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I've had issues with oil ingestion issues on a track boxster with a 996 3.4 engine swap and an LN +2qt deep sump. The AOS was replaced with the real Porsche part but the white smoke problem continues. I contemplated AOS deletion over the motorsports version given there are many posts noting the super expensive motorsport version will fail as well and others that simply plugged the lines, noting it was purely emissions related. 


However, one post made me rethink when someone mentioned the AOS helps seal the low-tension piston rings used in M96 engines and that the mechanism is used to collect and reburn blow-by gases. I made a diagram to help me better understand why the engineers put in there in the first place and also figure out how oil actually gets sucked up. We know if it wasn't needed, Porsche would have removed it to save weight (like they do in every aspect of the car).

 

Looking for validation that the diagram is correct and and changes edits needed. Thanks in advance. 

 

 

 

 

AOS996_986whitebackground.png

Edited by xmac
added white background to image
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14 minutes ago, xmac said:

I've had issues with oil ingestion issues on a track boxster with a 996 3.4 engine swap and an LN +2qt deep sump. The AOS was replaced with the real Porsche part but the white smoke problem continues. I contemplated AOS deletion over the motorsports version given there are many posts noting the super expensive motorsport version will fail as well and others that simply plugged the lines, noting it was purely emissions related. 


However, one post made me rethink when someone mentioned the AOS helps seal the low-tension piston rings used in M96 engines and that the mechanism is used to collect and reburn blow-by gases. I made a diagram to help me better understand why the engineers put in there in the first place and also figure out how oil actually gets sucked up. We know if it wasn't needed, Porsche would have removed it to save weight (like they do in every aspect of the car).

 

Very interesting post, thanks for starting this discussion.  I just replaced the AOS on one of my cars last week so this topic is of interest to me at the moment.

 

It is definitely true that the AOS helps collect and reburn blow-by, which helps reduce costly emissions and increase efficiency.

 

When the AOS fails or starts to fail the vacuum pressure increases massively which can cause oil to actually get sucked into the intake and raises the possibility that you could destroy your engine by way of hydrolocking it with oil:

 

 

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58 minutes ago, xmac said:

I've had issues with oil ingestion issues on a track boxster with a 996 3.4 engine swap and an LN +2qt deep sump. The AOS was replaced with the real Porsche part but the white smoke problem continues. I contemplated AOS deletion over the motorsports version given there are many posts noting the super expensive motorsport version will fail as well and others that simply plugged the lines, noting it was purely emissions related. 


However, one post made me rethink when someone mentioned the AOS helps seal the low-tension piston rings used in M96 engines and that the mechanism is used to collect and reburn blow-by gases. I made a diagram to help me better understand why the engineers put in there in the first place and also figure out how oil actually gets sucked up. We know if it wasn't needed, Porsche would have removed it to save weight (like they do in every aspect of the car).

 

Looking for validation that the diagram is correct and and changes edits needed. Thanks in advance. 

 

 

 

 

AOS996_986whitebackground.png

 

 

On the track,  it is not uncommon to see one of these engines blow some oil smoke.  Sometimes this may only happen in certain corners, and not in others.  Problem stems from too much oil being retained in the upper engine area (read cam covers), where it cannot drain down fast enough through the M96's oil scavenging system, and the AOS becomes overwhelmed.  This is one of the reasons Porsche developed the X51 package, which uses a "northwest passage" extra scavenging system to get the oil back down where it belongs.

 

One trick a lot of track rats learn is to drop the oil level in the sump a couple of bars to help control the oil level in the heads.  On the street, we always filled customer's sumps to at least two bars below the max line for this exact reason.  We also tested everyone of the cars that passed through the shop for sump vacuum levels, as the AOS deteriorates and then fails a lot more often than people expect.  It should be considered a maintence consumable item that needs to be replaced before it begins to fail.

 

One comment on your diagram:  The actual intake vacuum signal at idle can be as high as 26-28 inches of mercury, and one inch of Hg is 13.6 inches of water.  The only time the intake would drop to anywhere near the levels in your diagram would be a wide open throttle.  So the AOS is not only separating oil and returning it to the sump, it is throttling the intake vacuum down to around 5 inches of water in the sump to help the low tension rings seal.

