Originally Posted by The Radium King (Post 596545)

some good recent threads on the rennlist 996 forum on this. basically, if you replace the aos with a catchcan (or an aos with a higher-flow diaphragm) then you introduce too much crankcase vapour during high vacuum idle situations (ie when you come off throttle after wot) and screw up idle fuel trims.

if you put a catch can after the aos then it has to be of sufficient size/volume to make a difference should the aos fail.

one solution that seems to be effective is to vent the aos to the airbox. i have yet to research it to any degree (i am of the 'just use the most recent version of the aos and you will be fine' school) but apparently it doesnt foul the air filter too much, and the airbox is of sufficient volume to hold the oil should the aos puke up. not certain if there is sufficient vacuum at this location to positively vent the crankcase, or if this vacuum is even necessary.

Originally Posted by JFP in PA (Post 596547)

It DOES matter, without the five inches of crankcase vacuum, the low tension piston rings will not seal properly, leading to loss of power, blow by into the crank case, etc. I can see adding a puke can between the AOS and the intake vacuum port, but not rerouting the vacuum source elsewhere.

Originally Posted by The Radium King (Post 596551)

well, i do understand that porsche uses low tension piston rings, and that they require vacuum to work, but have yet to find a definitive source to confirm the 5" of vacuum value, and also what vacuum is present in the airbox. note that vacuum is present in the intake tract in two ways; at idle with the throttle closed, and as well due to venturi effect as air rushes by. the aos doesnt really want idle vacuum (too high, and vacuum more necessary at higher rpm) however aos returns at that location for emissions reasons.

there is also the more thoughtful perspective: the detrimental effects of blowby vs the detrimental effects (hydrolock) of total aos failure. again noting unless a catchcan can hold a substantial % of the 9 litres of oil in the engine then your 2.5/2.5/2.7/3.2 litre engine is still at risk.

Originally Posted by JFP in PA (Post 596556)

First of all, the air box is open to the atmosphere, there is no vacuum, only flow. Check it some time with a digital manometer.

Five inches of water is the nominal value for a new, factory AOS. Higher or lower values lead to problems, so you have to assume 5 inches of water is the system's designed "sweet spot", and which is actually controlled by the AOS itself.

Originally Posted by The Radium King (Post 596558)

flow across the end of a tube can create suction, so if the tube is introduced at an appropriate place in the airbox or elsewhere upstream of the throttle then there is opportunity for vacuum, but no idea if sufficient. note the 997.2 that appears to vent the heads directly to the intake tube while the crankcase goes to the plenum via an aos (if i am reading the parts diagrams properly).

Originally Posted by Quadcammer (Post 597399)

here are pics of my catch can solution.http://986forum.com/forums/uploads02...1560341693.jpg

Originally Posted by Quadcammer (Post 597424)

I do not have it drain into the oil sump. I'm not sure of a good way to do that.

Originally Posted by maytag (Post 597429)

Don't.
Drain it to the exhaust, same as we've done on race cars and race bikes for decades.

Originally Posted by B6T (Post 597516)

Whats the point of the catch can then? To throttle the collected oil back into the exhaust?