I am still trying to come up with the best way to deal with this. The more I read the more confused I get.

I have a 928Motorsports setup on my 83 4.7L. I currently have the crankcase vented to atmosphere to avoid putting oil mist/vapors back into the intake system. I know the instructions say to vent this back into the intake, but with the amount of oil being pushed out under boost, I would have a real problem.

I am looking at a oil separator/catch can system to remove the oil before it goes back into intake. This leads me to another problem.

Under boost conditions, Both the crankcase(were oil return line is) and the intake(were vapor line should be attached) become pressurized. If I have both ends of this system hooked up(closed system) they appear to be pressure fighting each other. Won`t this create a closed, pressurized engine with nowhere to release excess pressure other than blowing out seals and gaskets??

 

When you say oil return line, do you mean the line going down the front of the block to the pan, becomes pressurized with boost? Them something is pressurizing the crankcase.

How is everything hooked up?

Do you have the stock hose from the block going to the fill can, then the top hose from that can going to atmosphere? Or do you have more lines added in somewhere?

Have you done a compression test?

 

The oil line that normally goes from the filler cap to the lower part of intake, now goes to a T fitting and then drains back into passenger side cam housing/cover. The other part of the T should go to the intake to absorb vapors back into combustion mix. They idea is to allow oil drain back into engine plus keep a closed ventilation system. When the engine is under positive pressure, oil is being pushed back up either drain or oil cannister then out the available exit which happens to be the now open hose from T fitting.

Compression is good on all holes.

 

I would plug the lines going to the cam towers and see if anything changes. I'm trying to picture what would have to happen in order for the boost to travel back up into the head and out the cam tower. Anything is possible I suppose.

It's either that or blow-by past the rings pressurizing the crankcase.

We've seen time and time agian (not just with 928's) that detionation can and will cause excessive blow-by, even if everything checks out (like compression).

If you have the ability to take a close-up macro shot of the white porcelain on the spark plugs, do so and post them.

On that note, what plugs are you using? (how many times a day can I ask this? )

I have a gut feeling this is all detonation related, which doesn't necessarily mean you've damaged anything yet.

You could also have a damaged oil control ring with good compression numbers. Didn't someone recently go through that? Or am I thinking of the 944 board again?

 

928 intl has a oil breather plate that might help.

 

He needs to find the root cause of the problem first, this much blow-by is not normal.

 

I have the 928MS breather plate and system installed. Right now I am running the WR8DC plugs. The car runs well and Im not hearing any detonation. My big concern is keeping the oil out of the intake system and off of the engine. I`m thinking a good baffled catch can with a breather filter.

 

That is stock for a 78-82 and one range hotter for 83+

IMO too hot for any supercharged 928.

I've been standing next to engines on the dyno that are showing knock counts in the data logging. Nobody could hear any detonation.

When the knock counts increased so did the blow-by.

By the time detonation in a boosted car is loud enough to hear, it’s usually too late to save it.

Worst case scenario is pre-ignition, melted pistons are messy to clean up. I haven’t seen or heard of this happen to a 928 yet.

Your concern should be why you are pressurizing the crankcase so badly.

 

Do you have anything in the factory filler neck?

If yes can you snap a picture of what you have in there.

 

Among many other things, this might be caused by too much oil making it to the cylinder walls. Too much oil overwhelms the oil control ring. Then the excess oil makes it to the top ring, unseating it from the wall. This breaks the pressure seal between the piston, ring, and the wall, leading to large blowby. The excess oil will also cause detonation, if it makes it to the chamber. Just thinking out loud.

I am by no means an expert, but my first step would be to plug all lines that could psosibly pressurize the crankcase from the intake manifold. The simplest explanation (and the cheapest to solve) would be a breather line say from the intake manifold to oil filler neck somehow pushing the boost to the crankcase. (I ahve no idea how the OB hoses are run so don't take this example too literally.)

 

That's the way I understand it, I've seen this theory proven on the dyno with more than just 928's. Supercharged, turbo, high reving and even bone stock.
Detonation doesn't discriminate.


Agreed. I've seen cars setup this way (intake pressure routed directly to the crankcase.....with no check valve). Not necessarily 928's either, this can be backwards on any car.

Reason why I suggested to plug the lines going to the cam towers. I'm not very familiar with his setup so it's hard to picture what might be causing the crankcase pressure other than blow-by.

 

Shouldn't Carl be helping you with this? Just seems silly you have to come here for this. Murf and DR give great support on their kits.