makes a difference
compression
Thread Starter
6th Gear
Joined: Mar 2011
Posts: 6
Likes: 0
From: Liverpool New York
compression
I just did a compression test on my engine because I'm getting a lot of pressure in the valve covers. 6 cylinders came in at right around 150 lbs with 2 cylinders coming in at 120 lbs. I'm assuming that a 30 lb difference is not acceptable and is an indicator that these cylinders are bad, anyone have any thoughts?
Thanks
Thanks
Drifting
Joined: Jul 2002
Posts: 1,944
Likes: 20
From: The only Corvettes in Highett Victoria
Fscott,
Remove the positive lead from the coil or HEI, remove the plugs and squirt some oil down the plug holes of the two doubtful cylinders. Then compression test them and if the numbers come up to around where the others are, then it's a pretty good sign that the rings in those two cylinders are on their way to the retirement home.
Hope this helps.
Regards from Down Under.
aussiejohn
Remove the positive lead from the coil or HEI, remove the plugs and squirt some oil down the plug holes of the two doubtful cylinders. Then compression test them and if the numbers come up to around where the others are, then it's a pretty good sign that the rings in those two cylinders are on their way to the retirement home.
Hope this helps.
Regards from Down Under.
aussiejohn
Le Mans Master
Joined: Oct 2010
Posts: 6,936
Likes: 848
From: melbourne florida
2023 C3 of the Year Finalist - Modified
it is possible some valves aren't sealing well maybe a bad seal on the head. before you get too carried away do the test over and verify. might be a valve adjustment. lots of possibilities
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Compression testing is a good thing. It will tell lots of stories about what condition your engine is in. But you also need to provide some more info- Were the 2 cylinders that were low adjacent to each other? If yes, possible head gasket. How many strokes did it take for each cylinder to reach that maximum number? Did it build up to that maximum number slowly or did it jump up there quickly and stablize?
A leak down test is also helpful- put 100 PSI air in the cylinder with both valves closed and see what it will hold. If you hear air escaping at the tailpipe, possible exhaust valve problem, at the carb, possible intake valve problem, at the crankcase breather, possbile rings. IF a cylinder holds 75 PSI out of a possible 100, it's a 25% leak rate.
Small aircraft use the leakdown test as an annual inspection item- but they only use 80 PSI. That will work too, it's just easier to figure the percentage of the leak using 100.
A leak down test is also helpful- put 100 PSI air in the cylinder with both valves closed and see what it will hold. If you hear air escaping at the tailpipe, possible exhaust valve problem, at the carb, possible intake valve problem, at the crankcase breather, possbile rings. IF a cylinder holds 75 PSI out of a possible 100, it's a 25% leak rate.
Small aircraft use the leakdown test as an annual inspection item- but they only use 80 PSI. That will work too, it's just easier to figure the percentage of the leak using 100.
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Thread Starter
6th Gear
Joined: Mar 2011
Posts: 6
Likes: 0
From: Liverpool New York
Compression testing is a good thing. It will tell lots of stories about what condition your engine is in. But you also need to provide some more info- Were the 2 cylinders that were low adjacent to each other? If yes, possible head gasket. How many strokes did it take for each cylinder to reach that maximum number? Did it build up to that maximum number slowly or did it jump up there quickly and stablize?
A leak down test is also helpful- put 100 PSI air in the cylinder with both valves closed and see what it will hold. If you hear air escaping at the tailpipe, possible exhaust valve problem, at the carb, possible intake valve problem, at the crankcase breather, possbile rings. IF a cylinder holds 75 PSI out of a possible 100, it's a 25% leak rate.
Small aircraft use the leakdown test as an annual inspection item- but they only use 80 PSI. That will work too, it's just easier to figure the percentage of the leak using 100.
A leak down test is also helpful- put 100 PSI air in the cylinder with both valves closed and see what it will hold. If you hear air escaping at the tailpipe, possible exhaust valve problem, at the carb, possible intake valve problem, at the crankcase breather, possbile rings. IF a cylinder holds 75 PSI out of a possible 100, it's a 25% leak rate.
Small aircraft use the leakdown test as an annual inspection item- but they only use 80 PSI. That will work too, it's just easier to figure the percentage of the leak using 100.
Each cylinder took 4 strokes to reach max, cylinders 1 and 7 took many more strokes, but never came over 120. These are new AFR heads and the valve seating and adjustments is spot on. I'm looking to rings because of the amount of blow-by and oil pushing out of the PCV circuit...the oil external capture tank is actually filling to overflowing!