996 Turbo's AOS... where is it?

Hi all, It seems that dry sumped engines, like the 996 Turbo, don't have oil smoke problems. Is that true? Looking at the 996 Turbo PET i even couldn't find the AOS (air/oil separator) diagram! Can anyone tell me if this 996 107 926 00 is the part number?

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Not sure. I have only my time with my Turbo, one year (almost) and around 24.5K miles and the engine has emitted oil smoke, a cloud and not much of it, just a couple of times. More than once but I'm not even sure it it does it more than twice. In the same time frame my 02 Boxster has done this I estimate close to 10 times. And I know how to lessen the possibility and make every attempt to avoid giving the engine any chance to make this smoke. Near as I can tell the Turbo engine does not have an AOS at least not one like the other non-Turbo Porsche engines have. It appears the scavaged oil is routed to the oil/water heat exchanger which acts to not only cool the oil but remove the air. This has a breather hose that runs to the oil tank. Might also add the oil from the heat exchanger is then routed to the tank as well. I searched my Turbo parts numbers docs for the part number and I didn't find it. The part number is the Motorsports AOS. Expensive. And it doesn't look like it will even fit the Turbo engine. Sincerely, Macster.

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Thank you Mcaster! I find ridiculous to have that kind of concern in a modern car, let alone in such expensive car... i'm very disappointed with Porsche! Yeah! The conclusion is simple... whenever the Porsche has dry sumped engines there's no, or very little, smoking problems water cooled or not, it even doesn't need an AOS. (I think that Porsche air-cooled engines all have dry sumped engines therefore they don't have smoking problems.) Now, when Porsches don't have dry sumped engines they completely rely on the AOS quality which is, for my BIG surprise, generally poor for space and cost savings reasons. Why dry sumped engines don't need AOS i don't know for sure... it seems that the tank itself acts as a large one and that there is several distributed venting points throughout the oil lines - crankcase pressure is released through the oil scavenge pump which routes the oil to the tank allowing the air to be vented there and along the lines through those venting points. But then again, dry sumped engines are costier and need more space for tank install. With non dry-sumped engines the much more expensive Motorsports AOS seems to solve the oil ingestion smoking problem, not the oil starvation one though! Do you agree Macster?

Flat, opposed cylinder engines smoke, dry sump wet sump, air-cooled, oil cooled, water cooled 2 cyl 4 cyl 6 cyl xx cylinder. Do some searches, on an air-cooled forum. Air-cooled engines smoke at times for one reason or another. They smoke due to over filling, they smoke due to premature failure of the valve guides, they smoke due to oil leaking past the oil pump when started after not being run for awhile, they smoke from the low side bank of cyl after being on the side stand for awhile (BMW R bikes, Honda Goldwing) etc... To address your original statement dry sump Porsches have a built in AOS in the oil tank. Turbos smoke due to other reasons, turbo seals being one, over filling being another.

