L98 compression test
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Instructor
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From: Frederick, MD
L98 compression test
Getting a nicely consistent range of 152-157 psi across all cylinders in my 91’s L98 engine. This is a high mileage car (~180k). Is this reasonably good? It’s a stock engine with aluminum heads. Head gaskets were supposedly changed before I got it, but I don’t know how long ago (sometime in the last 40k miles I believe). But no major engine overhauls that I know of. Trying to guesstimate how much longer this one will last.
Team Owner
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From: Park City Utah
What makes it "good" is the 5 PSI spread between your best and worst. That is ~3% which is great..
Last edited by Tom400CFI; Jul 18, 2021 at 08:46 PM.
Pro
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From: Buchanan, Michigan
While it's not horrible, it's showing it's age. In original condition with low miles the 90-91 l98's cranked quite a bit higher than that. Out of all the l98's the last two years of Corvette production saw a hp increase, and it was primarily due to flat top Pistons with valve reliefs and no dish. A like new 90-91 l98 should be cranking in excess of 175, depending on if the test is done on a properly warmed up engine, all plugs removed, throttle wide open and done at sea level it could well be cranking around 180-190 psi. Your engine is fine, but it is tired. That many miles guarantees worn rings, worn valve guides, worn valves and seats, even worn timing chain allowing the cam timing to retard. All can, and do, contribute to lower cranking compression when tested.
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From: Buchanan, Michigan
Some reference from this forum back when these cars were newer. Again the 90-91 l98 had different pistons and higher compression than earlier years, hence their 245 hp rating from gm. Nothing to be afraid of, but if you plan on keeping it you may want to start considering a rebuilt short block and having your heads gone through. She's getting tired my friend.
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Pro
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From: Buchanan, Michigan
A new timing chain would probably jump compression up 15 pounds, maybe more. Also consider having a shop do a carbon blast of the intake, gummed up valves from cheap detergent used in modern gas can start to interfere with cylinder sealing if bad enough. The tpi is so torquey it's hard to notice because performance dwindles so slowly. A few inexpensive things could easily gain you another 10-20 psi cranking pressure and make her run a lot better.
Thread Starter
Instructor
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From: Frederick, MD
A new timing chain would probably jump compression up 15 pounds, maybe more. Also consider having a shop do a carbon blast of the intake, gummed up valves from cheap detergent used in modern gas can start to interfere with cylinder sealing if bad enough. The tpi is so torquey it's hard to notice because performance dwindles so slowly. A few inexpensive things could easily gain you another 10-20 psi cranking pressure and make her run a lot better.
In retrospect I didn't measure it optimally - it wasn't warmed up and I forgot to tie open the throttle. I did crank each cylinder exactly the same - counted 8 compression strokes per cylinder. The gauge would jump up during the first three and then not move after that. Seems like even though the method wasn't perfect it was probably good enough. Would a hot engine have measured higher or lower?
Thread Starter
Instructor
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From: Frederick, MD
Ah I noticed you mentioned the why with regard to timing chain.
Anyway I have it back together now and the #2 cylinder plug is a BITCH to get out/in so I'm not going to re-do it. I'm just happy it was well balanced.
Thinking about the timing chain though, I am somewhat worried it should be replaced soon, just since I can't see in its maintenance history that it ever was.
Anyway I have it back together now and the #2 cylinder plug is a BITCH to get out/in so I'm not going to re-do it. I'm just happy it was well balanced.
Thinking about the timing chain though, I am somewhat worried it should be replaced soon, just since I can't see in its maintenance history that it ever was.
Team Owner
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From: Park City Utah
In retrospect I didn't measure it optimally - it wasn't warmed up and I forgot to tie open the throttle. I did crank each cylinder exactly the same - counted 8 compression strokes per cylinder. The gauge would jump up during the first three and then not move after that. Seems like even though the method wasn't perfect it was probably good enough. Would a hot engine have measured higher or lower?
*How's oil consumption?
*How is fuel consumption?
*How is power?
*How is starting and drivability?
*Any pressure/flow from the breather?
If all those things "check the boxes", and you change your oil and don't overheat it....it should cruise right on by 300,000 miles w/o any issues.
Team Owner
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From: Park City Utah
Timing chains don't typically get replaced on an interval. I sold a pick up w/a SBC 350 in it, a couple years ago that had 300,000 miles on it, ran like new. Had the original timing chain....that that engine got beat way, WAY harder and worse than any 'Vette engine ever would.
Pro
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From: Buchanan, Michigan
Timing chains don't typically get replaced on an interval. I sold a pick up w/a SBC 350 in it, a couple years ago that had 300,000 miles on it, ran like new. Had the original timing chain....that that engine got beat way, WAY harder and worse than any 'Vette engine ever would.
Burning Brakes
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I plan on checking mine, though it will be a pain. As I'll have to warm it up and then quickly try and pull the spark plugs with #2 being a pain. I'll have to do one side first like the driver side. Then put everything back together and let the battery charge over night, and then warm it up and do the passenger side. Looks like my valve seals may be leaking to do oil getting on the spark plugs like something has dripped on them.
Drifting
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From: Northern California
I would have thought that previous owner who pulled the heads to replace gaskets would have also done the valve guide seals at the same time (they're easier to do when the head is off). But if you're seeing residue on the plugs, and those seals are original at 180K miles, I promise you they are letting oil by into the combustion chamber. Probably a lot. Mine were toast at 127K, and my car is about 2 years older than yours. I was going through a quart of oil each 1200 miles. I even had oil leaks showing from the #7 and #8 spark plug holes around the threads of the plug.
Now - all good.
Now - all good.
Team Owner
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From: Hartford WI
I would have thought that previous owner who pulled the heads to replace gaskets would have also done the valve guide seals at the same time (they're easier to do when the head is off). But if you're seeing residue on the plugs, and those seals are original at 180K miles, I promise you they are letting oil by into the combustion chamber. Probably a lot. Mine were toast at 127K, and my car is about 2 years older than yours. I was going through a quart of oil each 1200 miles. I even had oil leaks showing from the #7 and #8 spark plug holes around the threads of the plug.
Now - all good.
Now - all good.