REPRINT

[Joe C.]

Intake Manifold R&R/Winter Project (Original Post Friday 10/27/06)

Mike,

Thanks for the response, and all of the details. First, let me say that Duke is very familiar with this issue, and that's probably why he is conspicuously missing from this thread. He and I have hashed and rehashed this privately over a couple weeks this past month. AFAIAC, the results are still inconclusive, and Duke also had some very valuable input. I wanted to air it out on the FORUM, to see if there were any other good ideas out there. Here is some more background:

1. The car is equipped with GM NOS sidepipes.

2. Dyno tests (with sidepipes connected) show that the engine makes 4% better torque than a similar LT1 cammed 327. The power is substandard because it stops climbing at about 5700 RPM due either to valvetrain harmonics, or ignition deficiencies. One reason to believe that the engine is healthy (under compression).

3. Dyno tests with sidepipes disconnected (open ramshorns) show 10% improvement in power and torque, but the power still stops climbing at 5800 RPM due to the aforementioned issues.

4. During both dyno tests, blue smoke was visible out the right sidepipe, on the COAST, not the PULL.

5. Leak down tests revealed DRAMATIC, across-the-board improvement, when conducted at 100psi versus 17psi. Apparently, the higher pressure seals the rings. (I like your compression versus vacuum reasoning).

6. Number 2 and number 8 cylinders show highest leakdown percentages.

7. Examination of the AC 44 plugs is inconclusive. On removing them and studying them on three separate occasions, I have found 2 or 3 black-and wet, but never the same ones. A pattern is not discernable. The engine does not misfire, and runs well, except for the occasional tipping-in stumble during cruise.

Thanks also to Joe L., Bill, and Ken, for your responses to the original thread.

Joe

[Joe L.]

Re: REPRINT

Joe-----

Something I often thought about trying but never did (to determine if the problem was due to a leaking intake manifold gasket) is the following:

1) Close all of the valves by loosening the rocker arms;

2) Fabricate a plate and a gasket that can be used to cover and seal the carburetor flange on the manifold;

3) Include a fitting on the plate so that a vacuum/air line can be attached;

4) Using a hand vacuum pump, pull a vacuum approximately equal to the maximum engine vacuum. See if the vacuum can be maintained.

As an alternative, one could also apply air pressure to the fitting which is approximately equal in PSI to the inches of vacuum that the engine develops and see if it can be maintained. Also, with air pressure applied, one could listen (perhaps at the oil filler) to determine if any escaping air pressure could be heard. This would help determine if there was leakage past the intake gasket.

[george romano]

Re: REPRINT

If the oil ring(s) are weak, will a leakdown test confirm this? I didn't think a leakdown test gave any indication as to the condition of the oil control ring, only the compression rings.

George

[Joe L.]

Re: REPRINT

George------

You're correct; a leakdown or compression test really only deals with the effectiveness of the compression rings. However, it may IMPLY the condition of the oil rings.

On the other hand, this notion about "ring seating" is really an anachronism of days long past. For at least the last 25 years, piston rings have been "pre-lapped" at the manufacturing plant. They require NO "seating in".

As a result, GM engines installed in passenger cars and trucks for the last 20+ years, or so, have NO required engine break-in period. There should be little or no difference in oil control during the first few thousands of miles driving as with the next thousands. As a matter of fact, that's exactly what I've experienced with my last 3 GM cars. There was no noticable difference in oil consumption.

For instance, just recently I had a 2007 Impala with 3.9 V-6 as a rental car. When I got it it had 250 miles on the odometer. The oil level was "full" (presumably none had been added to bring it to this level). In any event, I put over 1500 miles on the car. I checked the oil just before I brought it back. It was still "full".

Also, many GM vehicles have engines factory-filled with synthetic oil and that oil is almost always 5W-30 these days (some may even be 0W-30 now). So, regular synthetic oil is used from the get-go; there is no special oil used for engine break-in.

The same is true for rebuilt older engines assuming that modern piston rings are used and machining is done properly. Many rebuilt engines (including all of mine) are bored and honed with deck plates installed for better ring sealing. Factory engines do not even receive this treatment. So, a rebuilt engine properly machined and using modern parts should have at least as good oil control as a factory engine.

Now, if someone were foolish enough to use 30+ year old pistons with 30+ year old NOS rings, then one might have a problem with ring "seating". I don't know of anyone foolish enough to do that---certainly not me.

[Duke W.]

Re: REPRINT

Excellent idea. I did something similar when I refreshed the head on my Cosworth Vega a few years ago. Even though the badly worn guides and limp OE valve seals were an obvious cause of the high oil consumption, I wanted to be sure that everything sealed, so I fabracated a plate to cover and seal the inlet port. It had a nipple to install a vacuum pump.

Each port held vacuum and oil consumption is now virtually unmeasureable. The test verified both the integrity of both the valve seats and modern stem seals.

The system you describe will detect any leaks in the inlet path, but won't necessarily allow you to isolate it to a specific cylinder, but well thought out tests are always better than guessing.

