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Rasta, you suck, and you owe me $100.
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Min  



Joined: 04 Nov 2002
Posts: 2368
Location: Vernon, British Columbia, Canada

PostPosted: Tue Apr 22, 2008 8:20 am    Post subject: Reply with quote

Lizard wrote:
Sorry to tell you this Chrenan,

That is not going to really work all that well.

it will work ok of you take off that drain valve and put a pump on the bottom to pull the fluid out of there and pump it back into the crank case.
But even doing this will not give you any vacuum inside the crankcase.


Since there is vaccum being generated by the suction of your intake, I find your comments hard to believe. Hook one end up to the crankcase, hook the other up to your intake, and it should work fine.

Min
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bass gt  



Joined: 02 Dec 2004
Posts: 971
Location: Johannesburg for now!!

PostPosted: Tue Apr 22, 2008 8:38 am    Post subject: Reply with quote

Min,

Lizard is not far off the mark. Try this- Take the dipstick out and place your finger over the tube. You will feel neligable vacuum. What vacuum there is will be sufficient to draw out oil vapours, but little else. To create true crankcase vacuum, you need to either dry sump the motor, or fit a vacuum pump such as the GZ items Dan linked to.
Remember, as two pistons rise creating vacuum, the two others are decending, creating positive pressure. The vacuum created by the intake is negligable, unless you run a 30mm pipe from the crankcase to the intake and draw the motors air through the crankcase. Not really workable. A small (10mm) pipe will simply not be big enough to move the amount of air required to create a vacuum or negative pressure within the crankcase.
Then add in the positive pressure caused by blowby of the rings when on boost. Even small amounts will be significant in terms of volume of air entering the crankcase.
As an example, my dry sump pump is a triple rotor design, with 2x scavenge and 1x pump. this is capable of pumping 2.76 gals per 1000rpm per 1" of rotor length. Now my pump is fitted with .850" rotors so at 6k it is displacing 28 gallons of oil/air per minute. Now this creates a vacuum!!

Steve
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Chrenan  



Joined: 15 Jan 2003
Posts: 3899
Location: Canada

PostPosted: Tue Apr 22, 2008 9:10 am    Post subject: Reply with quote

Standard setup:

crankcase breather ---> hose ---> plastic intake tube

With the standard setup I gaurauntee that there is vapour and oil mist being evacuated from the crankcase via the suction provided by the intake charge. How do I know this? Show me a standard 924 with no oil deposits in the plastic intake tube, subsequent rubber boot and throttle body. That 924 doesn't exist.

Modified setup:

crankcase breather ---> hose ---> catch can ---> hose ---> plastic intake tube

In theory, with the modified setup the oil mist will be captured in the catch can and can be disposed of properly. The air drawn from the crankcase and "cleansed" of the oil vapour and returned to the combustion chamber. Best of both worlds, slight vacuum on the crankcase and "clean" air returned to intake tract.
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Last edited by Chrenan on Tue Apr 22, 2008 9:18 am; edited 1 time in total
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Chrenan  



Joined: 15 Jan 2003
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Location: Canada

PostPosted: Tue Apr 22, 2008 9:17 am    Post subject: Reply with quote

bass gt wrote:
The vacuum created by the intake is negligable, unless you run a 30mm pipe from the crankcase to the intake and draw the motors air through the crankcase. Not really workable. A small (10mm) pipe will simply not be big enough to move the amount of air required to create a vacuum or negative pressure within the crankcase.


Incidently, the line running between the crankcase and the intake is 19mm.
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bass gt  



Joined: 02 Dec 2004
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PostPosted: Tue Apr 22, 2008 9:28 am    Post subject: Reply with quote

Chrenan,

I agree that there is negative pressure created by the intake, however, this is not sufficient to create a vacuum equal to that created by a seperate pump.
You are right that this will draw oil vapous out, but don't confuse this with crankcase vacuum. hook up a manometer or MAP sensor and you will see.
I would even bet that above Xrpm, there is actually positive pressure within the crankcase. Also, to create a vacuum, you need to seal the crankcase/head, so no external air can enter the engine. if it does, you will reach a point of equilibrium, and the pressure will drop no further as the air removed is replaced by the air coming in.
So as an effective replacement to the stock crankcase oil breather system, this is fine, but it wont create significant vacuum.
As another example, an engine which has been run in as a dry sump'd motor will smoke like a chimney if the dry sump is removed and a conventional wet sump system fitted. This is because the rings bed in much better with a significant vacuum in the crankcase, however, once this is removed, they fail to seal in the same way, and the resulting blowby is epic. If the intake system was able to create the same vacuum as the dry sump pump, there would be no change to the pressures, and no blowby. Sadly, this is not the case. New rings MUST be fitted and the motor re run in.

