Go Back   986 Forum - The Community for Porsche Boxster & Cayman Owners > Porsche Boxster & Cayman Forums > Performance and Technical Chat

Post Reply
 
LinkBack Thread Tools Display Modes
Old 05-31-2019, 10:02 AM   #41
Registered User
 
Join Date: Feb 2005
Location: It's a kind of magic.....
Posts: 6,261
Quote:
Originally Posted by The Radium King View Post
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.

__________________
Anything really new is invented only in one’s youth. Later, one becomes more experienced, more famous – and more stupid.” - Albert Einstein
JFP in PA is offline   Reply With Quote
Old 05-31-2019, 10:24 AM   #42
Registered User
 
The Radium King's Avatar
 
Join Date: Apr 2010
Location: Canada
Posts: 3,121
Quote:
Originally Posted by JFP in PA View Post
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.
The Radium King is offline   Reply With Quote
Old 05-31-2019, 11:14 AM   #43
Registered User
 
Join Date: Feb 2005
Location: It's a kind of magic.....
Posts: 6,261
Quote:
Originally Posted by The Radium King View Post
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.....
__________________
Anything really new is invented only in one’s youth. Later, one becomes more experienced, more famous – and more stupid.” - Albert Einstein

Last edited by JFP in PA; 05-31-2019 at 11:38 AM.
JFP in PA is offline   Reply With Quote
Old 05-31-2019, 11:46 AM   #44
Registered User
 
The Radium King's Avatar
 
Join Date: Apr 2010
Location: Canada
Posts: 3,121
Quote:
Originally Posted by JFP in PA View Post
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).
The Radium King is offline   Reply With Quote
Old 05-31-2019, 12:03 PM   #45
Registered User
 
Join Date: Feb 2005
Location: It's a kind of magic.....
Posts: 6,261
Quote:
Originally Posted by The Radium King View Post
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.
__________________
Anything really new is invented only in one’s youth. Later, one becomes more experienced, more famous – and more stupid.” - Albert Einstein
JFP in PA is offline   Reply With Quote
Old 05-31-2019, 12:35 PM   #46
Registered User
 
The Radium King's Avatar
 
Join Date: Apr 2010
Location: Canada
Posts: 3,121
vacuum is not really constant anywhere in the system is it (except perhaps at idle)? how does the 997.2 aos operate?
The Radium King is offline   Reply With Quote
Old 05-31-2019, 01:05 PM   #47
Registered User
 
Join Date: Feb 2005
Location: It's a kind of magic.....
Posts: 6,261
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.
__________________
Anything really new is invented only in one’s youth. Later, one becomes more experienced, more famous – and more stupid.” - Albert Einstein

Last edited by JFP in PA; 05-31-2019 at 01:17 PM.
JFP in PA is offline   Reply With Quote
Old 06-11-2019, 06:08 PM   #48
B6T
Registered User
 
Join Date: Aug 2015
Location: Waterloo, Ontario
Posts: 193
Nevermind...

Last edited by B6T; 07-29-2019 at 12:48 PM.
B6T is offline   Reply With Quote
Old 06-12-2019, 04:14 AM   #49
Registered User
 
Join Date: Dec 2016
Location: Clifton, NJ
Posts: 1,135
here are pics of my catch can solution.
Quadcammer is offline   Reply With Quote
Old 06-12-2019, 05:04 AM   #50
Who's askin'?
 
maytag's Avatar
 
Join Date: Mar 2018
Location: Utah
Posts: 2,446
Quote:
Originally Posted by Quadcammer View Post
here are pics of my catch can solution.
And? Looks good. Works well?

Sent from my SM-G970U using Tapatalk
maytag is offline   Reply With Quote
Old 06-12-2019, 06:06 AM   #51
2001 Boxster S
 
SpIcEz's Avatar
 
Join Date: Feb 2017
Location: Montreal, QC
Posts: 29
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?
SpIcEz is offline   Reply With Quote
Old 06-12-2019, 07:14 AM   #52
Registered User
 
Join Date: Dec 2016
Location: Clifton, NJ
Posts: 1,135
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.
Quadcammer is offline   Reply With Quote
Old 06-12-2019, 07:47 AM   #53
Who's askin'?
 
maytag's Avatar
 
Join Date: Mar 2018
Location: Utah
Posts: 2,446
Quote:
Originally Posted by Quadcammer View Post
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)

Sent from my SM-G970U using Tapatalk
maytag is offline   Reply With Quote
Old 06-13-2019, 01:48 PM   #54
B6T
Registered User
 
Join Date: Aug 2015
Location: Waterloo, Ontario
Posts: 193
Quote:
Originally Posted by maytag View Post
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?

Last edited by B6T; 06-13-2019 at 01:55 PM.
B6T is offline   Reply With Quote
Old 06-13-2019, 02:06 PM   #55
Who's askin'?
 
maytag's Avatar
 
Join Date: Mar 2018
Location: Utah
Posts: 2,446
Quote:
Originally Posted by B6T View Post
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.

Sent from my SM-G970U using Tapatalk

maytag is offline   Reply With Quote
Post Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -8. The time now is 12:50 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Content Relevant URLs by vBSEO 3.6.0
Copyright 2024 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page