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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.
Agreed. I guess anything less than ambient is "vacuum" but it's not much, and even less with a big mouth MAF. In my car, like many, I get positive pressure in the crankcase due to blow-by. It kept pushing the dipstick up out of its tube until I enlarged the hole at the top of the AOS. Any slight vacuum at the j-pipe is easily overcome by blow-by. And my motor is relatively healthy, doesn't burn oil, etc. Maybe that's why Porsche added the check valve, but I don't know.
Originally Posted by thom4782
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.
Porsche seemed to think it was a good idea. I do wonder if they were solving the problem I had in a different way. Even if the dip stick doesn't push up at factory boost levels, pressure in the crankcase prevents the turbo from draining well, so maybe they were just adapting to what they were seeing on the earlier cars. Pure speculation though.
Our 928 brothers have all kinds of issues with crankcase pressure but in my casual observation everybody (944 too) does it backwards...trying to vent out the cam tower etc.
Whereas again there are 60+ years of vacuum-based PCV systems working great in the rest of the auto world.
I'm about done playing with 944 engines but I think it'd be a worthwhile experiment for someone to try.
Hey all. I don't know if this thread is still active or not, but I just came by to give updates on the DEC PO82609B cat that was recommended to me by Tom 2 years ago. Thought i'd come by and post my results after two years for Cali smog. Car is a 1988 Turbo S 2019 Smog results
2021 Smog results
Heads up that I only put 2000miles on the car between the two smogs. Didn't take it out much over covid. I'll come by again in 2years with the same cat if people would like to see how the cat holds up another 2 years.
06-25-2020, 02:26 PM
