Cylinder Compression

[Dennis Wilkie]

I just finished my compression tests on all of my cylinders in a 1992 K1500 4x4, and the #1 is noticeably lower than all of the rest. It's about 10 psi lower than the average, but it's 25 psi lower than the #2 cylinder which is 190 psi. I also just had to replace the even side head gasket because it was blown between the #6 and #8...and the intake gaskets for the 2nd time in 3 weeks. I did all of the work myself, so I know it was done right this time, and nothing leaks. From what I understand, a 15 psi or less difference is ok. But what is the effect of 25 psi difference between the #1 and #2 on performance, and what's the best course of action to take on this? I'm just curious as to what it all boils down to, because it's a fairly new rebuild with less than 10k miles on the motor itself, which came out of an 87 Corvette. I'm not all extremely familiar with the inner workings of an engine, but i've become very familiar with this one, and am having to learn real quick and very expensively thru trial and error, lol.

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[L29Sub]

Pull both heads, check for surface square. Surface for "clean up" if doubts. Check valve seats and valves. Check cam shaft while in there for damage. Flat tappets are famous for wiping metal during break-in. Hope you find something in that area. Otherwise, roll the engine over and evaluate the cylinders for scars....that would be bad. Poorly installed rings can run for a while. Have you adjusting the valves correctly? Hydraulic should be about 3/4 turn down from zero lash when running. Solids are whatever cam specs call for. Check for rocker studs loosening. Understand how to set solid lifter valves.
Aluminum heads are easily warped or damaged. Critical that head bolt tightening sequence is both correct and to spec for aluminum heads. Head bolts and washers should be lightly lubed. Use quality torque wrench.

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[Erik the Awful]

The general spec for compression testing is 10%. That said, I'd probably run it until the compression gets bad enough to cause a loss of power or mileage.

Like L29Sub said, check the valves first.

You said you replaced the other side head gasket because it blew out. Are you using a quality gasket? Were the heads torqued correctly?

Aluminum heads are finicky compared to iron heads. If you have an aluminum head off a car, always straight-edge it.

 

[Dennis Wilkie]

Yeah, I got all the torque specs and put every bolt back in the exact place with the right sequence and a 1/2 inch drive Kobalt digital torque wrench. I adjusted all of the valves and rocker arms 3 different times, twice while it was running just to be sure. I replaced the even side head gasket thinking it might have been the source of an oil leak that I know now is the rear main seal, because i've literally replaced every other potential oil leaking source, including aftermarket guages. I recently heard about a blown head gasket that managed to not mix with the water, so I replaced it. And I don't use cheap chinese parts and or cut corners on replacements like the tweakers that owned it before me. I've spent 10 months and about $5000 reversing all of their f*ck*ps so far, and i'm the only person that's turned a wrench on it since i've owned it. The intake gaskets are the Fel-Pro MS93317, and my head gasket is the Fel-Pro 7733 PT-2. The numbers on the side I worked on are all within 15 psi of each other, and higher than I was expecting. It's only the #1 cylinder that's out of character at 165 psi. The others are all 175-190 psi, and this was just on the dry test.

 

[east302]

For reference, attached is the GM spec for compression testing (98 service manual)

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[Dennis Wilkie]

I had my dad, whose built multiple drag cars look over the head, cylinders, and valves when I had it all broken down, and he didn't have anything to say other than how new everything looked. These valve pictures are as soon as the head came off, before anything got cleaned

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[RichLo]

I personally wouldn't be concerned with those numbers. But you could pour some oil down the #1 cyl and retry the test. if it comes up you have a slightly scoured cylinder, if it stays the same, you might just have a pitted valve seat.

Either way it sounds like you've been through that engine thoroughly enough already and I would just drive it and enjoy it.

 

[RichLo]

I personally wouldn't be concerned with those numbers. But you could pour some oil down the #1 cyl and retry the test. if it comes up you have a slightly scoured cylinder, if it stays the same, you might just have a pitted valve seat.

Either way it sounds like you've been through that engine thoroughly enough already and I would just drive it and enjoy it.

 

[Schurkey]

Drive it until something gives you evidence of trouble. Your lowest number is about what I'd expect from the best cylinders. Your best cylinders seem high to me.







And for the record, I verify my compression gauges multiple times per year, by connecting them to compressed air and making sure that the gauges all say the same thing. A single gauge can be inaccurate which isn't good. They can also be non-repeating, which is much worse. With only one compression tester, compare to the gauge on the compressor.

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[Dennis Wilkie]

It's a brand new kit, and the needle was sitting on 0 right out of the box. But it's not the stock engine. I looked up the head numbers (10088113) and they come back to an '87 Corvette 58cc. (14093638) is '87-'95 and the engine suffix is - USA and is from '83. So it's definitely been completely rebuilt. I expected the compression to be higher than usual, but wasn't expecting all that, lol. It also had a comp cam kit and edelbrock performer 3704 installed with 3' shorty headers with no cats right before I got it, and it's barely past the break in stage. I don't know how much all of that would factor into the compression rate, but that's what's there that i'm aware of. These were the only pictures I could find from when I had it broken down. The antifreeze was NOT there before I pulled the intake off, just to clarify.

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