More stupid AOS talk...

[JFP in PA]

Originally Posted by The Radium King

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.

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.

[The Radium King]

Originally Posted by JFP in PA

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.

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.

[JFP in PA]

Originally Posted by The Radium King

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.

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, if anything you will find a slight pressure at speed, which is why I laugh at aftermarket cold air systems.

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.

A small catch can is only going to keep small amounts of oil out of the intake system; if AOS failure or track dynamic's lead to it inhaling liquid oil, not much is going to help as the small can will be overwhelmed, while too big a unit is going to result in you grenading the engine when all the oil disappears at speed. Either way, you lose.....

[The Radium King]

Originally Posted by JFP in PA

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.

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).

[JFP in PA]

Originally Posted by The Radium King

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).

Vacuum levels created by flow across an opening are totally dependent upon flow rates, which are never constant in the intake box. The heads on the 997.2 do not "vent" into the intake tube while the engine is running.

[The Radium King]

vacuum is not really constant anywhere in the system is it (except perhaps at idle)? how does the 997.2 aos operate?

[JFP in PA]

All the AOS operate the same way, they are a throttling device, not unlike the cooling system's thermostat, the take a relatively high vacuum level at the intake (well over 20 inches of mercury) and reduce it to a level (five inches of water) conducive to controlling the low tension rings pressure against the cylinder liners. Regardless of how high or low the intake vacuum level is, the AOS will hold that steady five inches of water in the sump. As a comparison, you cannot even see 5 inches of water on a vacuum gauge calibrated to inches of mercury. One inch of water is only 0.0734824 inches of mercury, so five inches of water is only 0.367 inches of mercury, not enough to even move the needle on a tool calibrated in inches or mercury, hence the need for manometer to read the lower vacuum levels accurately.

[B6T]

Nevermind...

[Quadcammer]

here are pics of my catch can solution.

[maytag]

Originally Posted by Quadcammer

here are pics of my catch can solution.

And? Looks good. Works well?

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[SpIcEz]

Did you connect the bottom drain valve of the Catch can to something or just leave it closed and check it once in a while?

[Quadcammer]

works well, not a perfect solution to stupid AOS, but will prevent big plumes of smoke if AOS sucks it in. Hasn't collected much oil yet, will check it again after 3 days at NJMP.

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

[maytag]

Originally Posted by Quadcammer

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

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

The idea is that under normal operation, no liquid oil is present, only vapor. The only time anything would be draining out the bottom would be if something goes wrong, so it's not like you're oiling down the people behind you (though i'd be lieing if I said I never did on a superbike, Haha)

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[B6T]

Originally Posted by maytag

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

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

[maytag]

Originally Posted by B6T

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

Yes.... sorta.
The catch can is to contain it. But if you don't want to check it and empty it regularly, you gotta get rid of it somehow.
No track will allow you to just drain that to the ground, but we've been venting/ draining to the exhaust for a very long time.
We're not talking about more than a drop of oil here and there. If you're getting more than that, something's wrong. And we're also not taking about pumping that oil through the motor and saturating it with fuel, as you suggested in your post before you edited it.

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