I just had the valve seals done on my 90 L98 and the mechanic said they were fine. He also said that it looked like someone had already been in there based on their condition and very new looking valve cover gasket. The really weird thing is he said compression was 220 across the board. I was expecting around 150 for a small block Chev. Any ideas?

 

If they did not have it written down on paper I would doubt they ever looked at it. Mechanics like to look good to their customers.

Me, I would do a compression test myself and record the results for future comparison.

Before doing the Compression test be sure to attach a battery charger to the battery and get it fully charged up.
Then you secure the Throttle Body "wide open" with a tie wrap or something.
Then you remove all eight spark plugs and on the l98 you can pull the fuse for the injection system Pump and disconnect the red power to the distributor. You don't want gas getting lit by a random spark while performing the test.
Crank it for a specific amount of time to get the highest number possible.
After doing the first round I will add a bit of oil to each cylinder to get a "Wet Reading" as this shows any rings leaking.

When I last tested my L98 they were all in the 140-150 range at 100K miles. In our 1968 C3 the 427 uses 12.25-1 Compression ratio and it is pushing 250 psi on a compression test. You are looking for a close grouping of the results, the maximum of 10% from the top to the bottom cylinder.

A better test is a Leak Down Test where you are introducing air into the cylinder. The leak down is used to monitor the life of a piston aircraft engine. You can find leaks in the head with this system as air leaking past a valve can be identified depending on where it is heard. In the intake it would be your intake valves, if you hear air coming out the Positive Crankcase Ventilation hole then you have cylinder blow by. If there is air coming out the exhaust you would know that cylinder has a leaking exhaust valve. A very handy tool!

 

How much did it cost to do those valve seals? I believe mine are leaking due to pulling plugs and seeing oil spots like oil had dripped on the spark plug.

Also, does the compression test need to be done with the engine hot or cold or does it matter? I've read people suggesting either. I think it would be hard to do it hot as you will have to quickly pull the plugs, which may be a little hard with the engine hot. Also I find pulling the plug wires off the plugs to be rather hard for some reason.

I have a battery charger, though it's the slow kind of charger. So not sure if that would be fine enough. But I was thinking of doing the driver side first. Then letting the battery charge overnight. And then do the passenger side. Though that would be hard to do with it hot as I'd have to put the plugs back in to get the engine to temp and pull them again for the passenger side.

 

It is not that hard to do on the Corvette. What you need is a air compressor to hold the valves in place or you can go the route of putting string or rope into the cylinder and then rotate the cylinder to TDC and do the seals. This subject should provide lots of posts as it has been discussed a lot! I apologize for not having any idea what a shop would charge to do that but at a $160 an hour around here it would be expensive as it takes a few hours when you are ready and have the parts. It would be probably three to four hours to do a V8 for a mechanic in my area. It is not a hard job it requires patience and steady hands along with a Spring Remover and the tool to take out the Locks, having magnets around then would be helpful.

The seals are relatively inexpensive and easy enough to do your self if you are mechanically motivated. You just Don't want a Valve to drop down into the cylinder or you WILL be removing the heads from the engine! Keeping the cylinder you are working on at TDC is really helpful

I like to do my compression test after running the engine and getting it warmed up and then cool it down enough (4-6 hours) that your hands don't get burned touching stuff. The pistons will expand a bit when they warm up. This is a real problem with some types of pistons as they actually can be heard slapping when the engine is Cold.

If you don't have a big battery charger just charge the battery for as long as it takes to get it fully charged before starting. While performing the test you should have the charger on the battery between cranking. It should be fairly close unless the battery is shot. If you have to you can put a second (fully charged) battery next to it and connect it with jumper cables to keep the voltage up. We are looking for consistency here.

Getting the plugs in and out of the L98 is a task that some engineer "failed" to think about. I can reach all eight of my C3's 427's spark plugs and swap them in 20 minutes. Splitting the test into 2 sections might affect the results. I would prefer to do all eight at one time to get consistent results. If you only spin the engine to get the highest reading you should be fine. Don't forget to have the throttle wide open and your ignition or fuel disabled while performing the test.

I am lucky as I have a daughter who is a "Rocket Engineer" and she has small hands and can reach the plugs easier than I can. She helped me with the plugs and wires last summer. Having a woman clean up your engine compartment is nice, she ran the wires so much better than I did. She wants my C3 after I am gone so she figures to learn as much as she can about Corvettes. For her first Car she wanted my C4 but instead she got a 2011 Camaro and she LOVES that Camaro. A future Corvette Nut in the making for sure!