TedH

I have yet to perform the leakdown test on my engine. My objective of course is to understand what is causing oil to be forced out of the pcv/hose and into the base of the carb.

From what I have learned from others, there are several potential causes for this phenomenon:

1. Rings never properly 'seated' - I installed Hastings moly rings. The cylinders were overbored .020 and there was a cross-hatch hone pattern on all cylinder walls. Were moly rings a bad choice? I heard they seat better than chrome moly and are more durable than the basic cast iron. Fact or fiction?

2. 1 or more Rings not installed correctly in the piston groves on 1 or more pistons (no two rings rings on a piston should be aligned - end gaps at same point).

3. Rings installed upside down?

4. Too much cylinder pressure under normal operation? Just fishing here. Just wondering if perhaps the problem is not the rings but something else...

5. Cylinders out of round. Cylinders not properly bored; resulting in taper or out-of-round condition. Normally a result of boring without torque plates. Thanks Lars for this feedback. P.S. '3D' the engine shop that bored the block in 1999 was SuPPOSED to use torque plates... although I cannot verify.

I have read that Olivier has had to plumb hoses from his valve cover breathers to his side pipes on his high perf small block. Is this a common 'problem' with high performance engines?

My engine has about 25K miles since the rebuild in 1999. It has exhibited 'consumption' problems since the rebuild. It has had an appetite for oil but does NOT have oil-fouled spark plugs. It also exhibits low compression as it tolerates 25 degrees of advance at idle and appro 36 degrees of total advance by 2500rpm without any detonation.

Input welcome! :seeya


[Modified by TedH, 11:25 AM 4/12/2004]


[Modified by TedH, 11:31 AM 4/12/2004]

lars

Ted -
The 406 crate motor that we tore down here in Denver due to excessive oil consumption suffered from cylinders that had been bored out-of-round. This prevented full ring contact with the cylinder bore. We corrected it by re-boring the block with torque plates installed. We found the problem by doing a leak-down test: the out-of-round cylinders had leakage in the 20-30% range. I can send you a photo of what an out-of-round indication looks like.


[Modified by lars, 9:23 AM 4/12/2004]

TedH

Lars,

That's one that I missed entirely. I recall that two of the cylinders (2 and 6) were identified to have a 'taper' condition that warranted the .020 overbore.

So, if the leakdown test shows 20-30% losses (or more), it is likely an out-of-round condition in one or more of the bores?

Appreciate your feedback. Once I perform the test, I'll probably plan a tear-down later this year...

TedH

Here's one: If I reuse the cylinder head bolts more than once, could this contribute to uneven loading of the block and cause distortion of the cylinders; resulting in loss of cylinder pressure?

Could a fresh set of head bolts from ARP help? I am currently using the original headbolts. I reused them in 1999 when I installed the 882 heads. Then in 2002 when I installed the DARTs and then in 2004 when I removed and re-installed the DARTs. The oil consumption has been present since 1999...

TedH

Lars,

A picture would be great. Thx for your feedback!

lars

Ted -
Cylinder leakage in the 20-30% range is, obviously, gross leakage. This type of leakage will cause the crankcase to get pressurized beyond the capacity of the PCV valve, and you'll get oil blowing out the breather. A new engine without the rings seated will be below 15%. A tight engine with a good ring seating job will be as low as 5%. Anything over 15 is a potential problem and is not acceptable for a performance engine.

Send me an e-mail to my office address and I'll send you some photos. If you have posting capability for the photos, why don't you post them up here once you get them. E-mail to:
lars.grimsrud@lmco.com

TedH

Lars,

Ya' got mail...

lars

Just sent you some pictures of cylinder bore scoring and out-of-round indications.

LemansBlue68

Do you know what the blow by is while running? I believe snap-on sells a meter that you can use to measure this in-vehicle. On dyno we use a volume measurement system much akin to a gas meter. At 20-30% leakdown it sounds like the bores/rings may be scuffed. Another contributor to low compression numbers could be that your valves are not sealing properly. You would probably notice low vacuum at idle and a "miss" to varying degrees if this is the case.

I'm a big believer in deck plate honing. The OEM's typically do not do this but they can control bore geometry reasonably well. I've seen Deck plate honed production prototypes run oil consumption tests that consume virtually no oil!!

Deck face flatness and head gasket construction also can contribute to poor assembled bore geometry. An out of flatness condition or older design head gaskets with prominent "fire rings" around the bores can also mess up assembled geometry.

One more thought, If you're getting a lot of oil through the PCV valve how well is the oil draining back in to the crankcase from the cylinder head? I've seen motors where the drain backs are constricted from sludge, misalignment, or are just too plain small to drain back the oil that is getting pumped up to the valve train. Also, intake manifold gasket leakage is common with SB Chevy's where air and oil is sucked in to the ports from the lifter valley filling the floor of the intake with oil.

TedH

LemansBlue68:

No idea what the blow-by is while running. Only know that I am consuming .5-.75 qts every 250 miles.

I get really good vacuum at idle and no misses; slightly lumpy idle from cam.

I truly believe the shop used a deck honing plate but I can only go on their statement.

Drain back into the engine from cylinder heads is not constricted by sludge, restricted or clogged drain back ports in the heads.

I just replaced and re-glued the intake manifold and installed a fresh set of ARP intake bolts. No sign of leakage or indications that it is pulling air/oil throught the intake gasket.