Sump pressure venting and oil catch can questions..

[yoeddynz]

I had asked this question last night on a ausrotary but wanted to see what other people are doing here on Oldschool. I think it's more relevant to boosted cars this. And not just rotaries - although I think rotaries produce less blow by, unless the oil control rings are kanckered.

Question was..

is there any reason why could I not just vent my crankcase to atmosphere? I realise that it will be a total lose setup with oil being lost to the underside of my car.

Is it strictly necessary to involve vacuum?

I presume you can't just run a baffled catch can with a filtered vent but no vacuum because it would then fill with volatile fumes which could be a hazard?

My current setup just involves a hose leaving the vent halfway up the filler tube being sucked into the inlet manifold after the turbo. The manifold has a one way valve.

There is also a small hole in the centre plate facing backwards about where the big Mazda writing is on the rear housing. This is open to atmosphere and hence after hard driving or long trips I have to clean the area under the oil filter of oily mess. This I want to stop...

Is that vent hole on the centre plate just venting the same circuit as the filler vent (basically the sump) or is it linked to some other area like the air gap around e shaft?

Can I just block that hole and just use the one vent on the filler tube?

I would rather not be burning oil into the manifold and would like to stop the above mess so I was going to install a simple catch can that once in a while I would just empty back into sump. Bad idea??

Plus I like the idea that by keeping the sump venting and oil vapours separate to the inlet side of the engine I completely rule out any worries of vacuum leaks and oil burning upsetting my wideband readouts and general tuning of the carb etc.

The user Bumpstart on ausrotary replied insisting that I need vacuum!!

My reply was..

Ok so I need the vacuum. But why? If blow by from the turbo forcing more past the oil control rings etc is pressurising the sump then surely just an open vent to atmosphere will stop the sump pressurising?

The reason I figure the vacuum is needed is to 'pull' the unburnt petrol fumes out from the sump and other voids. If these were left then over time they will mix with the sump oil and generally degrade it, plus being a potential volatile hazard?

My problem with my current system is that the vent pipe is drawn through a one way valve, into the manifold after the turbo. This works fine at low revs under a light load when the manifold is a vacuum. But when the turbo is boosting, and producing the most crankcase pressure, the manifold is under pressure- so shutting the one way valve which means that sump pressure can go no where.

But if I route the vent into the system pre turbo then I will be burning all those fumes- and oil. So some sort of oil catch can setup, with decent baffles to really help the oil drop out is needed. Then create a vacuum on this tank. Rather than just suck the fumes into my induction I would like to create the vacuum some other way. Maybe a Venturi within a pipe but that will only work when the car is moving eh. Hmmmm?

Ideas.

[yoeddynz]

Oh - and I think my one way valve is just that. A simple one way valve. It's not a variable opening PVC valve.. Which according to wiki....

"The PCV valve connects the crankcase to the intake manifold. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve by manifold vacuum, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. With a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, vacuum is much reduced, down to between 1.5 and 3" Hg. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway."

Anyway- I would rather completely by pass the induction side of the engine and keep the crankcase venting its own separate system.

[DJZ]

I don't know anything about rotaries but have been told by several people that motors make more power with the crankcase under slight vacuum. I thought about this long and hard on my setup as RBs are known to have problems with pressurising the crankcase and not allowing oil to return into the sump.

I ended up joining the two rocker cover breather vents together and venting them in front of the turbo, my theory on this is that under boost there will be some slight vacuum/suction there and under boost is when you want it venting the most, rather than having a PCV valve so it only vents when the inlet manifold is under vacuum. The RBs don't have a vent for the crankcase itself (like the old L series motors do) but I've even heard of some people removing the dipstick and running a line from there.

My rocker covers are baffled pretty well so there shouldn't be any chance of it sucking oil up.

There are things for venting the fumes directly into the exhaust as well, these are supposed to create some vacuum using a slash cut pipe I believe. Obviously you need a one way valve in there.

[mjrstar]

the other thing to think about is moisture/foriegn bodies making thier way in, some form of breather system is a better cleaner way to prevent "stuff" getting in.

I am not a great fan of plumbing the vents to the intake due to un-necessaray oiling of the intake pipes etc.. (the idea above using the exhaust looks interesting) but if it was me i would vent to a filtered "catch can" but not worry about giving it too much in the way of vaccum to assist with the ventialtion.

