dprantl

I don't know if this helps anyone or not, but since my valve cover gasket replacement, I replaced the plugs on the driver's side cover with the elbows, basically mirroring the passenger's side cover. The elbows on the driver side have now been running open and not connected to anything for a couple of months, and there has been no oil ejection from there at all even after 8psi WOT runs.

Dan
'91 928GT S/C 475hp/460lb.ft

Herman K

Perhaps this one is the scrubber to use..

Alan

No because there will be check valves (noted) to ensure unidir flow into the cam covers. At low blow-by throttle posistions the intake vac should be able to keep up and suck air in for circulation. It won't happen so much with more throttle and will stop as the crank starts to pressurize with blow-by - but it won't reverse. No oil to the MAF ever.
Don't know but it all depends on the degree of restriction to the breathing (which can be modified) and the degree of blow-by in a given engine too.
No I don't think so - this one goes straight to the intake, it provides more consistent vacuum at low/no throttle (~idle/decel) but its basically a known vacuum leak so it has to be calibrated. The 'leak' must be maintaned AS IS (or less) - and that was one of my questions - will maintaining the same orifice size in a different location/configuration still work adeuqately well?

ideally yes but its a combination of all the available exit paths - if you have the vac to the throttle body, the intake vac and the big Y - its the sum of these areas. At WOT they probably don't provide any more flow than an open atmospheric path of the same total size(maybe less than a short one) but I guess there is always the a reserve possibility of a direct blow-off pressure release to atmosphere.

probably but our oil choices are limited by other practical considerations - so I only care about what I run (15W50) which gets ingested freely...

I think the 928 is somewhat unique in config with the stock vac set-up - but it appears Porsche gave up on it and accepted excessive oil ingestion - probably reasoning that their target 928 drivers were a sane bunch of drivers who would not go to the track or care about regular oil purchases.

I've been reading a long time... time to get on with it and see.

Alan

Alan

Sounds like we agree on fresh air to the cam covers. I was considering a simple baffle plate in the crank vent as Herman shows - but I don't think that will seperate much. I agree at WOT the scrubber probably won't cope. My hope is it 'works' in the midrange and 'helps' at the top end. I always planned a big feed to a secondary seperator with oil drain back (Provent). Now the provent is pretty small capacity - but uses a filter element for seperation...?

You raise an interesting point on the restriction of the dense scrubber though. I've been more considering both the WOT restriction after the Provent and 'positive' evacuation of the internals at low throttle.

Alan

Alan

Thats interesting because it suggests the stock venting to the intake is ~keeping up (even on your SC) and the crank isn't pressurizing 'much'.. if it were it seem likely you be seeing at least some of the entrained oil exiting there. There may be some exhausting but if so it seems the oil has seperated out well by then.

Alan

dprantl

I should add that the passenger side elbows are connected together, which is connected to 1) hoses that run down under the car and end around the cats and 2) the oil filler. This leads me to believe as you say above that it is not necessary to do any oil separation on the driver's side, since the passenger side is sufficient (which is probably why Porsche put plugs on that side). This of course applies to a 32-valve 5.0 liter boosted to 8psi, so don't know how it would affect other 928 engines/setups.

Dan
'91 928GT S/C 475hp/460lb.ft

Alan

I'm not so sure.. the main reason we lose oil on the passenger side is that thats where the biggest outgoing vent is - secondarily it seems the engine rotation throws more oil (and mist) that way. On the face of it just switching the evacuation path to the drivers side may help with a cleaner venting OR maybe then the 'dirty' side just swaps based on flow. it would be an interesting test to compare oil loss by just swapping the same venting over to the drivers cam cover*. My gut says it would be a little better all the way up even at WOT due to better natural air/oil seperation on the driver side. However it still seems to me that having the primary WOT vent be out of either cam cover on a 928 is not ideal.

Alan

* however Erkka tells me that the domed GTS seperator fittings don't work without modification on the driver side.

Alan

Ahh just noticed you said this.. interesting. So with that draft tube also connected to the filler you have an extra blow-off path from the crank to atmosphere and at low throttle position some suction into the filler that may help to keep that draft tube outlet somewhat cleaner.

Alan

heinrich

Not correct. Passenger side is actually where you DON'T want venting .. we should vent from the driver's side only, because no oil is ever pushed up there. Passenger's side is a great way to allow oil to exit the engine.

Alan

BTW - this is interesting reading too:

http://www.republicsales.com/MANN/Provent_Brochure.pdf

Note that they have a variable pressure valve for closed systems (venting back to engine intake) that allows some control over the vacuum level (so as not to encourage more blow-by?).

Interestingly they talk of it as if its maintaing a pressure band e.g. regulating vacuum to a max level (they show ~ -7mbar) and pressure to approx 10mbar - these seem very low numbers to me.

There also seems to be an optional over pressure blow-off cap (not on the standard model) however this seems to blow-off on the feed side if I'm understanding the diagram - so seems its an emergency pressure relief not an operational one e.g. would also work if the filter is too clogged but it vents the provent input out the top of the cap. Can anyone with a Provent comment on what you'd have if there was a pressure relief in the middle/top of the cap?

Alan

bd0nalds0n

I got one of these, and maybe I'm crazy but it seemed like you need to remove the water bridge (or something around there) in order to install it. I may regret not taking the time to do so, but I didn't install it for that reason.

Alan

It goes under the fiiller neck & I'd assume you also have to take the intake off to get it in ...? if not it will be awkward and very tight...

Alan

PorKen

I was off a mind to leave them plugged when I was hot to pull a vacuum in the crankcase, but Louie's fresh air argument made sense.




A bit of an aside, but speaking of cam venting, I would like to see someone try making an oil drain out of the exhaust cam plug(s), with a pedulum switch triggering pump(s) to return oil to the sump in high speed curves.

I have to wait until I get a spare set of S3 cams for base circle reduction, at which point I might also have the rear bearing surfaces removed, a la S4+, as well.

Leon Speed

I am not sure I understand this feature, as I see it but the regulator closes when the intake pressure + crank case pressure < atmosphere by 7-10 mbar. It is operated by a spring so I guess it can be adjusted by using a different spring rate.

The cap itself has a relieve valve on the underside, so when it opens air flows out between the body and the cap.

Alan

I understood it to mean it would close the outlet (to vac) when the Provent is ~7mbar < atm.

On the other side of it as it pressurizes approaching WOT not clear what happens... if its already open at that point how can it regulate pressure to be lower unless it acts as a local blow-off valve (but I don't see how that would ever be emission legal).

Not sure what you mean about Intake + Crank pressure - it can only regulate the Provent Vs Atm right?

so that vents out before the filter? or bypases around it?

Alan