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My '88 951 has an original AOS with a hose on top that has what appears to be an air inlet or outlet near the 90deg elbow: it's a small circular grill. See arrow below. This hose is not available separately but comes with the whole vacuum metal "tree" I believe.
Is this what you guys are referring to as an "air control valve" for the AOS?
My '88 951 has an original AOS with a hose on top that has what appears to be an air inlet or outlet near the 90deg elbow: it's a small circular grill. See arrow below. This hose is not available separately but comes with the whole vacuum metal "tree" I believe.
Is this what you guys are referring to as an "air control valve" for the AOS?
Yes that's what it's called in the pet catalog my 88 has the straight elbow with out the T is yours a turbo S?
Can one retrofit the air control value to an 87 turbo. If so, do you happen to know if one need to buy an different AOS hose in addition to the valve? Thanks
No, mine is a model year 1988 plain old turbo. I believe it was built in August '87--at least that's what is stamped on the clutch disk I pulled a while back
Hi...I'm reaching out to those of you with far more experience than me. If I were to add an air control value to my 87 turbo AOS plumbing, would doing so likely have a positive impact on lowering emissions. I've just replaced the AOS with all new plumbing and had the turbo rebuilt. The car will be due for CA smog testing soon and I'd make the change before getting it tested if there's a benefit. Thanks...looking forward to hearing your opinions.
No. That item only serves to release excess pressure in the crankcase.
If your car looks and runs like "stock" you should have no problem with smog.
Agreed. Fresh ignition, stock tune, fresh O2, and a good cat, along with a no leaks or other issues, and the car will pass at the tailpipe. Confirming it's in closed loop, and that the O2 sensor hasn't gotten sluggish with age before the test, is always a good idea too. I once got super close to getting a friend's car to pass without his original 35-year-old cat. We tuned and tuned but one of the three numbers would always be just over the limit. We then put a new aftermarket cat on the car, and all three numbers were ultra low. Tweaking helps on the margin, but there's nothing like a new cat if you're struggling near the limits.
No. That item only serves to release excess pressure in the crankcase.
If your car looks and runs like "stock" you should have no problem with smog.
Why would you need a valve to vent crank case pressure when the line ends at the mouth of the turbo as long as that engine is running the AOS is seeing vacuum?
Well, my fears were for not. I passed today using the DEC cat from 2018, and think these were my lowest overall numbers yet! So, sample size of 1, but seems like the DEC cat is good for at least a couple rounds, maybe more. Admittedly I didn't put many miles on the cat in between tests, but I was worried just the age alone might reduce its effective. But, the results pretty much confirm it's working great still. As always, I did a full tune up fist, new ignition parts, new O2, and ran the test with all stock EMS parts. Very happy to have it behind me for another couple years!!
Why would you need a valve to vent crank case pressure when the line ends at the mouth of the turbo as long as that engine is running the AOS is seeing vacuum?
Ask Porsche why they put it there in 88 or 89+.
The intake end of the AOS sees only a very light vacuum in either the NA or 951 because it dumps on the atmo side where there might be a token venturi effect.
In studying the system I don't think Porsche ever really intended this to put a vacuum on the crankcase - they just wanted to route excess pressure (above ambient atmo) out of the block and back into the intake.
After all these years I am still confused as to why no one has tried a run of the mill PCV system on a 944 engine - it would put the crankcase under actual vacuum and help oil drain down from the head.
The intake end of the AOS sees only a very light vacuum in either the NA or 951 because it dumps on the atmo side where there might be a token venturi effect.
In studying the system I don't think Porsche ever really intended this to put a vacuum on the crankcase - they just wanted to route excess pressure (above ambient atmo) out of the block and back into the intake.
After all these years I am still confused as to why no one has tried a run of the mill PCV system on a 944 engine - it would put the crankcase under actual vacuum and help oil drain down from the head.
I'd beg to differ the AOS sees the same amount of vacuum the AFM barn door sees the tube dumps into the J-Boot 3 inches in front of the throttle body on the NA and the mouth of the turbo so it is under a constant vacuum.
Here's how porsche tried to fix it in a Boxster
I know it isn't 944 but they both work on the same principle separate the blow by gases from the oil in recirculate it back into the engine.
you skip thru the R&R of the AOS and he shows how the valve works n why.
Having your crank case in a near vacuum is actually and ideal situation every time a piston moves it has to fight 2 columns of air the one above it and the one below it you remove the column below it does two things allows the engine to rev faster and you have less aeration of the oil in the pan. http://nutterracingengines.com/racin...uum_facts.html
I understand crank ventilation and how the AOS works.
But I don't think there's really that much vacuum (measured in inHg) going on even in front of the turbo; more of a passive pulling of air out of the AOS tube by the larger mass of air going into the turbo or NA throttle.
I think if you put a vacuum gauge on the crankcase itself and watched it on a stock 944 it would be running very close to "ambient" atmospheric pressure and just venting positive pressure.
A real PCV connects after the throttle to provide real vacuum.
Several years ago I modified a spare oil filler cap to include a vacuum hose port that I connected to a spare MAP sensor. I logged crankcase pressure along with everything else. Crankcase stays mostly at ambient pressure as Spencer says. At high RPM and high boost pressure the crankcase pressure dips just below atmosphere but I'd certainly not call it vacuum.
Interesting discussion....to cut to the chase, is there any value in retrofitting an air control value to a standard '87 951 AOS set up? What would be the downside other that the cost of the parts and the time to install? Thanks.
06-22-2020, 01:00 PM
