engine problem (leak down) again

[L8apex32]

1981 911SC Euro, used for PCA e-stock racing only

I bought the car knowing it had leak down problems on the #1 cylinder. It burned some oil, maybe a quart or 1.5 quarts in a weekend. I drove it this way for 2 years.

In Apr of 2011, I had my engine rebuilt by a reputable shop. They found that the #1 cylinder had broken cylinder rings. And, that I was lucky that they stayed in the ring grooves. No damage to any of the cylinders was done.

During the rebuild, we opened the case and replaced the crank bearings. It was a pretty decent rebuild, but did not include new pistons or cylinders.

After the rebuild, it smoked as much as or more than before. But, it ran much more smoothly. It seemed to be more powerful, but we didn't dyno it. The builder said the smoke was because it was built a little looser than a new engine and should have more power because of this.

During the 2nd race weekend, I thought I saw some smoke coming out the exhaust after certain corners.

For the 3rd weekend, there was a LOT of smoke coming out and, sometimes, the engine would run really badly due to burning too much oil. The problem got so bad that oil came out the oil cap. This lead me to believe that I had another case of cylinder pressure blowing by the rings. I had to abandon that race weekend.

At home, I did a leak down and found that cylinders 1, 2 and 3 all have a major problem. One is 20%, one 30% and one 50% leak down. Cylinders 4, 5 and 6 are all great, better than any of the 6 were pre-rebuild - with less than 5% leak down.

I am fully convinced of major blow by for 2 big reasons. One, oil came out the oil cap. And for two, during the leak down, I could hear oil burbling in the case.

My engine builder and I are working out the details of how to proceed, but nothing further has been done yet.

My question is, what can cause 3 cylinders to have ring problems after just a few weekends, but NOT the other 3 cylinders? My engine builder assumes that something other than the rebuild is causing this - perhaps running lean or something. He of course wants to fix this, but charge me more if everything he did was done correctly the first time.

My thought is that there is nothing that can cause 3 cylinders rings to break in less than 3 weekend and that after a rebuild, the engine should not smoke to begin with. Am I right to assume that the rebuild is the problem?

Also, what can cause rings to break in any case? These should be brand new rings in a fresh rebuild.

Any thoughts would be great.
Thanks.

[kyle242gt]

Loose tolerances and insufficient break in could lead to the rings never seating properly. Just a WAG.

[yelcab1]

A leak down test can identify where the leak goes. Did it go through the intake, exhaust, or cylinder wall ?

[L8apex32]

The leak was past the rings, into the crank case. Not only could I hear the burble of oil, but I could hear it coming out the oil fill neck when I had my hand over it.

Beyond that, when running at the track, in certain conditions, the pressure in the case blew oil up the filler neck and past the oil cap. That is a lot of pressure which never occurred before.

[E Sully]

What type of cylinders, Alusil or Nikasil? Were they prepped in any way for the new rings? I've read that alusil is difficult to put new rings into.

[L8apex32]

As far as I know, these are the original pistons and cylinders.

I do not know what prep was done, but this shop has rebuilt many SC and other 911/356, etc, motors.

[Bob Kontak]

Originally Posted by E Sully

What type of cylinders, Alusil or Nikasil? Were they prepped in any way for the new rings? I've read that alusil is difficult to put new rings into.

Believe this is so. Might want to do some searches on Alusil ring seating.

Should be some interesting future discussions between you and your mechanic. Regardless of what is says, the rings don't seem to be seated. That would be his job to have done.

[Flat6pac]

Or the ring lands are too worn and breaking the rings. If youre breaking rings, something is worn out. Euro is nikisil for sure.
Bruce

[Eagledriver]

This is a race car with 9.8 pistons right? They are prone to detonation with aggressive ignition advance and pump gas. This will cause broken piston rings. That's probably what broke the first ring before the rebuild. Need to repair and run less advance and/or race fuel.

-Andy

[Dan J]

^^^^ What Andy Said, You can't race that car with street gas. Those pistons are cast and don't like to rattle

[john walker's workshop]

did the shop offer a warranty on a race engine?

[L8apex32]

Detonation? I raced the car for 2 years with usually 93 octane pump gas. I didn't change my behavior for the 3 races this year.

Also, why would 3 cylinders on one side have this problem and not the other 3?

And, speaking of detonation, is it easy to hear/detect in a 911 SC? I've heard detonation in other engines before and it was easy to detect and no damage occurred. I didn't hear any in the 911.