Just completed 5000 miles on a complet rebuild on my 72 LT-1 (#matching). Few weeks ago (chasing an oil dispearing leak) did I check the compression...

1. 160 2. 153
3. 155 4. 153
5. 160 6. 150
7. 162 8. 150

I took this test with all the plugs out, and throttle wired WOT. I also repeated the test on each cylinder 4 times to get the above results. Oh, the engine was hot!

As you can see the pressures are within reason, but for some reason one whole bank is lower than the other... I am wondering why this is so, and if need be, what to do about it.

I also just aquired a engine casting numbers book (1955-93) which shows my heads are original.

Last'ly...I can kneal down between the exhaust tips (right in the middle) and if listening closely, can hear a slightly stronger exhaust sound on the higher compression side. I do have an H pipe installed.

Thanks in advance. Muddy

 

Perhaps one side of the block or heads were shaved/surfaced more than the other?

 

Well, thats exactly what I was thinking... I did forget to mention...the block was not decked so as to retain numbers....so it has to be the heads... in your opinion...should I remove the heads, mill to the correct CC or is it ok to run like this? Muddy

 

If it's running good I wouldn't tear it down, unless I just needed something to do!

 

The block casting is machined when new. Same thing goes for the heads. Maybe one side of the block was a little different, same with the heads.

 

The engine is running good, excellent actually! I'm just a little concerned, that if one whole bank is not as strong as the other...that I'm dealing with an off balance engine which could or could not be detrimental to the engine at higher RPM's. Needless to say, I guess this is where blueprinting would have come into play! Thanks Muddy

Mandm1200: As I indicated above... I suppose this is where blueprinting an engine does come into play...or at least this is where the heads should have been checked for CC...

 

does your car have a heat riser installed on one side? Mine did the same thing although the delta was 5 psi, but all on the right side, consequently I also have a heat riser baffle on the right side...so I assume that the riser is a hair more of an exhaust obstruction on one bank of the engine leading to that bank's compression being 5psi higher than the left bank.

Other odd areas to consider, check your plugs and see if there is more fouling on the affected side, sometimes PVC systems that draw off of one side of the engine will allow the other side to foul the valves more in relation and leading to additional build-up affecting compression. Best result is plumb a breather into each valve cover.

hope that helps

 

Before you panic, consider this:

I'm going to guess you checked the driver side bank and then the pass side bank. The second side checked shows low, seen that happen before, even when nothing is wrong. When I've seen that, if you start the engine for a few minutes, then retest, starting now with the second side first, the low numbers may be on the other side. The deal is a reduction of oil at the ring/cylinder interface, which allows some ring leakage that wouldn't otherwise be there. Just cranking over on the starter, isn't enough to keep the cylinders lubed like actual running will. If you want to absolutely guarantee getting an accurate test with a compression test, it is best to start the engine, even if only for a minute, between each cylinder's test. That way, you'll be sure to have consistant conditions for each test. A compression test can be a valuable test, but in general, I prefer to do a leakdown test. With it, you won't run into this issue, and you can hear where any leaks are coming from.

 

Check valve lash,could have some effect on things .

 

It is not uncommon for one side to be machined from the factory lower than the other side by .010" or so. Don't worry about it.

 

75coupered: no heat riser or pvc... I removed the riser when I installed the new exhaust. In place of the PVC I'm using a small air filter. The other breather hole is hooked up to the main air filter. The plugs are perfect in color (all of them).

540rat: You are right... I did check dirvers side first! I'll have to recheck in the oppisite direction and see what happens. But...this does not explain why...when I listen closely, (rear of of the Vette), the higher compression side (drivers side) is distinctive'ly (to me) a louder exhaust sound. No one else can pick it, untill I point it out to them. To me: the sound alone is telling me...that the drivers side has more compression. The question is...will this minor difference in the reading, from one side to the other hurt the engine???

Vetteaddic: Like your name Valve lash is perfect! Thanks!!

 

Recheck starting at the other side, and see what you have then. If the numbers are reversed, it will put your mind at ease. Even if they would somehow end up the same as before, it won't hurt anything, the engine is not that delicate.

 

I bet it is the deck surface of the engine block. The factory setup was a production operation over 30 years ago. Just had mine decked. It was a 73 L82 engine. The new machine to deck the engine makes the cut parallel to the crank centerline and exactly 45 degrees from straight up. The block is then flipped 90 degrees and same procedure on the other side. The first pass took some off the front outside corner of the deck. Had to get to .008 cut before it was cutting all the way through. took .018 off after full contact to zero deck one side and .23 off the other with the same issue, not parallel, not 45 degrees from center. Had to cut .008 before full contact across the deck on the second side as well. The pass side needed .005 more cut to get to zero deck than the drivers side. Mine was made within a year or less than yours and it sounds like your block might have the same issue, Probably decked by the same guy on the same machine! Production tolerances were pretty lax then compared to now. Probably not an issue for function. They are all within 10%. Blueprinted engines will be closer but 34 year old stock, even rebuilt, is not going to be uniform. With this much variation on the block, CCing the heads might not matter that much.

 

63mako: Kevin... wow! What you describe above is simply amazing... and after thinking about it...sure makes sense in regards to whats going on here. I know and understand this issue is small, but after going through this LT-1 (bumper to bumper), with the level of detail I have given to it...I want this engine to run perfect'ly for it's own longivity... Mind you, it does run very strong, tweaked it like tweaking MS XP...it jumps to life at the flick of the key, does not miss a beat. Pulls very strong right up to 6500 and looks to walk right past those last digits. It shuts down perfectly, but...at around 6200 RPM, I do feel a slight vibration... I do feel it's coming from the engine and was thinking it was related to the compression out of balance from one bank to the other...Maybe what I should do..is swap out this #matching (air) engine for a stroker!!! Thanks!! Muddy

 

You could pull the heads and measure top of piston to deck at the outside and inside of each cylinder. Then cc the heads. if my hunch is correct on the deck and you want to keep this in, not deck it and have good balance on compression, mill enough to mathematicly balance the total combustion chamber volumn off 1 head. Should have to take less material off the head to get compression right since the head chamber is smaller across than the bore. a couple thousands could make a big difference. The heads might already have 1 side milled more than the other. Only way to find out is pull the heads and check a few measurements. Good Luck.

 

What Kevin describes happened all too often in production. Blocks as well as heads. Another case in point is my 400 block. I wanted it square decked, NOT zero decked, and my machinist found four different measurments corner to corner. He took the smallest # and decked the block to match corner to corner. Subsequently all my pistons are .010 down in the hole, (much better than stock) and all my pad numbers are still there. Numbers don't matter for me...but DO for you!
Might be an option for you....
Eddie

 

Kevin, Eddie, members Thanks heaps for the input! What you both suggest sounds like a plan One last Q. Do you think it is possible (the described slight vibration above), to originate from this compression issue? Again thanks!! Muddy

 

Yes

 

You say you tested it hot. Which bank did you test first? The passenger side tested lower across the board. Do you still have the heat riser valve in the exhaust system?

BigBlockk

Later.....

 

Ummmm, tha's what I was afraid of. To answer my own question... slight vib. is not a good thing...especially @ 6200

Bigblockk: No, the heat riser has been removed and I did test drivers side first! As 540 Rat stated above...I will test again...starting at pass. side. The engine was also balanced! Muddy