This may be kind of long but I want everyone interested to understand what I have in regards to my combination which impacts the results. I've got an L98, 350, stock lower end with a 74211 Lingenfelter cam, 1.6 roller rockers, AFR195 heads 64 cc combustion chambers. Not sure if the intake matters but it is a Hogan port injection. It seems to run fine other than a slight shake at idle, that it has developed over time. I had the motor gone thru around 30k miles ago, new rings, bearings etc. All the original hardware, pistons, crank is still stock. I have Moates equipment and Tuner Pro and don't see anything on the scans that indicate there is a problem, good vacuum at idle etc.
So I had the headers off getting them recoated and thought I would do a compression test. I did it cold with the thought that if all the cylinders came in close to equal, even thought it is not at running temp, that most likely everything was OK. My concern though is that they were, 110, 105, 110, 104, 110, 117, 108 and 105.. From what I see on the web that is low. I understand that the cam etc. can have an affect on static compression but this still seemed to be low.
It does not use any oil and other than the slight shake at idle everything appears to be fine. When I built the motor I used a thinner head gasket trying to regain what I would loose by having the 64cc chambers versus the 58cc on stock heads.
Any thoughts?

 

Try adding a little 30 weight motor oil to each cylinder right be fore you test it. Are you securing the throttle body wide open and charging the battery while testing the compression? It is critical to getting accurate results to have the battery being charged as you go. The same goes for removing all the plugs while you test the compression.
How much vacuum does this engine produce when running?
I would do it again with a bit of oil and see how much of a change you get.
You might want to do a "Leak Down Test" on your engine to learn where the leaks are that push your compression so low. At 30k miles your motor should be tight and running strong!

 

Its low. But at the same time its equally low. Which is odd... i agree, try a wet test and see what happens.

 

Your gauge could be wrong, try another gauge to see if the compression comes up.

 

Thanks for the response, please see below answers

I agree, thats why I have the concern. I never checked it after the build so I don't have anything to compare to.

 

Also may have run the test with the throttle closed. That would lead to low numbers across the board.

 

It's low but uniform. Normally when things happen most of the time, only 1 or 2 cylinders are effected. You lost a point of compression going to 64 cc heads. You have a cam that needs compression. Imo it's just your combination.

 

Your engine makes 40-50 inches of Vacuum? Maybe you are thinking of Fuel Pressure? Your vacuum should be between 12-20 inches at idle. I agree with Kevova that being "fairly uniform" is a good thing!

I am very anxious to hear what changes with a bit of oil in each cylinder. If your engine was stone cold when you did the test I might try doing it on a "warm engine versus a stone cold engine".
On my 427 that has 12.25-1 Compression I get 235-250 psi while cranking with the carburetor wired open. This 427 is capable of maybe 14-16 inches of vacuum.

I have a good hint if you are planning on saving the Corvette for a few years start a file with the compression readings. I have a little book for each of my Corvettes and I record oil changes and test results like these.

 

Sorry, in tunerpro it reads in kps, looks like that is around 14 inches of vacuum. The car runs good, I'm inclined to agree since they are so equal it may be the combination.
I did have the throttle body open.
I'll get the headers back on and do a test hot and see how it looks.

I appreciate everyones input!!!!

 

Well your compression ratio dropped from 10:1 to 9.4:1 using those 64cc heads. But my guess is that would put you in the 140psi range at worst. Somethings not right with the test or the gauge. Could be whoever put the new rings in messed them up - but that would make for lots of blowby and a show on a leakdown test. Or maybe the rings never seated.

 

Would it possibly be a Cam Shaft not Zeroed or Zeroed improperly? The fact that they are fairly consistent show that whatever it is it affects all cylinders equally.

The Leak Down test would be the most important test to perform on your engine. Try the Compression test with oil added to each cylinder while the engine is warm first if you can as It might tell us a lot.

If you hire someone to perform the Leak Down Test be sure to watch and verify their findings as most automotive mechanics don't do them very often. An Aircraft mechanic that works on Piston powered aircraft would be the best as they do it all the time and are very familiar with the results.

 

Ok kind of embarrassed I didn’t have the throttle body opened far enough
checked it again at temp and got 200 psi
i think the rough idle may be dirty injectors. I have some new ones ordered so we’ll see
Tried techtron and it helped a little. Hopefully the new injectors will do the trick

 

Glad you figured it out.

 

Yeah l used a screwdriver to prop the throttle body open but obviously it wasn’t enough. It had it open about 1/2 way. I thought it would get enough air.

 

Honestly, I would think so too. At cranking speed? I'd think 1/2 throttle would be plenty. Weird. Good you got it though.

 

I'm gonna check it again under the same conditions to see if is the same. I would like to get 2-3 test on the same cylinder to see if they are consistent.

I'm hoping the rough idle is the injectors. On the tune it is rock steady on timing and BLM's very close to 128 at idle as well as the vacuum being steady.
No vacuum leaks, the Hogan is a one piece sheetmetal intake. I looked it over before I put it on and it is a work of art, no leaks there. Checked the plugs while running by pulling the wires one at a time and I can tell it has an affect. So the injectors may be the culprit. I ohmed them and they test OK, but may be dirty.

I'll report after I get them replaced.
Thanks again to everyone for their input!

 

I frequently disconnect the throttle cable and wire it open. Then I put a battery charger on the battery for several hours BEFORE the test. Having the battery fully charged and the throttle body wide open you should get good data. I would start keeping records of them. Whenever you change plugs just do the compression. It is great to watch the wear and keep it under control. Even doing a Oil analysis once every few years would not hurt either.

We are glad you figured it all out! Good Luck and get out and Drive that Corvette!

 

New injectors make a night and day difference esp if you have carbon buildup on the end of them
It will run the way it was supposed to the entire time. Never had good luck cleaning them others have.

 

Good idea!

 

Hopefully so, I keep everyone posted.