[morkster]

Is that vent hole on the centre plate just venting the same circuit as the filler vent (basically the sump) or is it linked to some other area like the air gap around e shaft?

Can I just block that hole and just use the one vent on the filler tube?

Yep both those sleeves vent the same area; the sump. I've got two hoses on mine (it's NA but) running down into a chassis rail (shhh). There's been a lot on ausrotary about crankcase ventilation and when they're blocked (especially on the later stuff) they blow smoke (especially on deceleration). Vacuum is probly best, venting to atmo is ok, but I'd say blocked is not that good. In Au from what I can gather (not sure about here) it is illegal to vent to atmosphere.

[Unclejake]

I will answer based on my generic experience with piston engines and not specific to Wankels

Is it strictly necessary to involve vacuum? No

I presume you can't just run a baffled catch can with a filtered vent but no vacuum because it would then fill with volatile fumes which could be a hazard? It will fill with oil eventually. It isn't a hazard and is mandatory on wet sumped race cars. Mount the can as high as possible

I was going to install a simple catch can that once in a while I would just empty back into sump. You can permanantly pumb the can back to the sump, but I wouldn't bother.

The user Bumpstart on ausrotary replied insisting that I need vacuum!! Vacuum is best, but it isn't mandatory. Perhaps you want a dry sump? That is the best of everyting

Ideas.

The old Holden Starfire motors had a belt driven vacuum pump that some racers have plumbed into their engines. It is hard to imagine the gains being greater that the parasitic loss in that scenario though

[yoeddynz]

Cool- cheers for answers fellas. If anyone else has experience or scientific calculations etc etc and reasons for and against vacuum please do tell.

What I do know is that yesterday at work i removed the one way valve off the manifold and plugged it with a bolt. I vented the filler tube to atmosphere. When I drove home the car was smoother low down and didn't bog like it sometimes does when I try to pull away in too high a gear.

I thought about it last night and realised that if the vent hole on the centre plate is indeed just venting the sump (which morkster has confirmed it does) I then had a vacuum leak. The old setup was pulling air all the way through the sump via that unplugged open centre plate vent.

I reconnected the vent pipe from my filler back to the manifold today as I was paranoid that not having any vacuum on the sump would create a mini havoc situation... And now the car is back to the old bogging down really low. As soon as there is boost then the oneway valve closes and the vacuum leak is gone.

I will draw a little diagram later of the setup I plan on building (cause I like doing that) and post it up.

I like the idea of using the exhaust as a vacuum source. Would need a one way valve that is all metal construction.

[Unclejake]

Things (like the underside of a piston) can move faster in a vacuum than they can when presented with the friction of air. That is the reason to run vacuum: Performance.

Positively pressurizing your crankcase too much will not only rob power, it will also cause things like crankshaft seals to leak.

I think your problem is that you have a buggered PCV valve. It shouldn't leak but it sounds like it does. You need a new one.

[Spencer]

Listen to the UJ your PCV valve is stuffed; they are a replaceable maintenance item commonly overlooked

I can’t comment on your system exactly but I would say setup the PCV system as factory, the factory setup is not there to put the crankcase under vacuum for performance gains but to ventilate it, mostly for emissions. It’s like a controlled vacuum leak sucking through the crankcase, draws out all the moisture/acid & shit that gets past the rings and chucks it in the intake. Better to burn this stuff that let it leak into the air, also drawing it in under manifold vacuum means you suck allot more of the shit out rather than it settling in your oil.

[yoeddynz]

Im pretty sure PVC valve works fine although see note at bottom of post...

It will only let gas flow one way which is towards manifold. I have tested it in the old fashioned sucky blowy way with my mouth..

I think the problem is that when it's needed most - at wot when the turbo is boosting and pressurising blow by is pressurising the sump also happens to be the time when the manifold is under pressure and the PVC is shut.

I need to re-route the intake to in front of the carb or after the carb but before the turbo.

If it gets sucked in after the carb then it may well upset the mixture.

And either way it'll mean oily fumes will be getting sucked through the carb/turbo.