Steve
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Chrenan  



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PostPosted: Tue Apr 22, 2008 9:36 am    Post subject: Reply with quote

bass gt wrote:
So as an effective replacement to the stock crankcase oil breather system, this is fine, but it wont create significant vacuum.


I think we're on the same page, this is all I'm looking for, an effective replacement for the stock crankcase breather system that avoids the majority of the oil mist going back into the combustion chamber. I'm likely using "vacuum" in the loosest sense of the word, whereas you are using it in the correct scientific and highly strung engine tuning sense of the word. What I really should be saying is "negative pressure" as you describe above.
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Rasta Monsta  



Joined: 12 Jul 2006
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PostPosted: Tue Apr 22, 2008 9:39 am    Post subject: Reply with quote

Yeah fellas, its just an 'improved stock' solution. I think it looks good, and am considering one for the 931. I presume the stopcock is threaded in there, so if you wanted you could use a fitting, and plumb the drain to the oil pan a la factory.
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bass gt  



Joined: 02 Dec 2004
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PostPosted: Tue Apr 22, 2008 10:00 am    Post subject: Reply with quote

Chrenan,

You're absolutely right. I wasn't trying to teach you to suck eggs, but just worried that you were hoping for vacuum, as opposed to negative pressure
Also, there is a major upside if you can filter out the oil vapour. This seriously reduces the quality of the air charge, and can lead to detonation on engines running higher boost. Anything you can do to purify the air charge is all for the good.
As an aside, i think the GZ pumps are well worth looking at if a motor is staying wet sumped. You would create major negative pressure which is a great thing for an engine, even adding HP, and could dump the oulet into a catch can, so keeping the vapours out of he motor. The GZ pumps quote some impressive figures.

Steve
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Chrenan  



Joined: 15 Jan 2003
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PostPosted: Tue Apr 22, 2008 11:44 am    Post subject: Reply with quote

bass gt wrote:
You're absolutely right. I wasn't trying to teach you to suck eggs, but just worried that you were hoping for vacuum, as opposed to negative pressure


Thanks Steve, I didn't think you were being mean at all, its good to hash these things out sometimes, it always results in learning and I appreciate the input. I'll always listen to anyone with a cleaner more powerfull engine than me, just makes sense!
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ideola  



Joined: 01 Oct 2004
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PostPosted: Sat Sep 06, 2008 4:08 am    Post subject: Reply with quote

Ozzie wrote:
Dont forget without a PCV you need a restrictor in the breather pipe
Ozzie, can you elaborate? I'm not running a restrictor in my 941 at the moment...what adverse effects could this have?
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Rich H  



Joined: 10 Jun 2007
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PostPosted: Sat Sep 06, 2008 4:49 am    Post subject: Reply with quote

Surely the vacuum on the manifold with suck the fumes out, the mesh will help it then condense and the resultant oil will collect in the base. Looks fine to me... Will need emptying but that's not a problem, not sure I'd want to reuse it anway and it will give you an idea of how much blow by your getting.
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ideola  



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PostPosted: Sat Sep 06, 2008 6:40 am    Post subject: Reply with quote

Reason for my question: when I replaced my vac lines a while back on this care with silicone, I replaced the breather line too as the old one looked pretty dodgy. I did not use the "factory" part, which apparently has a restricter in it. My aftermarket replacement line does not. What is the function of the restricter, and would there be an ill-effects of not running it? Bear in mind, this is on a Series I 931, so the line from the block breather goes to top-side inlet on the air-oil separator.
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Smoothie  



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PostPosted: Sat Sep 06, 2008 8:36 am    Post subject: Reply with quote

Pretty sure only the 924NA breather line has the restrictor, and its' main function is to lessen the amount of oil that gets by, and into the intake. Similar in function to the air-oil seperator..
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Ozzie  



Joined: 12 Mar 2005
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Location: Townsville, Qld. Australia

PostPosted: Sat Sep 06, 2008 9:09 am    Post subject: Reply with quote

To my thinking.
The line from the breather is about 20mm and connected on ROW cars to the join in the main air induction.
Without a restrictor this will put the sump into a high negative pressure.
Open the oil fill cap and you will have a massive vacuum leak.

Oil vapour has a greater chance to be drawn into the induction and burnt.
At high revs you will be leaving a smoke trail.

On US cars the air box will likely have oil in it, dripping everywhere.
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ideola  



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PostPosted: Sat Sep 06, 2008 11:18 am    Post subject: Reply with quote

Well, I just checked my airbox and it is essentially devoid of oil, so I guess the stock air-oil separator is doing it's job.
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