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Old 08-22-2019, 10:04 AM   #1
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check my thinking here...AOS related

so, it seems I'm one of the lucky ones that has regular AOS issues, despite running the oil level low, which exacerbates my oil pressure drops.

I get the crankcase needs vacuum for sealing and low tension rings, so you can't or wouldn't want to just delete the AOS. However, I'm wondering if there isn't a way to get around feeding it to the intake.

Specifically, I'd like to run an electric vacuum pump that generates approximately 10in/hg of vacuum on a motor.

Basically, AOS stays, the "outlet" gets routed to a catchcan, and the outlet of the catch can gets routed to the vacuum pump. The intake port would then just be plugged.

By doing this, you retain vacuum to the AOS/crankcase, which should render it a moot point. The issue tends to be heavy throttle (i.e. no vacuum), followed by sharp lifts at high rpms before turning (i.e. lots of vacuum). The electric pump would generate steady vacuum.

My understanding is that these vac pumps don't love oil through them, so hence the catch can and hence buying at FCP if it fails. I'd also consider running this only at the track, since on the street I have no issues at all.

The only problem I can see is that since the vac pump is exhausting metered air, it may cause a lean condition.

Any thoughts on why this wouldn't work or is a bad idea?

Thanks!
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Old 08-22-2019, 11:10 AM   #2
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air in the crankcase isn't really metered air, so i wouldn't worry about trims or anything. i too have read that electric vac pumps don't like dirty air. one thing i have thought about, both for the aos and the brake booster, is the 987 tandem oil scavenge pump. there is a scavenge pump on each head, and the 987/986 pumps are interchangeable. the 987 pumps are 'tandem' because they also have a vacuum pump. the 987 uses them to provide vacuum for the brake booster (instead of taking it from the intake) but should work for the crankcase as well (they are just a mechanical impeller so should pass oil vapour, etc. ok). problem w most vac pumps is that they seem to all be designed to vent to atmosphere and don't have anything to connect a catch can too.

i think the bigger question is why you are chewing through aos's? i think the official party line is to confirm part # and always install the latest iteration of aos available (porsche has been frequently updating the part in an effort to address aos issues). so don't buy one that has been sitting on your local dealers parts shelf for 15 years and is not the latest version. and always go oem.

next, understand how the system works so as to troubleshoot. i got a great deal of info from a pair of threads on the rennlist 996 forum a few months ago. firstly, your intake system makes vacuum in two ways: at full throttle the air rushing through the intake pipe creates a venturi effect and a smaller pipe inserted perpendicularish into the intake pipe would see vacuum. this is the extra hoses you see on the intake on tip cars for example. 996's have these as well.

at idle the engine is trying to draw more air than it is being allowed (throttle closed) so vacuum is created; these are all the hoses on the intake that are after the throttle body. the aos connetion immediately after the throttle body tries to capitalise on both vacuum mechanisms (without dumping oil on the throttle plate).

so, crankcase wants vacuum, but not a whole lot of vacuum. in fact, the vacuum created at idle is too much. this is why the aos has a rubber diaphragm; as vacuum increases it sucks in the diaphragm and closes the connection between intake and crankcase. note that you want to be evacuating the crankcase at higher rpms and not at idle anyways; this is when the oil is getting churned up, turning to vapour, expanding, and creating back pressure on downward-moving pistons and blowing past rings or reducing power.

so why is the diaphragm failing for you more than others? something else going on w the system?

Last edited by The Radium King; 08-22-2019 at 11:29 AM. Reason: i swear the forum has a routine that randomly adds spelling mistakes to posts ...
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Old 08-22-2019, 11:29 AM   #3
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I have thought about the tandem pumps as well. Not sure if that can generate the vacuum necessary and might be a tight fit to the crossmember

I'm not sure what cause my car to be a problem. My AOS is plumbed in the standard way and the intake crossover tube is factory. I push the car fairly hard on the track on NT01s and while I don't see huge smoke clouds on track, at start up after every session, I get some smoking (not valve seals). And thats with a catch can. So either my car has tons of blowby (doesn't really seem to be the case), or there is something else going on.
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Old 08-22-2019, 11:36 AM   #4
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getting any ticking/tapping - perhaps scoring or oval cyls allowing some oil past (flat engine, so opportunity for oil to seep past rings while sitting if seal bad)? ie, confirmed that it is aos? i think smoke bombs are a result of scavenge pump in the head getting overwhelmed in a high-g sweeper and liquid oil (instead of vapour) getting sent out the vent in the head right when you step on the throttle out of a sweeper and open up the diaphragm again.
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Old 08-22-2019, 12:38 PM   #5
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certainly possible.

I might try the tandem pump.
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