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Well, I'm not that upset by the fact that upon occasion my Boxster engine smokes upon startup. I wish it never would of course, but it happens so rarely than I can live with it. And this characteristic is not sufficiently negative in my opinion to cause me abandon the brand for another brand of car. My second hand info and admittedly rather thin info at best is that the air-cooled engines don't appear to smoke upon engine start as often as the water-cooled engines do. Same goes for Subie engines. Now a problem is that many air-cooled engines have alot of miles on them and alot of miles for an air-cooled engine is not the same as alot of miles on a water cooled engine. This is my clumsy way of trying to point out that reports of air-cooled engines smoking upon start up may be due to engine wear, rings or more likely valve guides/seals and have less to do with the efficiency or lack of or presence of an AOS. I just don't believe the majority of oil smoke upon start up by any Porsche flat 6 engine water cooled engine is from oil getting past the rings. If this were the case these engines would smoke at every startup. There are 6 (obviously) cylinders. Each one receives a copious amount of oil while the engine is running courtesy of an oil jet aimed right at the piston. Thus when the engine shut off each piston and cylinder is very wet with residual oil. Now if the piston ends up at the bottom of its stroke the odds are good the oil will simply drain away and flow down onto the floor of the crankcase above the oil sump. If the piston stops at the top of its stroke this oil will still drain of course but it will drain down to the exposed cylinder wall surface where unless the car parked on a heck of a slant where it will drain down onto the floor of the crankcase. If it went the other way, past the rings through the tiny space/clearance between the piston and the cylinder wall -- from gravity cause there's no other force driving this oil in that direction -- then these engines would smoke every start up. Thus I believe the oil smoke upon startup is the result of an inefficient AOS coupled with an engine design that places a bigger load of oil vapor in the crankcase fumes that allows too much oil vapor to make it through the AOS and into the intake manifold where the oil vapor impacts the intake manifold walls. Now while the engine is running this oil vapor or any oil that is deposited on the walls is pulled into the engine's combustion chambers and burned with no visible trace. However, upon engine shut down any oil on the intake manifold walls is then free to drain down and collect on top of the intake valves or even flow past them of they are open and and collect in the combustion chamber at the bottom of the cylinder, where of course upon engine start it will produce oil smoke. Dry sumped engines generally benefit from a lower crankcase pressure courtesy of the scavage pumps. There is little need to provide lower crankcase pressure by using engine vacuum. The wet sumped engines need a lower pressure to help induce the water and unburned gas to boil out of the oil that resides in the sump and the hot oil as it is slung about the crankcase and under the camshaft covers. The oil tank -- or the engine crankcase or both -- do need some vacuum applied to provide lower than atmospheric pressure to help entice the more volatile water and gas to boil out of the oil but this ventilation system does not have to deal with high levels of oil vapor -- the oil vapor is handled by the scavage pumps which collect this vapor and route it via return lines to the oil/water heat exchanger which allows the air to leave the oil or the oil to leave the air and then this oil is fed back into the oil tank. Thus there is no need to have a huge amount of air flowing through the crankcase to provide the speed necessary to cause the oil laden vapor to swirl around the chamber of the AOS to help separate the oil from the fumes. So for the dry-sumped engines a simpler positive crankcase ventilation mechanism probably suffices. I have no experience with the Motorsports AOS. All I know is it is expensive. I believe I have read posts from people who have used it and they report good things about its performance and reliability. Were I tracking a Porsche equipped with wet-sumped engine I might be inclined to replace the stock AOS with this Motorsport one. I don't know. I'd do some research first to ensure there was a problem for which it would provide a solution. The oiling problems are not going to be solved by any AOS. In fact that the AOS comes in for some of its negative press is due as much to the engine's oiling system shortcomings as it is to the AOS's shortcomings. The oiling problems are inherent in an enigne designed for use on the street being subjected to the realities of track speed and the very high g-forces that arise from this speed. My track experience is very dated. But I learned early on with engines on the track that before you make them produce more power, you make them reliable. They have to survive the experience. And for most street engines this means addressing any oiling issues and almost all street engines have oiling issues when they are on the track. A deeper sumped oil pan, longer pick up of course, replacing the stock oil pump with a higher performance part, fitting an oil cooler, fitting an oil catch can a sort of AOS to keep the amount of oil being fed to the engine by the crankcase ventilation system to a minimum, and to prevent any chance of any oil being ejected by the engine and oiling the tires of one's car or possibly the tires of a following vehicle. The Boxster or 996 engine is no different in this regard. They are pretty good engines for the street. They can be pretty good engines for the track as long as one intelligently addresses their weaknesses and oiling and crankcase ventilation are some of those weaknesses. Sincerely, Macster.

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I totally agree with you on those aspects. However, I have one doubt: do the oil/water heat exchanger have venting ports to the atmosphere? Is that way it allows the air to leave the oil? As for the flat, opposed cylinder engines i once had an Alfa Romeo in a time when Alfa Romeo engines were flat and i didn't remember to ever have seen a smoke out of the tailpipe - it had double carburetors though. Also, it seems that Subarus are much less prone to this problem. In any case, I wonder why today only Porsche and Subaru use flat, opposed cylinder engines... reliability issues, like those we are discussing? Are they more expensive to make?

