Kingtal0n

All I checked were junkyard 2002-2007 Truck engines, LM7 LM4 LQ4 I dont know all their names. All iron block though and some had ridiculous high compression like 220psi.
They all had in excess of 120,000 to 150,000 miles and pretty much old and scattered around containers for many years without being run. which I think is a part of the compression settling over time, valves get stuck open and you get super low compression on 1 or 2 cylinders per engine it seems when they sit.

grinder11

Probably nothing worse for an engine, and most other mechanical devices, than sitting for extended periods without any usage.....

Bikespace

Another update.

My son and I got the other head cleaned up, and bolted back on for a compression test. I redid all the cylinders, since my undersized lithium battery is significantly lower now.

1. 160
2. 175
3. 180
4. 175
5. 175
6. 160
7. 180
8. 160

Close enough for me!

Here is photo from today. Also, if anyone knows what the markings on the (completely flat) pistons mean, please let me know. 7566. and 56/22.

vette4fl

7566 is the rated horsetorques. Careful with the go pedal!

congrats on the engine fix, and let’s know how she runs. 👍

grinder11

Horsetorques.....LMAO. I thought it meant to drive 75mph on Route 66!!

fatbillybob

Funny how the numbers are so different using the same tester. My understanding is 10.5 compression ratio x 14.7 barometric pressure at sealevel = about 155 psi. If the gauge is accurate that's what we should see. I can only assume the reason we see the big numbers like 200psi is because of gauge accuracy? Maybe that's why people only look at relative numbers on compression tests. What is the general rule of thumb for acceptable difference on compression test wet vs. dry before people think about doing rings? I bet that wet vs. dry delta decreases with warm vs. cold motor that might have been sitting for weeks? The reality is who wants to try and do compression and leakdown on a hot motor. The job is painful enough when the car is cold.

Kingtal0n

Valve timing and air momentum play a role in cylinder fill. Cranking speed and late intake valve opening can drastically change the compression. Long intake runners allow momentum energy to keep flowing air into a cylinder after the piston reversed direction. That is how long duration cams work to increase cylinder VE despite piston ascension. It is based on RPM- crank slowly enough and the air can escape before the valve closes, that is why long duration cam kills low end torque at low rpm. But this interplay revolves around intake runner diameter and length and other issues such as overlap and exhaust scavenging and expanding combustion gas and chamber design on a running engine etc... which I digress...

Compression testing for random samples has a face value with low diagnostic utility besides an obvious inclination. However for an experienced intelligent person that understands how the engine functions compression testing can give far more useful information and details when it is done a specific manner with a high quality tester and well established conditions.

For example, I always like to provide examples, these are details how I like to perform compression testing
1. Top off battery and keep battery charger on the battery during testing to maintain similar cranking rpm for all testing
2. Use a high quality tester- never a rented unit
3. Use two compression testers on a couple cylinders to compare them looking for obvious deviation
4. get the same number of 'hits' on each cylinder, I usually go 1... 2... 3... stop 3 hits or 4 hits max to compare
5. Make a conscious effort to write down the numbers exactly with a decimal... 152.3, 155.6, 164.7, 158.3, etc...
6. Repeat the tests two or three times and average the results for statistical interface
7. If you collect 3 tests per cylinder and average them you can perform a student T-test to get P-value will confirm significant difference between cylinders is less than 95% confidence for example

Things to keep in mind
A. If the heads were recently replaced and the engine has not been run yet there may be significant difference between cylinders which will clear up once the engine has been run for a bit, for example a bit of debris in the cylinder rings from head work you just did, or cylinder ring behavior from some oil film issues because of recent cleaning efforts, the oil film replaced on the running engine will not easily be replaced from just cranking unless the engine have piston oil squirts in which case it may take several revolutions to start them spraying and suddenly the cylinders may read differently about half way through the test because of recent oil squirts late into the testing.
B. Testing on a recently run (warm) engine may be helpful to even out the results to remove inconsistency in oil film and ring behavior and valve sealing

Bikespace

Thanks for the extra info! I'll be sure to make use of that if I really want to dial in an engine.

For now, I'm optimistic it will work. Certainly the leakdown test found the issue, and the first compression test showed that I should clean both heads. I found several additional valves with gunk on them, so cleaning and lapping was well worth it, even if they weren't really leaking. And it seems the battery state of charge has a lot to do with the absolute compression numbers, as well as whatever all of the Seafoam has done to the rings.

I'll update once I get it installed and running.

vettenuts

Are you going to document your installation into the C3? It would be interesting to follow along.

Bikespace

I'm documenting as I go, and I'll have a big post when it's done. I'm not doing a build thread, but I hope to do a "completed" thread.

Bill Curlee

YES,,, I have ARP Head Studs and have had the heads on and off the engine inside the engine bay without any issues!

Ive done numerous cylinder head jobs and have TONS of solid tips on doing this job correctly! If you want to discuss, PLEASE PM me. There are several MUST DO things to do the job correctly!
YES I AGREE, ARP Head Studs are the correct way to go. Even if you dont care about high compression/boost ect..

Have you tried to soak the exhaust valve with Carb Cleaner and Mystery oil? Give it a try and see if there is ANY improvement in compression. Soak all cylinder valves, Give it some crank over time and see if you get any increase in Compression.

Hell, I might just save you a head job.

Bill

Bikespace

Thanks! But did you miss the part where I already removed and rebuilt the heads?

Lots of Seafoam was used to soak the valves, and several Scotchbrite pads valiantly gave their lives to clean them up, before lapping them.

At least two exhaust valves had large chunks of carbon crap bigger than the tip of a pencil, including #4.

Bill Curlee

Yes, I missed it.

Bikespace

No worries. Still solid advice. Thanks!

Bill Curlee

Are your heads back on the engine?

Bikespace

Yup! Post #43 has the summary, with the after-rebuild compression numbers (on a low-battery).

romandian

" TONS of solid tips":

bill, maybe you could share some of them?