GregBBRD

Certainly could be done....although once the oil is out of suspension and sitting on the plenum floor, it probably does not go back into suspension and on to the combustion chambers.

Seems like every time the plenums are off (spark plug changes), the oil runs out of the driver's side plenum. It's a bit more difficult to get the oil out of the passenger side plenum, because it has no low fitting for the power brake hose like the driver's side has.

What I'm trying to eliminate is the oil that stays in suspension and actually makes it to the combustion chamber.

Again, because of the poor design of the crankcase ventilation system by Porsche, it is a tall task.

The Forgotten On

The crank case is always positive pressure unless you attach a vacuum pump to it due to pistons displacing air and combustion gasses getting my the piston rings.

You will need 0 vacuum on the catch can/provent to have it do it's job as they were both designed to vent to atmosphere or attach to the stock points on the intake.

Crumpler

Thanks Blake, that helps me quite a bit. Appreciate it.

Alan

The pistons don't displace any air in the crankcase (net), they create pressure pulses - mostly inter-bay - but that is about it.

Its just blowby from the rings (and seals) that tends to flow combustion gasses into the crankcase. Given the stock breather set-up there are times when the crankcase will still see vacuum from the intake & throttle body venturi (idle, & small throttle openings) - but that's not constant and the blowby can easily overwhelm it at higher RPM or WOT.

You need a net difference between the crankcase pick-ups and the catch can outlet pressure to create flow. This can be between a positive pressure crankcase and ~ambient (e.g. atmosphere* or ~CAI) or you need a vacuum source (throttle body venturi, intake** or vacuum pump).

* Please don't
** its difficult to use much intake vacuum without engine management issues (it looks like a vacuum leak).

Just venting back to the crankcase won't work as there will be no incentive for net flow in your loop (unless you pump it).

Negative crankcase pressures (~10"Hg) do seem to help keep the oil mist down - but you need this most at high RPM/WOT when its also most difficult to maintain.

Alan

GlenL

A fair number of threads on this.

I'll add that I've got a catch can and have found it very worthwhile. Originally, it served to protect the engine from the octane-robbing oil mist. As I got my engine sorted, it served to measure the oil being ejected. Now that the ejected oil is practically eliminated, I enjoy draining it to find just water coming out with very little oil at all. If this changes, I'll know somethings amiss.

Crumpler

Good Morning All.
I had a little free time to expand on the catch can concept.
Space is definitely the issue as GB alluded to. In fact, my fix only fit because I have an electric fan set up and air pump delete.

I ended up routing hoses from cam cover in line and then through catch can, with can at lowest point in closed loop.

Then hose from catch can back to filler neck.

I left the vacuum line that sits high on the neck installed in an effort to maintain CC vacuum.

Two larger vacuum ports on TB and one on filler neck are capped.

I'm not crazy about the materials used to make corners but it's what I could get to short term that would stand up to engine temps.

In terms of "working"?
The catch can has fill level indicator, we will see how effective it is.
In terms of engine management, I have AFR and Vacuum gauges in the cabin.
On the shakedown drive last night I could appreciate normal power and idle. The vacuum readings ran a few pounds higher and seemed to have more reserves, at least took more throttle to bottom out vacuum.

I did notice the AFR levels stayed closer to home at 14.7. Traditionally, I will see more fluctuations as I run through the gears.

Alan

What do you think is making anything flow around your catch can loop? I don't see any reason for any net flow?

The heads and the crankcase are basically at the same static net pressure (they are connected together by 4 large oil drains). Since you capped off both the largest (but still probably too small) throttle body/venturi ports you now only have one even smaller (4mm?) port to vent the whole crankcase. It is highly likely you will now significantly pressurize the crankcase at high RPM. This will also create a larger than expected intake flow (=vacuum leak) that possibly may upset the ECUs

Even then I don't think there will be ANY net flow through your catch can.

I think you are endangering your engine and should seriously rethink things! Blowby gasses have to have a place to go - you have mostly eliminated the exit path.

Alan

GlenL

Here's a pic on "why" and "what."