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Not clear from the drawings and I'm too lazy to go downstairs and look at my Turbo's engine -- not that could see the heat exchanger in amongst the tightly packed engine compartment anyhow I suspect (and I have pics of a Turbo engne out of a car (Cab) but I didn't think to snap detailed pics of the vent hoses and their connections (next time I get a chance though I will!)) -- but I can tell you no modern engine sold in the USA has any fumes from the crankcase vented to atmosphere. All fumes from the inside of the engine are routed to the intake manifold after the MAF and throttle body to be burned in the engine. I don't know the history of Subie engines but Porsche inherited the flat engine gene from VW and for most of its engine needs stayed with what it started with, with what it knows best. Although it has done a few more conventional engine layouts and done a very good job of them too. A flat engine is not quite as strong as say a V8. Obviously the Porsche engines are strong enough but in making then stronger they get a bit heavier. And having an engine split in two through the vertical line through the center of the crankshaft is well at least a bit more expensive to tool up and make. IIRC the 2.5l Boxster engine weighed in at around 402lbs. That's near V8 territory. The Turbo engine weighs in at 570lbs. That's heavy for a 3.6l engine (granted turbo-charged). A flat engine has a lower center of gravity, which is nice. A big old V8 fitted with DOHCs is a big wide thing especially up top, which means the center of gravity higher. Those overhead cams really add the weight. Pop the hood on say a modern GTO (I owned an 06 for a couple of years) or Corvette or even I guess the new Camaro and the engine is really quite compact with its OHV layout. Tiny valve covers compared to the huge camshaft covers of its SOHC/DOHC competitors. But still the V8's tall and the weight's up higher. Back to the flat world: A flat engine is wide. Hard to find a wide enough location to well locate the engine. Porsches with the rear engine location have plenty of room. Not constrained by the rear (or front) wheels/suspension/steering. Subie engine is mounted in the front but it is a very tight fit. Mid-ships is ok but this means the car will be strictly a 2 seater (exception the new Lotus Evora). You didn't ask but some wonder about the flat 6's future, and what new engines will Porsche use. Nowadays designing a new engine and getting it through all the various certification tests required by the various agencies in the regions where the engine will be used is too expensive and too long a process for the small numbers of cars Porsche sells. So Porsche will likely if it moves away from the flat 6 engine or expands its engine line will source engines it needs from VW (Audi) of course. There is already the rumor of a 4 cylinder Boxster (sub-Boxster) using an Audi turbo-charged straight 4 cylinder engine and I've even read reports (rumors really) there could even be a 3-cylinder Porsche. In some markets Porsche can get away with this and it needs to smaller engined cars to reduce its fleet average CO2 emissions and improve its fuel economy. Porsche had hoped to piggyback on VW's larger fleets for its numbers but the EU ruled against this. Porsche must stand on its own in this regard. Sincerely, Macster.

Opposed engines are more expensive to manufacture. Take an engine block as an example. A straight 6 would require one machining setup for the one piece block bore & hone operation. Then all 6 bores can be run automatically. An M9X flat 6 has two blocks (or 6 cylrs in air-cooled Porsche plus two sides of the case plus the case mating halves) Two heads or 6 in air-cooled. (V6 negates this still one block) Many other parts are also multiple pieces with multiple setups. Then all of the pieces must be assembled. It starts to add up. As far as smoke on startup there are many reasons this can happen. It does not always have to be worn components. Else why does Porsche spec 1 qt of oil per 600 miles. If your car is burning that much oil it will probably smoke on startup sometimes. Is it worn out? Porsche doesn't think so. Macster, Oil at the bottom of a static horizontal engine cyl has no more incentive to run towards the crank case then towards the heads. It will have a harder time getting past healthy piston\oil scrapers\compression rings but not impossible. Usually there is not enough to produce visible smoke for any length of time. I can't debate the AOS on our cars I don't know enough about how they are intended to work.

