AOS modification.
#1
Intermediate
Thread Starter
AOS modification.
Hello all!
I have had a 996 for about 7k miles now requiring an AOS replacement. I do not know how long this AOS was in use for, but I have read threads covering the issue and talking about how frequently it happens. I wanted to know how the AOS works. Does the oil in the chamber fill up to a point where it floods the membrane? Even if the oil fills up to the bell-mouth, it affects the airflow and hence how pressure is applied to the membrane. I have been thinking about adding a one-way flow valve to the midsection of the AOS, connected to a small canister that can be drained after some use, and I would like to know if it would help the situation. I would like the community to chime in and tell me their thoughts on this.
Thanks!
I have attached a link to a 997.1 thread that has a cut section of the AOS. I have assumed that the 996.1 AOS would be of similar construction.
997.1 AOS
I have had a 996 for about 7k miles now requiring an AOS replacement. I do not know how long this AOS was in use for, but I have read threads covering the issue and talking about how frequently it happens. I wanted to know how the AOS works. Does the oil in the chamber fill up to a point where it floods the membrane? Even if the oil fills up to the bell-mouth, it affects the airflow and hence how pressure is applied to the membrane. I have been thinking about adding a one-way flow valve to the midsection of the AOS, connected to a small canister that can be drained after some use, and I would like to know if it would help the situation. I would like the community to chime in and tell me their thoughts on this.
Thanks!
I have attached a link to a 997.1 thread that has a cut section of the AOS. I have assumed that the 996.1 AOS would be of similar construction.
997.1 AOS
#2
Rennlist Member
The AOS is really just a polite and eco-friendly replacement for a draft tube, vent, or catch can as a sealed system.
If you never want another AOS failure, plug the vacuum side and run the oil suction side into a catch can that then vents to atmo. Or get the motorsports AOS.
If you never want another AOS failure, plug the vacuum side and run the oil suction side into a catch can that then vents to atmo. Or get the motorsports AOS.
#3
You can reroute the AOS vent line to the air filter box. No more excessieve vacuum (at zero throttle or idle) applied to the AOS diaphragm. The AOS will stay always in open position and never fail. In fact in theory this is also how it is done on the gt3 models. There the vent line is not subjected to huge intake vacuum as in the NA m96/97 engines.
No more greasy throttle body, no more oil and gunk in your intake. Only drawback is the air filter gets dirty a bit more at the spot where you let the rerouted line enter the air filter box. Further advantage is a colder air intake temperature as hotter AOS fumes are rerouted and have more time to cool down. Makes for some extra noticeable bhp. Engine runs smoother at idle as no unmetered air gets into the engine.
As said before the main advantage is no more oil in your intake. In case of an internal problem which causes the AOS lines flooding with oil and the positive crankcase pressure pushing the oil out, it ends up in the filter box rather then the intake to possibly cause a hydrolock.
You could put an additional catch-tank to filter out any oil getting past the AOS. Saves you some cleaning of the air filter box every time you change the air filter.
I have this set up and it saved my engine half a year ago when my head 1 scavenging pump gradually lost its pumping capacity. At some point the AOS spitted out oil every right hand turn I made. In the beginning just a tiny bit. But got worse and worse and the car started smoking of the oil ending up in the air box and starting to drip on the hot exhaust, I did not have an additional catch tank at that time. At some point I connected a small bottle to the line and only one right hand curve at moderate 35mph resulted in 200ml of oil in the bottle. Imagine that going into the intake. Hydrolock festival
It turned out the scavenging pump in head 1 was not able to pump the oil back fast enough and the AOS vent line in bank one got flooded with oil which found its way out via the AOS due to the overpressure in the crankcase.
To prevent the engine from this overpressured cranckcase scenario. There is now a second connection from the oil cap to the additional catch tank. So any overpressure in the cranckcase when the AOS gets flooded can escape via there.
if you analyze the setup of the 997.2 and it’s oil filler tube you will see it is exactly the same what Porsche has implemented in its later models. As well as the rerouting of the AOS line to a ‘lower vacuum at zero throttle’-position on the intake. It makes sense, not reinventing the wheel again.
