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Old 07-14-2011, 04:29 PM   #1
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Running without AOS?

Hi guys,

Simple question (hopefully a simple answer!)

Can I run my car with the AOS blocked off? As in, can I just remove the J tube and block the intake and AOS outlet points? Car is running much better without it..

Appreciate your help!

Sam

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Old 07-14-2011, 05:56 PM   #2
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You do want to ventilate the crankcase. Without it, you'll notice a frothy brown substance built up on the dipstick and in the oil filler neck affectionately known as lung mustard. Basically, all the blowby combustion products and vaporized oil mixes the oil into an unattractive froth that isn't doing much in the way of lubrication.

Does it really need to vent into the intake manifold? Not really, but the vaccum helps to minimize aeration and is friendlier to emisions.

Last edited by blue2000s; 07-14-2011 at 06:00 PM.
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Old 07-15-2011, 03:01 AM   #3
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Ok thanks. I will decide if I install a new stock AOS, a motorsport one or a vac pump and catch can I have here...

Thanks again,
Sam
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Old 07-15-2011, 07:47 AM   #4
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it is my understanding that the GT3 porsche motor sports AOS can be installed with little modification and it is much more reliable and better than stock.

Catch can is cool and all, but as I understand it, offers not real benifit other than it replaces the AOS that might fail over time.
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Old 07-15-2011, 09:38 AM   #5
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Motorsports AOS is designed for 2003 & newer Boxster as they have a newer version AOS. OK to block off AOS temporarily if it fails (I keep a plug in my console) but it will run rich as this is a large source of unmetered air the DME counts on. Also the vacum in the case helps the piston rings seal.
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Old 07-15-2011, 11:41 AM   #6
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good dialogue, as i've had the same ideas kicking around in my head. i also *think* i understand the function of the aos - to vent any gases from the crankcase back into the intake for emissions purposes and to reduce oil aeration. the aos tries to pull out any oil that might be sloshing around and want to put itself into the intake. some questions, however:

- while the air introduced by the aos is downstream of the dme, the dme would also read the air/fuel mixture at the exhaust and adapt, so i'm uncertain if removing the additional air provided by the aos has any negative effect.
- lindsey racing also notes that the air introduced by the aos is laced with oil vapour which is non-combustible, so is actually a detriment to performance. at a minimum, it is not a participant in the combustion process, and removal would result in a lean condition (until the DME adapts) as additional air that can participate in combustion fills the void.
- i can see how vacuum would reduce aeration of the oil, but am uncertain re sealing of piston rings? would sealing of piston rings not best be achived by equal pressure on both sides?

from the above perspectives, you have to weigh the increases performance of not introducing crankcase vapours into the intake against the decreased cooling of frothy oil caused by lack of vacuum. then factor in the potential catastrophy of aos failure.

some half measures include a catch tank that vents to atmostphere; it reduces pressure in the crankcase but does not create vacuum. it does not have a 'seperator' but catches any splashed oil and has to be emptied occasionally. both canton racing products and lindsey racing sell catch cans.

another option i've seen is a closed catch system. it's a tank between the aos and the intake that catches any oil should the aos fail. i've seen them fabricated from a small tank and some wire mesh that serves to stop the oil while passing the vapour. a form of failsafe is all it really is.
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Old 07-15-2011, 12:22 PM   #7
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another thought,

The AOS catches excess oil that flies around the motor and vents the crankcase.
BUT keep in mind that the excess oil is not lost really, it is put back into the crankcase.

A catch can will remove oil from the system, and If you forget or have a lot of excess oil hit the system, you will be low on oil in the crank case.

The can seems to me to be more of a headache than its worth. If I had a dedicated track car- maybe, but for a regular car, its one more think to mess with.

AOS are cheap and easy to replace every 50K miles.
Just my opinion.
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Old 07-15-2011, 02:00 PM   #8
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We use vacuum pumps in racing and they give a surprising performance gain. Piston ring seal is better (very hard for oil under vacuum to pass rings) and the crank can actually spin in the crankcase with less restriction under vacuum.

I wasn't planning on running just a catch can, but an all out vac pump and catch can setup.

Failing the effort involved, I could just replace it with a new stock one...

Sam
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Old 07-15-2011, 03:01 PM   #9
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Quote:
Originally Posted by The Radium King
- i can see how vacuum would reduce aeration of the oil, but am uncertain re sealing of piston rings? would sealing of piston rings not best be achived by equal pressure on both sides?
The rings are seated within the piston so that when the pressure of compression and especially combustion pushes on them, they push harder on the cylinder walls and help to keep pressure from escaping into the crankcase. The little amount of vacuum from the ventilation system has a small additional sealing effect.
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Old 07-15-2011, 03:13 PM   #10
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i've also read that too much vacuum removes benficial oil mist from within the crankcase, so how would one size an external vacuum pump?
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Old 07-15-2011, 03:26 PM   #11
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Originally Posted by The Radium King
i've also read that too much vacuum removes benficial oil mist from within the crankcase, so how would one size an external vacuum pump?
That's a new one to me. I'd be surprised if you could have too much vacuum.

To Sparker's suggestion of the vaccum pump, cars that spend alot of time at WOT or cars with positive intake pressure (turbo) need to use a pump for ventilation.
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Old 07-15-2011, 06:52 PM   #12
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We've ran up to 25 inches of vac in a motor before, just makes it produce more and more power.

Sam
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Old 07-17-2011, 06:10 PM   #13
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another question i have is regarding whether porsche has upgraded the stock aos in any manner. i've seen an aos part number that ends in '04' so can i presume that it is on it's 4th redesign. has anyone checked to see if the aos failures that occur have been with old ones, new ones, etc. it may be that just going with the latest aos solves all the problems.
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Old 07-17-2011, 06:43 PM   #14
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Interesting point - would like to hear from people with failed ones as to which 'version' they had.

Sam

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