Crackership

You mean in/hg... right? 20psi would be about 40in/hg... which is a lot... near impossible I would think. What are you using to measure boost/vacuum?

SirLapsalot

Yes must be in/hg. I'm pretty sure 30in/hg is the "perfect vacuum" which is theoretical not actually possible if I remember correctly.

Thom

Reason for these questions is that I have just fitted an AOS top from a 968 which features a vacuum line to the intake manifold.
Granted there will never be any pressure in this line on a NA 968, but when adding the blue/black check valve in this line on a 951, will that create enough vacuum when the engine is idling to avoid excessive crankcase pressure?

User 41221

I am curious, do any of y'all have the breather set up from an '89 951? It had a small valve near the AOS (from the factory, its Item #2 and connects to teh top of the AOS. For those of us who have deleted the venturi mess, Lindsey sells a hose that replaces it nicely.

Thom

Scott, you may be refering to the valve dicussed here?

My understanding is that it should open when there is excessive pressure in the line, which may happen when the recirculating valve opens (as throttle is being closed).
However, as the standard AFM/air box set up represents a higher flow restriction than the typical cone filter/MAF pipe set up, this valve may open only when using the AFM/air box set up. There might never be enough pressure build up in this line on a car using a MAF pipe/cone filter.

User 41221

Yep, thats the one I was referring to. I am not sure I entirely agree with the assessment that the valve wouldn't open up when running a MAF/cone filter set up. I am fairly well convinced that if you are getting positive crankcase pressure, that valve should open up from the pressure coming from the AOS, regardless of what the MAF/cone filter is doing and it *should* solve the problem. My question is, are any of the folks having this issue running that set up?

Lindsey sells a hose that replicates the control line fitting that runs from the top of the AOS to the inlet boot with a fitting for the check valve. Its not cheap (approx $60) but I think it the right direction to look in.

zerMATT951

You're right - it must be in/hg. I'm using my aftermarket VDO boost/vacuum gauge that registers something like 30 vacuum to like 25-30 pressure. I always assumed PSI, but in/hg is more likely.

Oh, and I guess I should definitely do the vacuum cleaner test before running any new hoses... I wonder if my 5hp shop vac would create too much vacuum in the crankcase?

fast951

Hi Scott,

The issue is related to not having enough vacuum pulled on the crankcase. vs. relieving excess crankcase pressure.. The crankcase requires vacuum! The factory air box creates enough restriction in front of the turbo, which translates to negative pressure or vacuum. When going to a free flowing intake, like a MAF + Cone filter, the vacuum in front of the turbo is reduced. The lack of vacuum is very pronounced at idle, which can cause problems..
When the turbo is spooling, it creates enough suction to help siphon out the crankcase...

By adding a PCV system to operate at idle, we are basically re-introducing vacuum (which was lost when the factory airbox was removed). This PCV system operates only under no boost conditions.
The stock crankcase vent system must remain in place, which operates mainly under boost.

Some cars need the PCV while others do not. It depends on how much crankcase pressure there is in the particular engine.

User 41221

Gotcha, thanks for clarifying! Thom, my apologies for being too thick headed today to read what you were correctly saying the first time.

minho78

I see blue and white smoke.

Thom

Scott, you are welcome, it took me a long time to understand how this breather system works, and I still have this issue with smoking at idle!

Thanks John for the proper explanation, and what do you think about using a small vacuum line plugged into the AOS top? As I tend to mix up air flow with vacuum in my head, I'm not sure what to expect with such a set up.

fast951

Using the AOS top is one option, however it does complicate things a bit.
The size of the hose is dictated by the size of the PCV and the one way valve. Using vacuum just by itself off the manifold is not a good approach, the air flow must be regulated by the PCV itself. The activation of the new PCV is handled by the one-way valve, as it will have a internal spring to regulate at what pressure the valve is open.

Another item to deal with is air entering the intake through the crankcase vs. through the TB. Not a big deal, but something to deal with..

As you can tell, there are many variables to deal with. As simple as adding a PCV + hose is, it must work as part of the existing system or it will have side effects...

Thom

Thanks.

I am not sure why it would even be needed to use an aftermarket PCV in addition, as the standard blue/black check valve fills the one-way function already by itself...?

- Engine running under vacuum in the intake = one-way valve open, intake pulling pressure from the crankcase (if there is too much pressure being pulled from the crankcase then another one-way valve can be put between the turbo and the AOS so that the turbo won't pull any pressure at all when the engine is running under vacuum. Eg the standard one-way valve between the brake booster and the intake manifold can be used there)

- Engine running under boost = one-way valve closed, turbo pulling pressure from the crankcase.

Thom

Mikjel, what was the actual value of the vacuum you measured?

mikjel

In my case I needed only a very slight vacuum from the intake manifold to the cranckase. I use a thin hose which I made restrictions in to keep the air flow as low as possible. I put two pcv-valves (opening in one direction and closing the other direction) in series to be sure I will not get boost pressure in the crankcase.
This is air entering the intake without going through the MAF (air not being "measured") and turbo (not spinning it - not good for draining), so it is best to keep the air flow at absolute minimum. For me it solved the problem with smoke at idle comlpletely, but I am still having some smoke on trackdriving and higher oil consumption than before I mounted the new turbo + maf. Do you ever see any smoke when driving Thom, and how is your oil consumption?

Lots of european and japanese cars have PCV-systems like this. I mounted a pcv-valve from a volvo v40 turbo and a "one way check valve" (from a hardware store) in series on my car. Two valves are safer than one..

+1

I think it will! I did not know the 968 had a hose / aos top like that.

Yes, my car has this valve. I think you must have lots of leakdown to open this valve (really high cranckase pressure).

I did not measure it. The vacuum was very low, not easy/possible to feal with my hand when I held it over the oil filler hole with the cap off. I "measured it" by holding a piece of a thin plastic bag over the oil filler hole. I could see that the plastic was sucked down on my friends stock setup (vacuum) and pushed up on my car with maf setup (positive pressure).