No more greasy throttle body, no more oil and gunk in your intake. Only drawback is the air filter gets dirty a bit more at the spot where you let the rerouted line enter the air filter box. Further advantage is a colder air intake temperature as hotter AOS fumes are rerouted and have more time to cool down. Makes for some extra noticeable bhp. Engine runs smoother at idle as no unmetered air gets into the engine.
As said before the main advantage is no more oil in your intake. In case of an internal problem which causes the AOS lines flooding with oil and the positive crankcase pressure pushing the oil out, it ends up in the filter box rather then the intake to possibly cause a hydrolock.
You could put an additional catch-tank to filter out any oil getting past the AOS. Saves you some cleaning of the air filter box every time you change the air filter.
I have this set up and it saved my engine half a year ago when my head 1 scavenging pump gradually lost its pumping capacity. At some point the AOS spitted out oil every right hand turn I made. In the beginning just a tiny bit. But got worse and worse and the car started smoking of the oil ending up in the air box and starting to drip on the hot exhaust, I did not have an additional catch tank at that time. At some point I connected a small bottle to the line and only one right hand curve at moderate 35mph resulted in 200ml of oil in the bottle. Imagine that going into the intake. Hydrolock festival
It turned out the scavenging pump in head 1 was not able to pump the oil back fast enough and the AOS vent line in bank one got flooded with oil which found its way out via the AOS due to the overpressure in the crankcase.
To prevent the engine from this overpressured cranckcase scenario. There is now a second connection from the oil cap to the additional catch tank. So any overpressure in the cranckcase when the AOS gets flooded can escape via there.
if you analyze the setup of the 997.2 and it’s oil filler tube you will see it is exactly the same what Porsche has implemented in its later models. As well as the rerouting of the AOS line to a ‘lower vacuum at zero throttle’-position on the intake. It makes sense, not reinventing the wheel again.
The following 2 users liked this post by Silk:
Einsteiger (01-06-2020),
JeremyLynn (02-01-2020)
#4
Rennlist Member
This is a really useful post. I always thought there should be something akin to a catch-can setup that would prevent the AOS from being an issue, but I never took the time to track down the vacuum / vent line routing to see how it was plumbed together. Thanks for the post.
#5
Rennlist Member
You can reroute the AOS vent line to the air filter box. No more excessieve vacuum (at zero throttle or idle) applied to the AOS diaphragm. The AOS will stay always in open position and never fail. In fact in theory this is also how it is done on the gt3 models. There the vent line is not subjected to huge intake vacuum as in the NA m96/97 engines.
No more greasy throttle body, no more oil and gunk in your intake. Only drawback is the air filter gets dirty a bit more at the spot where you let the rerouted line enter the air filter box. Further advantage is a colder air intake temperature as hotter AOS fumes are rerouted and have more time to cool down. Makes for some extra noticeable bhp. Engine runs smoother at idle as no unmetered air gets into the engine.
As said before the main advantage is no more oil in your intake. In case of an internal problem which causes the AOS lines flooding with oil and the positive crankcase pressure pushing the oil out, it ends up in the filter box rather then the intake to possibly cause a hydrolock.
You could put an additional catch-tank to filter out any oil getting past the AOS. Saves you some cleaning of the air filter box every time you change the air filter.
I have this set up and it saved my engine half a year ago when my head 1 scavenging pump gradually lost its pumping capacity. At some point the AOS spitted out oil every right hand turn I made. In the beginning just a tiny bit. But got worse and worse and the car started smoking of the oil ending up in the air box and starting to drip on the hot exhaust, I did not have an additional catch tank at that time. At some point I connected a small bottle to the line and only one right hand curve at moderate 35mph resulted in 200ml of oil in the bottle. Imagine that going into the intake. Hydrolock festival
It turned out the scavenging pump in head 1 was not able to pump the oil back fast enough and the AOS vent line in bank one got flooded with oil which found its way out via the AOS due to the overpressure in the crankcase.
To prevent the engine from this overpressured cranckcase scenario. There is now a second connection from the oil cap to the additional catch tank. So any overpressure in the cranckcase when the AOS gets flooded can escape via there.
if you analyze the setup of the 997.2 and it’s oil filler tube you will see it is exactly the same what Porsche has implemented in its later models. As well as the rerouting of the AOS line to a ‘lower vacuum at zero throttle’-position on the intake. It makes sense, not reinventing the wheel again.