So I am planning a catch tank with really good baffles to separate the oil and then create a vacuum in the tank by venting it to the air filter base..or the exhaust.

I need to check for any other vacuum leaks as tonight it was running lean as I went into neutral and would die as I pulled to a stop. Weird. Then after filling up at petrol station and giving it a really good rev it would idle fine. Maybe the PVC valve is a bit sticky and under strong vacuum it opens and creates the vacuum leak due to the above circuit I have noted in last post. Then revving the car hard creates a bit of boost which jams it shut so no more leak. Intermittent fault?...

Tomorrow I will again disconnect it and just vent the sump to the atmosphere for the trip to Kaikoura and see how it goes.

[sentra]

wait a minute… there is no air being displaced inside a rotary engin crank case/ no powergain to be had.

bumpstart is a wealth of knowledge, i wonder what is reason for caring is.

(ive never bothered with it, the oil gets changed regularly, who cares? rather eliminate a vac leak)

[slacker.cam]

I think I disagree with Spence and UJ. Your PCV valve isn't stuffed. If you have done the suck/blow test then you can be confident that it's installed around the correct way and at least sealing reasonably well. The problem is in the setup now that your motor sees positive (w.r.t. atmospheric) pressures in the inlet tract where your current breather system vents to.

You're 100% right in your logic that you need to vent the sump to a point before the turbo to ensure that it is always under vacuum. Either that or vent to atmosphere and deal with the slight performance detriment. In addition to this (I have no knowledge of rotaries either so don't take this as gospel) it sounds like you do need to block up the ports on your center plates to stop the vacuum leak. If you decide to vent to atmosphere then you don't need to worry about this aside from getting rid of the mess.

Your logic sounds accurate to me with regards to the PCV valve being shut when you need it most (under boost).

Most cars you see out at the race track will not run a vacuum assisted breather setup. I think the main reason they do suck it into the inlet from the factory is done is to aid emissions, not so much to increase performance. I think the extra volatiles in your oil and (in a piston engine) there being more air molecules that the crank has to push its way past don't make a very large difference at all. If it did you would see everyone at the race track sucking back in to the inlet rather than venting to atmosphere. There is always debate about this when it comes up in discussions so you'll have to decide what you do and don't care about. For me personally I'm more than happy with venting the crankcase of my piston motors to atmosphere.

[Unclejake]

Slacker.cam's post has had me thinking (cheers man) ........so: Under normal driving loads, is there a vacuum in the inlet tract of a turbo engine, and if so where?

A NA PCV valve should resist being sucked open by inlet tract vacuum and should only open if the crankcase pressure is above 'N'psi. On an older NA engine the gas discharge end of the PCV circuit can often be found in the airfilter housing, where the vacuum is low.

If the introduction of boost is pressurizing the PCV at both ends of the PCV circuit - then the plumbing needs changing......... but, if the inlet tract end of the circuit is being fed pressurized air then you can't possibly have a vacuum leak. LOL.

I think we need a diagram of how this motor is set-up.

[Roman]

I dont think there's much of a performance loss to having the PCV etc setup correctly.

If you're going full steam then the PCV valve is shut anyway, when you're cruising along it's cleaning rubbish/vapours/etc out of your engines crankcase and filling it with fresh air instead.

I guess it depends on how likely it is that your engine spits oil out the breather lines, some motors have got a baffled rocker cover which works as a catch can anyway.

My motor spits ZERO oil out of the breather lines. So cant see the advantage of not having them hooked up, and plumbing them into the intake seems less work than a catch can anyway.

[yoeddynz]

Yeah tonight I'll draw some diagrams of my current setup and what I am thinking of building and post them up.

Before then here is the latest, slightly abrupt.., but still useful reply from bumps tart on ausrotary....

