240Z, 260Z, 280Z Performance / Technical Discussions related to performance motor enhancements, upgrades.

Normal Compression?

Old 02-13-2008, 05:53 PM
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Normal Compression?

Well. After running the car for a while, and setting all the valve lash properly, and playing with the triple Weber set-up, I did another compression test. I was shocked. Not only did they all balance out beautifully, but I did not expect to see 190-195psi across the board.

Is this normal? I have played too long with low compression engines to know. My motorcycle is above that, but it's got some high compression pistons in it.

1974 260Z. Factory block and head. Not sure there has ever been anything done internally to the engine. But, now I am curious.

I thought I had a tired engine. But, it it really is true, it only has 40,000 miles on it now.
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Old 02-13-2008, 06:02 PM
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lucky dawg. much more then that and you couldn't run on pump gas aye? i would say that the only way that could be bad is if it was high because of carbon build up.
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Old 02-13-2008, 06:13 PM
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The 280zx Haynes manual says the non-turbo normal compression pressure should be around 171psi.

Did you do a wet or dry compression test? Numbers will be different.
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Old 02-13-2008, 06:39 PM
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That was dry. I saw no need to do it wet after that, and for the fact that I burn no oil.

By my simple math..

A compression of 190psi, from atmospheric pressure of 14.7psi would make it a 12.9:1... This does not sound right to me at all. I would expect it to be more around 150PSI.
But, because I don't do many compression tests on gasoline engines, I am probably doing the math wrong.

But, it seems to me, that if you were to compress atmospheric pressure roughly 12.9 times, you get 190 psi.. Is that wrong? There is a variable I am missing, or I need to get a new compression gauge. The math may make sense, but my car is not at 12.9:1 compression. Not and run smooth on 89 octane.
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Old 02-13-2008, 06:44 PM
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your engine most DEF isnt 12.9:1 you sure aboot that calculation? cause a 9:1 is like 180psi i believe and a 8.5:1 is like 160psi im pretty sure. now stock, thats much higher than normal. could bea a shaved head, flat top pistons, thin head gasket idk.... but still.... thtas a bit high. you might wanna re-chack those numbers do it dry for sure, make sure. lol. and hold WOT
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Old 02-13-2008, 07:09 PM
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Well. Thought about it for a while. From what I understand, they calculate compression ration by volume, and volumetric efficiency. Not actual compression of the atmosphere. So, even though I am "technically" compression the atmosphere 12.9 times, that does not give me a compression ratio of 12.9.

Take more math than that to figure it out without measuring components, cylinder volume, squish, head gasket thickness, and combustion chamber volume. I could sit down, and figure it out, but it's really not worth it to me.
I think my engine is running pretty good for the age, and seems to have plenty of life left in her. That's what really matters. Guess I just wanted to question my thinking. Thanks for helping me out. haha
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Old 02-13-2008, 09:29 PM
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My Chilton manual says 171-185 so 190 with a little carbon build-up is easy to believe but you should try a carbon cleanout treatment and test it again. for those who don't know you should remove all plugs, coil wire off, hold carbs full open and results should be read after three compression strokes.
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Old 02-13-2008, 11:57 PM
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Don't forget to add, that the engine should be at normal operating temps.
If your engine is not at operating temps, you rings are not expanded like they should be.

I have a very clean engine now. Already gotten rid of carbon a couple times. Seafoam works well. Used it on many engines now. Makes some nasty smoke for a while, and pisses the neighbors off, but it works well.

I will tear the engine down at some point, and check it out. See what is factory and what is not. If someone put the money they did on the rest of the car, (Carbs, header, ignition etc.) then it is possible they did some work internally.
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Old 02-14-2008, 12:21 AM
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Is your compression gauge accurate? I've gotten varying readings on my engine from different gauges, but they've always been even across all 6 cylinders.
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Old 02-14-2008, 12:32 AM
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I have very close to even readings on all cylinders. Just higher than I thought it would be. I will bring home some more gauges that I made from work that I know to be dead on. Do a leak down test too. This is a 260. With the L26, and E88 head. Not a L28. So 185 would be normal. I am not far above that. Could be gauge that is out. Might be true. Not sure yet.
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Old 02-14-2008, 05:15 AM
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you can do it cold to. just a static or dynamic reading you get.
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