I have a 357ci LT1 running 12lbs of boost. The engine has approx 1,000 miles.

The pistons are Diamond, installed with .004 clearance. Rings are Diamond, Moly top, j-hook second, low tension oil. Gaps for 1st and 2nd ring are .024.

Under normal to moderately aggressive driving, it uses no oil. Not a drop.

Under very aggressive driving, and extended WOT runs, it uses oil. If it was driven very hard for 100 miles, it would probably use 3/4 to 1 quart of oil.

I did a leakdown test and when it was up to temp (but not right after a run) it tested 9%.

I would assume that if it was poor ring seal, that it would use oil all the time, and not just when it was under a high load. (not to mention it makes very good power).

The only thing I have for crankcase evacuation is a breather on the passenger side valvecover. I'm thinking that if that can't flow enough, then the built up crankcase pressure could cause oil usage.

My options are -

Install more breathers.

Install vacuum pump.

Re-ring engine.

What is an acceptable leakdown rate for an engine like this?

Am I also correct in assuming that just because the rings do well in a leakdown test, that the engine could still use oil? (and vice-versa), or are they always directly related?

I don't get any oil out of the breather, and an almost undetectable amount of smoke. Is excessive crankcase pressure under a high load something expected with an engine that makes 2hp per CI?

Any thoughts or experiences appreciated.

-Jim

 

http://store.summitracing.com/produc...earchtype=ecat

Simple, you need less pressure in the crankcase.

With low tension oils this can happen. Try that for a fix and see what happens. $30, plus some welding and some labor. It's the first thing I would try.

Bret

 

Thanks for the response.

I used that same header evac system on my 70 Chevelle with a roots blown 396. It worked very well, but the exhaust system was just 3 in pipes off the headers into low restriction mufflers and turndowns.


On my SS, I have longtube headers, dual 3 in pipes into an X-pipe, which reduces to dual 2.5 out of the X, into a single spintech muffler (stock location) with dual in dual out, and the stock two on the left tailpipes.

It is a low restriction exhaust as far as these cars are concerned, but is it free flowing enough for that header evac system to work?

 

That should be enough for it to help. I would only use low tension oils if they were wide enough and you have a good windage and evac system.

Bret

 

What do you mean if they are wide enough?

The rings are 1/16 1/16 3/16

 

I was hoping that size. A smaller low tension one might hurt even more.

The crankcase evac system is what I would do. See if that helps.

 

F-bud: I am not sure why you percieve what you have described as a problem. Oil isn't that expensive! But Bret's idea seems like a good one to cut down on the oil consumption. Is the car running well, making good power?

As far as leakdown goes, was the 9% the worst, or did you check just one cylinder? 9% would be too high for a new motor, but if there are a lot of miles on it would not be unexpected. If the motor is relatively new, and one cylinder has a much higher leakdown percentage than the others it indicates a problem. Leakdown tests are not well standardized, that's why an outlier means more than an absolute number.

Rich Krause

 

rskrause- Yes, the car is running perfect, and making good power. I'm not sure if what I describe is much of a problem. That's why I was looking for comparisons to see if this is a normal condition on a setup like this.

The leakdown was 9% on all of the cylinders. I would like to test it again when the engine is hotter. I would expect a lower leakdown rate on a naturally aspirated engine (smaller end gaps), but I didn't know what is really common for a leakdown rate on a blower motor with larger gaps. The engine has about 1,000 miles on it.

If it should still be in the 3-5% leakdown range with .024/.024 gaps on the rings, then I will most likely remove the engine and re-hone/re-ring it.

 

I know my experience isnt as technical, but if you flip through the roadracing section on this board, you'll see recommended equipment for a track day includes extra oil even for stock cars. It leads me to believe that while the car might not normally burn oil, when you beat the **** out of it, it will.

 

A new motor is typically 4% or less. The test is not standardized, as I stated. That's one of the reasons why it's difficult to interpret a single cylinder reading and why I asked you about all of the cylinders. Even though it's not really standardized, a leakdown is typically conducted with a cold engine. If they are all 9% but it running well it most likely represents normal wear, though the only way to know this for sure is to have had readings (performed using the same technique) when it was fresh.

It's your time and money, but unless I was bored, had nothing better to do, or was racing for my living, I wouldn't pull a motor that was running good, making power, but had a 9% leakdown and minor oil consumption. Not saying that there is anything wrong with doing it, and you gain some hp with a better ring seal. Just that it will be pretty low on the scale of money or time/gain. Of course, if you have other reasons to pull the motor then this could be added to the list.

Good luck.

Rich Krause