No more greasy throttle body, no more oil and gunk in your intake. Only drawback is the air filter gets dirty a bit more at the spot where you let the rerouted line enter the air filter box. Further advantage is a colder air intake temperature as hotter AOS fumes are rerouted and have more time to cool down. Makes for some extra noticeable bhp. Engine runs smoother at idle as no unmetered air gets into the engine.
As said before the main advantage is no more oil in your intake. In case of an internal problem which causes the AOS lines flooding with oil and the positive crankcase pressure pushing the oil out, it ends up in the filter box rather then the intake to possibly cause a hydrolock.
You could put an additional catch-tank to filter out any oil getting past the AOS. Saves you some cleaning of the air filter box every time you change the air filter.
I have this set up and it saved my engine half a year ago when my head 1 scavenging pump gradually lost its pumping capacity. At some point the AOS spitted out oil every right hand turn I made. In the beginning just a tiny bit. But got worse and worse and the car started smoking of the oil ending up in the air box and starting to drip on the hot exhaust, I did not have an additional catch tank at that time. At some point I connected a small bottle to the line and only one right hand curve at moderate 35mph resulted in 200ml of oil in the bottle. Imagine that going into the intake. Hydrolock festival
It turned out the scavenging pump in head 1 was not able to pump the oil back fast enough and the AOS vent line in bank one got flooded with oil which found its way out via the AOS due to the overpressure in the crankcase.
To prevent the engine from this overpressured cranckcase scenario. There is now a second connection from the oil cap to the additional catch tank. So any overpressure in the cranckcase when the AOS gets flooded can escape via there.
if you analyze the setup of the 997.2 and it’s oil filler tube you will see it is exactly the same what Porsche has implemented in its later models. As well as the rerouting of the AOS line to a ‘lower vacuum at zero throttle’-position on the intake. It makes sense, not reinventing the wheel again.
The following users liked this post:
*Eastep* (06-03-2021)
#6
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#8
Rennlist Member
#9
#10
I always wonder if the AOS can be moved to make it easier to replace in the future.
#11
Remember with the setup as explained above you completely take away the excessive vacuum created by a closed throttle body in combination with high rpm. The diaphragm is always open as it should be ideally and it will never fail. Unless there is another problem within the AOS you do not need to replace it.
I understand the probability that in the U.S. you might not pass vehicle inspection. In theory it should be okay as the air is recirculatid to the intake only via an alternative route.
#12
Rennlist Member
#13
Rennlist Member
#14
The AOS under driving circumstances (diaphragm open) is in fact a catch can, so I fail to see the influence on it during normal operation. Adding aan additional open flow catch tank just helps to catch condens any oil which is dispersed in the flow and which gets past the AOS under extreme driving circumstances.
Remember with the setup as explained above you completly take away the excessive vacuum created by a closed throttle body in combination with high rpm. The diaphragm is always open as it should be ideally and it will never fail. Unless there is another problem within the AOS.
#15
Intermediate
Thread Starter
Thanks all for chiming in. And thank you Joey for sharing the link to Stefans work.
I was thinking of integrating a removable catch can design to the OEM part. Stefan mentions that the bellows deteriorate due to exposure to acidic oil. I think adding a removable catch can to the existing separator chamber via a one-way vacuum shut-off valve will ensure that the AOS is always under the intended design condition, but will simply drain off the collected oil after each drive when there is no vacuum in the separator chamber. As the AOS works under intended design use, the operation conditions do not change at all, and hence there is no need to worry about the fuel trim. The device will keep functioning as stock, but gives the flexibility of draining out the oil after use. Draining the oil will ensure there is no change to flow characteristics inside the chamber, or oil churning up to turn acidic. Good oil maintenance practice, which is generally expected, will ensure that the bellows are not exposed to acidic oil. The one-way valve will also ensure that the oil from the additional catch is not sucked back in.
I certainly have not had a lot of time with the vehicle, and I have not spent enough time understanding the situation. But let me know what you guys think about it.
I certainly have not had a lot of time with the vehicle, and I have not spent enough time understanding the situation. But let me know what you guys think about it.