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Checking psi of cylinders


jj280z

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I ran a compression check of the L28 engine and found that it ranged from 125-145 psi. The manual indicates a compression ratio of 8.3:1, but I could not find anywhere what the psi value should be. Is what I found with my compression gauge normal or is it low?

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 More info please. Throttle open or closed, did the engine have oil pressure, proper valve adjustment, rubber hose on the c. tester are some of the things that will affect the readings. I would hope for something in the mid to high 160s with no more than 10% variation between the cylinders.

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Yes, it's not so much "I must have X# PSI", it's the difference between the high and low cylinders. 125 PSI is 86% of 145 PSI.  You're good according to the manual, although since the alternative is rather extensive work, I wouldn't mind if pressure was less than 80% of the high cylinder.  

I usually do a wet & dry test to have some idea whether any lower pressures are due to rings, or to valves. 

Edited by Pilgrim
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FWIW, I just did a compression check on my motor using a C.T. from Auto zone. I was pleasantly surprised to find cylinders 2-5 at 180 and cylinders 1 and 6 to be between  178 and 180. It was hard to extrapolate the latter due to the gauge using 5# increments, and the variation could be due to slightly off valve adjustment. I'm actually very pleased with the results as the F54 block came out of a high mileage 280 ZX ( are these engines strong or what?) and a rebuilt N 42 cylinder head from a 280 Z FI car with a Delta stage 2 cam. I didn't do a wet test as the numbers were so close and in a good range. Now all I have to do is change out the elderly valve stem seals, but I think that will be after it's long winters nap.

Cheers, Mike

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When I was trying to get my 1983 to run ( i purchased as a non-running car) I did a compression test and all cylinder were between 182 and 190! I do not know if there was any engine mods done prior to my owning it but due to the damage that was on the end of the crankshaft balancer woodruf keyway, someone had the engine apart at one time. It runs pretty darn good on 93 octane.

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One thing in addition to the info Mark suggested, wether or not the engine is at normal operating temperature vs. cold will affect your readings.

But as was mentioned, you're more concerned about having the numbers not be too far off from each other.

I have a cheap Harbor Freight compression checker and it reads about 18 psi lower when checked alongside a Snap on compression checker. It's even worse on my small 90cc Honda CT90 dirt bike.

Some say the position of the Schrader valve has something to do with it.

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I have read that excessive carbon build up on the tops of pistons can increase readings when doing compression test. I like to do a leak down test as well. this will let you know the health of the rings and valves.

another good test is a vacuum test, I think all of these things help with getting a picture of the overall condition of an engine.

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The pressure seen on the gauge depends on the size of the volume in the gauge and its fittings that the air is being compressed in to.  A long hose, or a big gauge, or an adapter can lower the pressure.  The smaller the volume the higher the number.

Pop's Z's Z has a small chamber head (280Z head, probably N42) on a block with flat top pistons.  Should be around 10:1 CR, give or take some.  Higher than the 8.3:1 of the typical 280Z engine.  So, higher cylinder pressures.

Like SteveJ wrote, it's the comparison that really matters.

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Zed Head is absolutely spot on. That is exactly what I have thus the high compression (approx. 10:1) and high #s on the gauge all very close together. I get by on 91 octane gas with no problems except the price.

Cheers, Mike

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