Duke

[Clem Z.]

joe that is the way we looked for cheater manifold

setup on race cars that were restricted either by carb size or a restrictor plates to make sure they were not pulling air in somewhere else

[Joe C.]

Good Idea, Joe!

Thanks for that, and, actually, I only have to close the intake valves. This test will not only isolate the intake gasket and/or casting as the culprit (if hissing is heard in the crankcase), but would also test the integrity of the intake valve seals (if hissing is under the intake valve springs). Better than a complete teardown, at this point.

Joe

[John H.]

Re: REPRINT

Joe -

I don't know what GM engine plants are doing these days, but the Chrysler OHC 3.7L V-6/4.7L V-8, both displacements of the Hemi, the new 2.0/2.4L Engine Alliance 4-cylinder, the 2.7L V-6, and the Viper V-10 are all honed with torque plates in production. Not driven by performance, but by emissions - the new generation of low-tension rings demand absolutely round bores to seal effectively. In all cases, the compression rings are moly-faced, and seat fully in less than five minutes.

[KEN BUTCHER]

ReJoe

Hey Joe,
So I guess when you spend the BIG BUCKS on all the internal's, it's a waste of money, because like I said earlier my L-82 didn't stop using oil until I got rid of the synthetic,10K miles later. So we all learn from our mistakes!!!!!

Ken.

[Chuck S.]

Isolated Cases of LS2 Oil Burning...

I remember some time back, John, you had commented on modern engines not needing a break-in because of changes in modern piston rings.

I'm interested in the moly facing on piston rings that you mentioned...Most new C6 owners making their first oil change early, say 1000-2000 miles, are finding the oil abnormally BLACK. When it happened to me at 1200 miles, I thought "Awwww, man, this sucker is a loser!", but the second change at 3200 was crystal clear amber.

I asked here on the Tech Board to find out if Chevrolet had added that EOS additive to the initial fill for break-in, but was informed that NOTHING but Mobil 1 was added. Do you suppose the BLACK color of low mileage "break-in" oil could be a moly facing worn/washed off the ring faces?

Occasionally, a new C6 owner will complain on CF about oil consumption (no laughing matter on a brand new $50K-$60K car). They relate their trials of trying to get GM and the dealer to address the problem. I believe the "fault" condition is like a quart every 1000(?) miles...which is not a big deal economically, but every time you pour in a quart, you're reminded of how much you paid for this car.

One or two owners with severe oil-burning were able to get the problem addressed to GM's credit (they paid through the nose)...in one case, they tore the engine down and re-ringed two cylinders that couldn't pass the leak down test. After all of that, I don't think their efforts were successful...it STILL burned oil; as you said, imperfect cylinder bore roundness is likely the key issue. A engine guru on CF also mentioned the low tension rings, and said they will be more prone to burn oil if not perfect.

I guess the moral here is...it doesn't happen often, but if you happen to get a late model engine that burns oil, you might as well trade it, because the only true solution is boring the case for over-size pistons.

[Joe L.]

Re: Isolated Cases of LS2 Oil Burning...

Chuck-----

I don't think that the coloration of your engine oil at first oil change had anything to do with the moly coating on the rings. I don't know why it was different than later, but I really don't think that ring "sloughing" had anything to do with it.

Moly faced compression rings have been used for years; they are really not new technology, at all, although the underlying technology may be improved.

Low tension piston rings are a much more modern development. I would not use low tension piston rings on a vintage engine under any circumstances.

[Michael H.]

Re: Isolated Cases of LS2 Oil Burning...

GM first listed "low tension rings" in about 1970-71 for competition use. Designed to be used in absolutely straight round cylinders with a smoother than production finish. The top ring had a deeper inside chamfer on the upper inner edge than conventional top rings.

[Chuck S.]

Re: Isolated Cases of LS2 Oil Burning...

Perhaps I misunderstand the meaning of "moly"...it's obviously a shortened version of "molybdenum". In my work life, "moly" was used for "molybdenum disufate", a graphite-like lubricant made by Dow Corning.

They sold it in a liquid form, preferably in 55 gal drums...it was pricey, but you could put it in a noisy gearbox and it would quieten it right down. I assumed this material could also be applied to the perimeter surface of piston rings as a surface finish to assist in piston ring break-in.

Molybdenum is also a element, metal I think, that's used to make high-strength steels...I suppose this must be "moly" material you guys are talking about.

So...what's causing this BLACK oil on a 1000-2000 mile engines?...Blow-by? Even though it WAS black, I was somewhat relieved that my C6's oil was not metallic black...I've had other first changes that looked like metallic silver paint, but the engine is still going like new with 135K miles.

[Joe L.]

Re: Isolated Cases of LS2 Oil Burning...

Chuck-----

In the case of the piston compression ring facing the "moly" does refer to molybdenum in the metallic state as opposed to the salt (e.g. molybdenum disulfide).

[Joe C.]

Re: Isolated Cases of LS2 Oil Burning...

Chuck-ster,

If a block was originally honed without a deck-plate, and is absolutely perfect, then a re-hone with a deck-plate requires at least .010" overbore to achieve perfect-circle.

Joe