I drained this out of my catch can. That's about 2.5" of water and 3/4" of oil. That's from street running and 1 20-minute track session.

Doesn't look bad. The engine uses about 1/8 quart of oil in a 25 minute session. About half of that ends up in the separator. I find that all pleasing.

Another thing to note is the amount of water. That's typical for street driving. Cruising along, the water vapor condenses out. Some people route the output from the separators back into the sump. Perhaps some separators don't condense or catch the water vapor but I really doubt it.

Alan

I have no idea what your configurations is - But, obviously it works.

What Crumpler has implemented makes no sense to me at all... there is no pressure differential to create flow around the catch can loop -
and much worse - there now is almost no way out of the crankcase.

General consensus seems to be that the total size of all the stock crankcase vents is actually marginally sized at best for the size of the engine - now its operating with <1/5 of the stock total vent size.

Alan

GregBBRD

That's an extraordinary amount of water....if that was going back to your pan, you would have a milkshake, instead of oil.

Alan

I agree with Greg. It seems very unusual for you have that much H2O. I have a vapor catch from my first AOS where most water would condense out and I accumulate very little water, a very very small fraction of what you have even after 6 months of daily driving, However in Phoenix we have very low humidity so that may be a factor also. Some of mine probably does makes it though the whole flow and back into the cold air intake to be ingested - but that's likely a small fraction - as it would also need to get through the Provent filter membrane.

Alan

GlenL

That's the point I'm making: there's a lot of water in blow-by and you don't want to drain it back into the oil.

The exhaust, and the blow-by, is 30% water vapor. When cruising on the street, the external separator is below 100C so condenses the water vapor. I'm sure the provent does the same thing.

Feel free to do the math on the volume. Burning a gallon of gas produces almost a gallon of water. If that's 1/3 pint and there's 1% blow-by, that's what can result from burning 5 gallons of gas. Or thereabouts.

Crumpler

I can't discount any of that Alan, thanks for staying subscribed.
Like a few other projects, it would likely be easier for me to not do it, but it does force me to learn more about each system.

Thanks also to Glen and GB for taking the time to help me with concepts today.

I guess originally, I thought the small vacc line on the neck would be enough to remove pressure built up by the crank case. With the system posted above, I knew if it captured oil vapor it would only be some, since the cam cover vents were the only thing inline to the can prior to routing into the filler neck.

As I go through my notes, I was likely not accounting for the other systems I looked at (like the Sharkvent) having vents to the atmosphere.

I guess in part I assumed the over pressure relief valve held closed one of the venturi's going to the TB most of the time anyway, and was only open when crank case pressure was uber high.

I will go back to the lab as it were, and see what I can do better. I will come up with a bigger vacuum and put the catch can after all crank case vents.

Any thoughts on cam cover ports seeing inlet air only and all vapors being extracted via oil filler neck?

Ducman82

some catch cans actually route coolant to it to keep it hot so water does not condense in large amounts. and just cycles back to get burnt out.

Alan

Evacuating from the heads is considered normal for most engines - because the heads allow some chance to separate the oil out of a mist (vs. evacuating direct from the crankcase).

However on a 928 this may be counterproductive because we know the heads can pack with oil under some conditions. If the net blow-by flow is primarily out of the head breathers then blow-by gas flowing up the head drains is competing with the oil trying to get back down to the pan. This to me is just a bad idea knowing what we know.

Evacuating from only the filler neck/main crank vent avoids that issue - but then there is more entrained oil at that location - so you need a better oil separator there.

Its a challenge - Porsche never really came up with a very good solution IMO.

I have Greg's crank vent separator (But I'd like to design a more elaborate stacked version to replace the whole filler neck since I already have a separate filler to the sump). I also have a vacuum pump to create negative pressure in the crank and to positively drive flow though my separator loop to strip the remaining oil. Net cleaned gasses get fed back into the cold air intake for a closed system (and oil back to the sump).

Think about how to drive flow around your separator /catch can loop - it has to be a static pressure differential. The head and the crank are ~the same so it isn't that. And make sure you have ideally an ~3/4" exit path from the crank for max blow-by.

Alan