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Well, I got curious and went downstairs and looked more closely at my Turbo's engine, what I could see of it. I can see a portion of the oil tank and you know what I think? There's more to that tank that just storage. It is big, tall, and the oil filler tube doesn't intersect it up high but down low down nearly as best I can tell close to about half its height. See pic of view of Turbo engine from the tranny end and this shows the back of the oil tank. See how tall it is? Tall sucker isn't it? And rather thick too. At least 2.5" and maybe closer to 3". Oil capacity is around 9 quarts and there's more than 9 quarts of room in that tank. As I pointed out above the oil filler tube connects to the tank (on the other side of which I have no pics) about half way up the tank's height. This means -- if my reasoning not horribly flawed -- the tank's oil level is somewhere below this oil filler tube level. That leaves alot of tank room. I bet there's an AOS of some kind in that tank. All vent hoses connect to it or connect to hoses that connect to it. One can see some of the hoses connected to the tank's right side near very near the top of the tank. I'd love to cut a Turbo oil tank open but I'm not about to buy a new one to cut open. I bet one of those suckers is very expensive. Sincerely, Macster.

Do Aircolled engines smoke? https://rennlist.com/forums/search.php?searchid=5847060

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I don't think worn components accounts for any significant oil smoke upon startup. In fact a worn engine or one with worn components would if it smoked upon startup would surely smoke at other times and I can't recall any who's posted a report of his engine smoking upon startup smoking at any other times. And you make my point again regarding the likelihood of oil getting past the granted stationary rings and piston into the combustion chamber. My take on this smoking thing is it is mainly (I would like to say only but there'll be an exception) due to oil getting into the engine via crankcase fumes routed to the intake manifold, crankcase fumes the AOS has failed to remove from the fumes a significant amount of oil vapor. I've mentioned it before but I'll mention it again: The Porsche techs I talk to about this tell me it is common to have an owner bring in a car serviced elsewhere and sometimes this means another Porsche dealer with the oil level high, at or over the max level mark. And more than a few engines the techs have stripped down to the point they can reach the inside passages of the intake find quite often the surfaces are wet with oil. Sincerely, Macster.

https://rennlist.com/forums/search.php?searchid=5847060 I thought this one would work. I just went to the air-cooled forum and did a search on "smokes at start" and there were many "why does my 911 smoke at start?" threads. And just as many opinions.

Macster There must be a cutaway view of your oil tank somewhere. The early tanks had a screen above the oil level that the return oil flowed through to act as an AOS. See #10 in the PP illustration http://www.pelicanparts.com/914/Part...system_big.htm

I was thinking the most common reason for smoke on start up would be worn valve guides letting oil by when the engine is off and oil draining back to the sump. Then figuring the cylinders are horizontal the flow wouldn't be down the length of vavle guides since they are horizontal also.

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Spoke to Porsche tech today (this AM) and he's seen a Turbo oil tank cut away at training. Says it is quite complex with lots of baffling/channels to separate the oil vapor from the fumes that get routed to this tank. There is obviously not as much vacuum applied to this tank either cause one can remove the oil tank filler tube cap while the engine is running and the engine does not react. Remove the oil filler tube cap from a regular Porsche engine and the engine will stumble and run rough cause the cap being removed represents an air leak. Air going through the open tube gets routed straight to the intake manifold. Sincerely, Macster.

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The area the valve guides reside in is not connected to the oil sump. AFAIK the only path to the sump for oil that ends up under the camshaft covers is via the scavage pumps, one under each camshaft cover. The exhaust and intake valves are not horizontal either IIRC. They are canted so the intakes point down and the exhausts point up. That is the stem/lifter end of the intake valve is higher than the head end and the stem/lifter end of the exhaust valve is lower than the head end. While the engine is running engine vacuum can pull oil past worn intake and exhaust valve guides and seals into the combustion chamber. It can also pull oil past the rings/piston. Sincerely, Macster.

they will blow a huge puff of smoke if they are started, not allowed to run for long and shut off... so will just about any engine... is this whats happening to you?