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| compression testing https://slantsix.org/forum/viewtopic.php?t=53047 |
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| Author: | '67 Dart 270 [ Tue Aug 13, 2013 10:00 pm ] |
| Post subject: | compression testing |
I have a newly rebuilt long block. I've set the valve lash (cold) a few thousandths more than cam card indicates for break in. Will set to spec hot after break in. I tried a compression test and the compression is really low, like 60 psi per cylinder. I haven't put the engine in yet, nor added any oil to the crankcase. I've never run a compression test on a new engine, no intake or anything on it yet. The only time I've ever done one was on a warmed up engine. Why is my compression so low? I'm pretty sure the test unit is getting a good seal, does a compression test have to be run with the engine turning over quickly (with the starter turning it over)? I'm just turning the crank by hand using the bolt on the harmonic balancer. Do I need to put a little oil in the cylinder? Forgive my naivitee. bg |
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| Author: | billdedman [ Wed Aug 14, 2013 12:01 am ] |
| Post subject: | |
We usually got a reading of 160 pounds on our turbo engine... time after time. Then, about 2 months ago it showed 50 pounds on #1 ans #2. WE assumed a blown head gasket, but subsequent dis-assembly and a careful inspection of all related parts revealed no blown head gasket.... nothing. We went over EVERYTHING with a fine-toothed comb and found..... nothing. No reason for the low compression numbers... We decided that the waay-too-rich mixture we had been running had washed all the oil off the cylinder walls, and a "dry" ring won't seal. We aare putting it all back together now, with plenty of oil on the cylinder walls so we will see... Stay tuned. Your "new" engine might have the same problem... not enough oil on the cylinder walls for a good seal. Just sayin'... Good luck!!! Bill |
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| Author: | DadTruck [ Wed Aug 14, 2013 5:00 am ] |
| Post subject: | |
Quote: I'm just turning the crank by hand using the bolt
the piston rings need some blow by to get to the inside diameter of the ring to fully expand it against the bore,, you won't get enough dynamic pressure turning the crank by hand to accomplish that. if you have a large air wrench,, you may be able to spin it fast enough using a bolt on the crank front. if you really want to test it before installing in chassy,, put a bell housing on and wire in the starter.. just be careful, I would not do that on an engine roll over stand. and yes,, on a new build the bores / rings / piston pins need to be pre lubed. |
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| Author: | '67 Dart 270 [ Wed Aug 14, 2013 7:38 am ] |
| Post subject: | thanks... |
I figured it might just be not fast enough turning and/or the oil seal isn't established. I'll put a couple drops of oil in the cylinder and see if it changes dramatically or not. Otherwise I'll retest after installed. bg |
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| Author: | '67 Dart 270 [ Wed Aug 14, 2013 10:44 am ] |
| Post subject: | more thoughts... |
I would assume a compression test isn't really meaningful until after break in anyway; i.e., so the rings and valves have a chance to seat/seal properly. |
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| Author: | VDART [ Fri Aug 16, 2013 4:08 pm ] |
| Post subject: | |
you would get different results with a factory starter versus a mini starter due to the speed the starter spins the motor. so your hand crank is good for 60psi@!! lawrence |
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| Author: | WagonsRcool [ Sun Aug 18, 2013 12:24 pm ] |
| Post subject: | |
A starter motor spins the engine at 100-150 rpm. Trying to read compression when spinning the engine by hand will get you some exercise but absolutely no useful info about engine condition. |
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| Author: | CNC-Dude [ Sun Aug 18, 2013 12:52 pm ] |
| Post subject: | |
If you haven't even run the engine and broken it in yet, then a compression test is a moot point right now. You will have to break the engine in to seat the rings before a compression test will give you any meaningful info. |
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