"SIGHS

IN ENGLISH

the oil recovery system from rotors and from turbo is designed to work with gravity AND VACUUM from the outset ,, all the way back beyond 1970

rotors fill up with oil for cooling,, they fling it back down the centre plate,, its hot,, it has also picked up petrol fumes when recovered by the oil control rings

turbo is fed pressure from the top,, and gravities out the bottom \\\

scenario 1 --- ( the moron with open vents )

so you have pressure,, flows down to atmo ( if open vented ) ,, if the vent isnt big enough some slight pressurisation is happening because of the oil temp/ vapour pressure

this is acceptable in an NA engine ,, you can get away with it,, simply because the volumes of oil returning and the temps arent insane

/

on a turbo engine,, or one with hiflow oil pump,,or high internal oil temps different story

youve all seen the oil pushed out over the mazda font,,, signs the open vent is not coping the with the speed and volume of the airflow out the little vent outlet

OK ,, scenario one ---

x pressure flows to atmo ,, the pressure drop is X- atmo

,, the higher the pressure drop then the more the oil will return from the rotors and turbo without backing up

and forcing a new path across your oil control rings to the inlet side of engine ( which is under vacuum ) or via the rear turbo seals into the exhaust or compressor cover

scenario two ------- vents are sealed over\ sump space is over atmo pressure

so

pressure drop ( y ) = x ( pressure in rotors )- ( atmo + overpressure )

since the sump is sealed,, and its hot,, then that overpressure will skyrocket,, and cripple the flow of oil returning from the rotors and turbo

doesnt take much brains to work out this is not good,, and things will soon fail

scenario three

- a catch can

( sighs,, ok one set up with a breather ,, stop fucking asking the open ended questions and apply BRAINS to your individual can and the umpteen permutations of nipples and breathers )

set up as a mist seperator ( the correct term for its correct use )

, ie , the can is higher than the fill neck, the bottom of the can drains back to the lowest filler neck nipple , the other filler neck nipple hoes to the next highest nipple on the can,, and the highest nipple on the can routes to the air cleaner

( there ,,its not fucking hard is it ????? )

then pressure drop y= x- ( atmo - slight underpressure )

[ note the underpressure is very low , but is slightly below atmo,, due to the mini filter acting as a vacuum break ]

scenario four

as above ,, but catch can lacks the little air filter

pressure drop is y= x ( oil pressure in rotors ) - ( atmo - high underpressure )

[ this setup can induce a high vacuum in some setups and carry over if there is no can in between ]

out of all those,,( and i can think of about 4 other variations with other cans/ filters or lack of filter combinations ) scenario three and scenario four flow significantly more than others and can never back up with level or over pressure the sump,, even for a moment

right,, compare this to the FC stock system ,, with a passive purge control valve

under light load air flows FROM aircleaner , THROUGH the sump space and is consumed in the engine when its sucked in at the TB

heavy load,,, the one way valves and purge valves in the above system shut,,, and over pressure ( without a vac break ) flow in the reverse direction TO the aircleaner

if you observe what is happening you will see that the catch system ( the one hooked up right ,, that works ) can is mostly an emulation of the OEM full load scavenge path

variations ( umpteen possible ways you can make a catch can, good and bad ) will use the airbreak ( a small filter on can or on one of the filler point nipples with a one way valve preventing venting ), but flow towards the aircleaner

THERE IS NO PERFECT DIAGRAM THAT IS GOING TO SUIT EVERYBODY

dont blame me

i dont make all those non functional catch ( wank ) cans that sell but cant possibly work , and nobody follows the rule book,, and you cant expect a welder to actually THINK about what he is making

your buying them,, how about you THINK before you buy something with a nipple setup that can never work"

[Unclejake]

Jesus. The guy has some social issues doesn't he.

That doesn't mean he is wrong of course, just arrogant and condescending. He reminds me of a couple of guys that used to post here.

Good luck

[yoeddynz]

yep- he certainly knows a lot of stuff but its sort of gone to his head me thinks. Fuck its funny when someone disagrees with him!!!

I don't spend much time on that forum anymore- i always have a feeling sometimes that they don't like kiwis so much. i dunno.

I had put up my viva thread on there thinking they might appreciate it as its something different to the usual rx2,3,4,7s etc etc but absolutely fuck all interest from them. Of all places with it being a rotary and having an different approach to induction. Left me cold.

Still - lots of useful info on there though once one trawls through all the crap.

[KKtrips]

We still love you Alex..

[yoeddynz]

awwww shucks. Now i have a stiffy.

[sentra]

you cant blame the guy, hes is trying to fight ignorance in autralias version of rotahavik

i am a little confused as to the massive roundabout way of saying "vent hole too small for amount of blowby"