gaskets may be rotted out. Try to get it running. If it runs to temp you can get a glass of cold water and while holding the throttle at 1500 slowly pour the water into the carb. This will turn to steam as it is run through and will clean the valve stems, heads, plugs and combustion chamber. Then do a compression test and see what you have. Good luck.

 

ive never heard of pouring water into motors before, wouldnt sucking throttle cleaner through do same and be healthier for engine?

 

is this the same motor i read about that also had a knock? or am i thinking of another post?

if it is, i wonder if maybe the piston skirt broke and possibly took out a ring or parts of them.

 

Use a leak down tester. Check percent of leak down. If low, listen at carb, if you hear leakage it is an intake valve. Listen to the exhaust, if you hear leakage, it is an exhaust valve. If leak percent is below 90 percent, and you did not hear anything at the carb or exhaust, it is a problem with a piston or ring. If you do noy have a leak down tester, you really need on . Put the compression gauge away,

 

Cool.. Ill Try The Lube Penetrate on the Valves First. Hopefully that get its moving. Could The "knock" The Engine is Making be Water In The Cylinder?

 

Its junk, rebuild it. Lippy

 

I guess you really did not want to be schooled on anything!!

 

Thanks for The knowledge Fellas.. Ive learned alot Here, and by research and reading. I appreciate the help. Thanks

 

Pouring water down a carb on an engine thats all warmed up,at a high idle,does work for knocking carbon loose from combuston chambers,tops of pistons etc.
Use a plastic water bottle & control the amount of water going in with your thumb.
Put at much in as you can at once without killing the engine.You'll be surprised how much water you can pour in at once without it affecting the way the thing is running.

 

120 is not really low. Pretty average for a stock engine with some miles on it.

 

I can tell you what recently happened to me. I could not get a brand new engine to start. Finally I had a friend come over to help. He did a compression test and 3 of the cylinders were under 90 psi. I thought the engine was junk. It was supposed to be fresh and never started. So just for shits and giggles we pumped some oil in the cylinders..... bigger than shit, it started and we broke in the cam. There was nothing wrong with my new engine.

I'd squirt some oil in the cylinders and test it again. I doubt that you are getting an accurate reading testing it dry.

 

X 2 especially if its been dormant for a while.
A good 307 chevy I checked before any work had 140 across all 8.