Compression Test - Need Help
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Compression Test - Need Help
So I just did a compression test on my stock '81 just to see what health my engine is in, and I need help interpreting the results. Here are the numbers:
Cyl 1: 70
Cyl 2: 90
Cyl 3: 90
Cyl 4: 90
Cyl 5: 90
Cyl 6: 90
Cyl 7: 90
Cyl 8: 110
Test was done with the engine hot (mid way I actually stopped, ran the engine some more, and continued because it took me so long). I've never done this before, and being the good millennial I am, I read everything I could on it an hour before hand and went at it. Problem is contradicting info. Some say do it cold, only thing that matters is how different the readings are, some say do it hot, some say do it dry, do it wet (didn't bother with the wet test, because I don't care if it's the valves or piston rings, either job is more than I can afford to take care of right now)... so you see where I'm at.
Can someone interpret these results for me?
Cyl 1: 70
Cyl 2: 90
Cyl 3: 90
Cyl 4: 90
Cyl 5: 90
Cyl 6: 90
Cyl 7: 90
Cyl 8: 110
Test was done with the engine hot (mid way I actually stopped, ran the engine some more, and continued because it took me so long). I've never done this before, and being the good millennial I am, I read everything I could on it an hour before hand and went at it. Problem is contradicting info. Some say do it cold, only thing that matters is how different the readings are, some say do it hot, some say do it dry, do it wet (didn't bother with the wet test, because I don't care if it's the valves or piston rings, either job is more than I can afford to take care of right now)... so you see where I'm at.
Can someone interpret these results for me?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
So I just did a compression test on my stock '81 just to see what health my engine is in, and I need help interpreting the results. Here are the numbers:
Cyl 1: 70
Cyl 2: 90
Cyl 3: 90
Cyl 4: 90
Cyl 5: 90
Cyl 6: 90
Cyl 7: 90
Cyl 8: 110
Test was done with the engine hot (mid way I actually stopped, ran the engine some more, and continued because it took me so long). I've never done this before, and being the good millennial I am, I read everything I could on it an hour before hand and went at it. Problem is contradicting info. Some say do it cold, only thing that matters is how different the readings are, some say do it hot, some say do it dry, do it wet (didn't bother with the wet test, because I don't care if it's the valves or piston rings, either job is more than I can afford to take care of right now)... so you see where I'm at.
Can someone interpret these results for me?
Cyl 1: 70
Cyl 2: 90
Cyl 3: 90
Cyl 4: 90
Cyl 5: 90
Cyl 6: 90
Cyl 7: 90
Cyl 8: 110
Test was done with the engine hot (mid way I actually stopped, ran the engine some more, and continued because it took me so long). I've never done this before, and being the good millennial I am, I read everything I could on it an hour before hand and went at it. Problem is contradicting info. Some say do it cold, only thing that matters is how different the readings are, some say do it hot, some say do it dry, do it wet (didn't bother with the wet test, because I don't care if it's the valves or piston rings, either job is more than I can afford to take care of right now)... so you see where I'm at.
Can someone interpret these results for me?
All of the pressures are extremely low. Are you using a screw-in compression tester? For a stock L-48 the pressures should be around 140-150 psi.
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Yea, that's the conclusion I'm at, but wanted to hear what you guys think. And yes, standard, rented at o'reilly, compression tester. O-rings on it were fine and I hand tightened them (first one they gave me were iffy looking, especially when I did the first cylinder and it read 70 i figured the thing must be broken, but then I rented a secondone and same result.)
Burning Brakes
Joined: Aug 2011
Posts: 983
Likes: 35
From: Redding California
Yep... she's getting tired. But, as you mentioned, no funds to really do anything, so... the if it's not broke, don't fix it theory applies here. And if it's a runner, I'd save $$$ until you have enough to do the entire motor right.
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Would fitting new rings still be beneficial?
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
unless the bores are badly worn, it'll get you by. if there is a big ridge at the top of the cyls, then not so much. usually only gets that way if run for a long time with no thermostat. how many miles on your car? can you rent-borrow a bore scope to take a peek in thru plug holes? 100 bucks rings-rod bearings. 100 bucks gaskets...
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Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
all plugs out it cranks faster. throttle open so it fills cyls completely. did you reinstall plugs 1 at a time as you did test? or was 1 (lowest) the slowest cranking, and 8 (highest) the fastest cranking?
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Even if, does it make that much of a difference? I guess I could try doing just one now, since I know the difference between them to get a read for all of them.
Intermediate
Joined: Nov 2014
Posts: 32
Likes: 1
That car would be smoking really bad and have no power with those readings.
I think you did the test wrong.
You need the carb throttle open all the way with all plugs out.
Fresh battery, the motor needs to crank over at least 4 - 5 times when testing each cylinder.
I think you did the test wrong.
You need the carb throttle open all the way with all plugs out.
Fresh battery, the motor needs to crank over at least 4 - 5 times when testing each cylinder.
Instructor
Joined: May 2005
Posts: 200
Likes: 4
From: Richmond Virginia
Well this sounds familiar...
It might not be the biggest help, but go check out my thread from two years ago trying to do the same thing. In my case, it was the result of a weak cranking battery not giving me full compression. I went from 65psi to 165psi by changing out the battery for a new one. I was trying to do the compression test off a set of heavy duty jumper cables connected to my pickup
https://www.corvetteforum.com/forums...ting-a-c3.html
It might not be the biggest help, but go check out my thread from two years ago trying to do the same thing. In my case, it was the result of a weak cranking battery not giving me full compression. I went from 65psi to 165psi by changing out the battery for a new one. I was trying to do the compression test off a set of heavy duty jumper cables connected to my pickup
https://www.corvetteforum.com/forums...ting-a-c3.html
Thread Starter
Instructor
Joined: Jun 2011
Posts: 197
Likes: 4
Well this sounds familiar...
It might not be the biggest help, but go check out my thread from two years ago trying to do the same thing. In my case, it was the result of a weak cranking battery not giving me full compression. I went from 65psi to 165psi by changing out the battery for a new one. I was trying to do the compression test off a set of heavy duty jumper cables connected to my pickup
https://www.corvetteforum.com/forums...ting-a-c3.html
It might not be the biggest help, but go check out my thread from two years ago trying to do the same thing. In my case, it was the result of a weak cranking battery not giving me full compression. I went from 65psi to 165psi by changing out the battery for a new one. I was trying to do the compression test off a set of heavy duty jumper cables connected to my pickup
https://www.corvetteforum.com/forums...ting-a-c3.html
Instructor
Joined: May 2005
Posts: 200
Likes: 4
From: Richmond Virginia
Yeah, I was posting the link to my thread more so you could read through everyone's suggestions of everything ELSE it might be going wrong in case there were additional suggestions not mentioned in this thread.
Le Mans Master
Joined: May 2006
Posts: 5,814
Likes: 414
From: Pittsburgh suburbs Pa.
Don't count on most of the compression testers being accurate . We had a new one bought at Advance that was totally inaccurate , then rented one at Auto Zone that was totally useless . Then we borrowed an expensive one from a high end repair shop in our area and it was right on. Just saying ! This could be moving you in the wrong direction. As someone else said if those readings were accurate the engine would be throwing a lot of smoke or would barely run.
Last edited by bj1k; Sep 14, 2015 at 04:15 PM.
Le Mans Master
Joined: Jan 2006
Posts: 6,504
Likes: 139
From: Butler Pa
Would fitting new rings still be beneficial?