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3 minutes ago, JFP in PA said:

We also tested everyone of the cars that passed through the shop for sump vacuum levels, as the AOS deteriorates and then fails a lot more often than people expect.  It should be considered a maintence consumable item that needs to be replaced before it begins to fail.

 

I agree with this and never gave this much thought until I started to research it in depth a few weeks ago.

 

In the AOS I replaced last week in a different car I wasn't getting any CEL's and the car drove just fine.  But when I pulled the intake hose a bunch of oil streamed out everywhere.  When I inspected the AOS I could see it was no longer working 100% and allowing oil to bleed into the intake.  I will always replace my AOS every 40 or 50K miles going forward even if there is nothing wrong with it.

 

Also meant to say in my last post my 996 4S used to blow some oil smoke on the track, especially after the hardest laps.

 

  

3 minutes ago, JFP in PA said:

So the AOS is not only separating oil and returning it to the sump, it is throttling the intake vacuum down to around 5 inches of water in the sump.

 

As far as the throttling the intake vacuum down is this concept where the "breather" term comes from?

 

 

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A "breather" is simply and atmospheric vent, not unlike an open window.  The vacuum signal "throttling" done by the AOS is much like the thermostat in your home: It keeps the environment in the sump at a controlled level.  If there were no vacuum level, the low tension rings would not seal, pressure would build up, and engine oil seals would blow out.  Too high a vacuum signal and the intake starts inhaling oil as Jake showed in his video and the engine kills itself.  The AOS vacuum throttling function keeps the sump "just right". 😉

 

That part in the diagram marked "breather" is actually how the sump vents any pressure that might build up after the engine is shut down and there is no vacuum.  The pressure would wend its way around the system and back into the intake manifold, and back out into the world.  It all has to go somewhere; the engine cannot function as a sealed vessel. 

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Thanks.  Yes from what I saw taking apart the AOS the diaphragm with the tension from the spring is what throttles the vacuum to the specified level.  In the event of a torn/cracked diaphragm I have seen some people don't replace the part and only replace the diaphragm/spring.  While much cheaper, I personally think this is not a very good idea since I have seen problems caused by this where the diaphragm was a different material or the spring had a different size or tension, which can cause things not to work correctly.

 

There are several reasons actually but one of the reasons I replaced this part preemptively in my car is because when a AOS fails it can cause a lot of excess pressure to build up in the case which can damage the rear main seal (which is a PITA to get at).  I have never seen that cited on the M96 but in theory I would think a bad AOS on a M96 could blow out a RMS (or any other seal for that matter) too.

 

Nice diagram by xmac.  That's a good high-level visual of how it works.  Thanks for doing that.

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One question I had in the revised drawings is on the hard turn I see a lot of the oil being forced over.  In my car there is a baffle in the sump which keeps the oil from doing this.  Does the M96 have this as well (I know xmac said he's running the LN deep sump)?

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The baffle maintains oil near the pickup so that there is no interruption, but the oil outside will still slosh. I have the LN add-on with the windage tray that is supposed to further help. From what I gather from Jeff, I bought a +2qt oil pan to run less oil on the track by underfilling. LOL


Mac

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I just looked at what you have -- that is a nice setup.  Yes, you are correct it would still slosh outside the baffle.  I guess they are not mutually exclusive but I agree with you that is pretty funny about buying a +2qt only to underfill, it sounds funny at face value at least.

 

LNENGINEERING.COM

<p>The Bilt Racing 2 QT Deep Sump Kit provides similar oil pressure drop protection as an Accusump kit, but takes oil control to a whole new level. We've had Pro Racing teams use our kit to solve their oil starvation and aeration problems completely, when

 

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2 hours ago, xmac said:

The baffle maintains oil near the pickup so that there is no interruption, but the oil outside will still slosh. I have the LN add-on with the windage tray that is supposed to further help. From what I gather from Jeff, I bought a +2qt oil pan to run less oil on the track by underfilling. LOL


Mac

 

As each line on the oil level gauge is a small fraction of a liter (somewhere around  0.2 L), by adding a two quart (or liter) sump extension, and then lowering the level two bars from the full line, you have still increased your total oil capacity by around 1.6 L; and as any increase in oil capacity is generally a good thing, you are still ahead of the game. 😉

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