DaleClark

iTrader: (55)

Hey all -

Digging around the Internet looking at stuff about catch cans got me thinkin', and that's a dangerous thing .

For some time now I've been running a vented catch can on my FD. Very simple - two hoses going to the two nipples on the oil filler neck, both going to the catch can, which has a filter on the top. The inlet on the primary turbo inlet duct that the PCV system plugs into is capped, along with the PCV connection on the UIM. In general, it's worked fine - had some oil in the catch can at oil change time, and no more oil getting sucked into the intake.

In the piston world, it seems to be a Good Thing to have vacuum in the crankcase, as that helps the rings seal better and all that good stuff. But, as the guts of a rotary are sealed from the oil pan, I don't think it's that much of an issue.

Anyhow, long and short of it, is it worth pulling a vacuum on a rotary "crankcase"? Will it help anything at all, or is it a waste of time?

Dale

slo

iTrader: (1)

I seem to get more oily watery residue in the catch can when pulling vaccum through it, which is probably a good thing to be evacuating from the crank case.

mono4lamar

iTrader: (32)

For us single guys it seems pretty important to run a catch can. I've opened my hood to see my oil dipstick pushed up out of the tube. The thing that bothered me most was the fact that the primary intake runner was CAKED with oil inside.

Main thing I can think of is that venting the pressure out can prevent the oil pan gasket from being blown out (common problem with our cars).

Another plus is that it gets some of the fuel diluted oil out of the car

Scrub

iTrader: (25)

Dale,
The stock system is setup with vacuum assist minus the catch can. If you take the opened end of your catch can and run it to the intake (before the turbo) you have vacuum asssist I don't know what benifit if any doing this will have....but I know it sure as hell can't hurt anything.

I was actually thinking about running two catch cans, one for each nipple on the filler neck.

-Dan

DaleClark

iTrader: (55)

Good feedback so far.

I do know for a fact that you don't want the PCV system to pressurize - I've known FC guys who have capped the PCV system without understanding it and have had smoke under boost, popped out dipsticks, etc. Undoing the caps fixes the problems immediately.

Mazda had to vent the PCV system to the intake due to emissions regulations - any emission the car gives off HAS to be burned by the engine and scrubbed by the cat. That's why there's the charcoal tank and all that jazz as well.

The big question is whether or not vacuum will give any advantage on a rotary. There's no sense plumbing something up if there's no benefit. The oiling system has the pickup submerged in oil on the beginning side, and the oil basically dumps back into the pan on the return side. The only thing I could think of is helping the turbos' oil drain work better, as the drain side is gravity-fed and under low or no pressure. That would help pull oil out of the turbos better, in theory.

A catch can is definitely a must, though, as well as ditching the stock PCV valve to the intake manifold like the '95 cars. This can greatly help in keeping the intake clean and oil-free.

Dale

mono4lamar

iTrader: (32)

^ You summed up all the stuff I forgot. I could not have said it better myself (obviously).

Kento

While I doubt it would hurt to pull vacuum in a rotary "crankcase", it won't provide anywhere near the benefits that doing the same to a piston crankcase offers. There's none of the internal pumping losses that piston engines suffer from existing in a rotary.

Any pan pressurization that is enough to blow out a pan gasket with a standard case vent won't be avoided by any type of case vacuum pump. It simply means you have blowby problems that need to fixed.

wstrohm

Depends on how you define "pumping loss." Any throttled engine, rotary or otherwise, has moving parts that must be forced to move against a vacuum. Only an unthrottled engine, like a Diesel, avoids these "pumping losses." Even eliminating the physical throttle butterfly, ala BMW Valvetronic, just moves the vacuum from the intake manifold to the cylinder itself, due to the barely-opening valves at idle, "part-throttle," and during deceleration.

Kento

Definitely true, but I was referring to the pumping losses in the crankcase, which is the main reason why case vacuum pumps provide such comparatively huge benefits in a piston engine, and was the basis of Dale's original question.

sereneseven

agree with kento, In a rotary there isn't really a "crankcase" since there is no unused side of the rotor. plus being that the oiling system is completley sealed from the rotor housing it would make no difference if it was in vacume or not.

slo

iTrader: (1)

Hey dale can you please be more specific in how the 95 cars pvc system is set up.

You peeked my curiosity on that one, is it just like the previous models but with no PVC valve or connection to the UIM?

Where does the oil line vent too just the turbo intake?

Thanks

DaleClark

iTrader: (55)

There are 2 nipples on the oil fill neck - one goes to the large grey/black PCV valve which is attached to the upper intake manifold, the other nipple goes to the metal hard line that runs over to the primary turbo inlet duct. The '95s got rid of the large PCV valve - just cap the nipple on the oil neck and cap the nipple on the UIM, done.

So, I'm thinking the consensus is vacuum on a rotary crankcase isn't really necessary. My only other idea is if it would be beneficial for the turbos or for a large single.

Dale

sereneseven

Dale, I don't see how the this would help the turbo....unless your thought is that a vacume in the pan would help pull oil out of the turbo return line...which is kinda an odd scenario since you are also pulling a vacume on the oil pump inlet so I would think the two would offset eachother=no benefit. the only why I would see this as being a benefit is if you were to run a dry sump where the oil out is seperate from the scavenge....

rynberg

I have been running my oil vent lines like they used to do in the old days....vented to atmosphere pointed down at the ground...

Dale is exactly right on the 95s, we don't have the PCV line from the filler neck, it is capped.

zli944

iTrader: (2)

@DaleClark I was exploring the PCV in another thread and on doing some additional searching this old thread popped up.

My issue currently is that the lack of vacuum on the oil system is causing the dynamic seals on my BNR Stage 3s to leak onto the turbo, causing smoking even on idle.
I read up that BNR Stage 4s for FC owners suffered similar issues and it was a big complaint back in the day.

so I changed my open can system to a closed can system. Vacuum from the turbo inlet alone did not alleviate the leaking so I hooked it up to the big nipple on the UIM where the stock PCV had gone but just without the check valve. My cars on jackstands so I wasn't worried about boost. It SEEMED to solve the problem but at a cost. My idle is wonky due to the additional air and there is a God awful wailing noise coming from the intake.

Looking into it, a stock PCV valve is essentially closed at high vacuum, and slow opens as vacuum lessens. However, once the pressure is near 0 or in boost, the valve is closed to prevent pressurization of the crankcase.

this puts me in a conundrum. I was considering putting the stock PCV valve in, but then I would lose the vacuum needed to prevent the oil leak at idle. However, without it, my idle is crazy high (1800 RPM) and there again, is that very loud decibel wailing coming from the intake which i still can't explain.

Anyway, just though I'd share my 2 cents and resurrect a very old thread

j9fd3s

iTrader: (3)

are you sure you have the PCV valve the right way around? and then B, mine was bad. i cleaned it and got it to be inconsistent, so there is that too

i'm not sure why they have so many part numbers for PCV valves that are externally the same, but they do the oil filler to intake is N3A1-13-890

ptrhahn

iTrader: (9)

I've had a vented catch can with a -10 line off the neck forever, TT's and single T's. I've never seen a drop of oil in it from regular driving, only on the track, and if the oil level is full I can generally fill or nearly fill the 1q catch can every session if I'm going fast (less sticky tires and/or less aggressive me can see a significant drop in the amount of oil in the can). Sometimes I've run the oil halfway down the dipstick and that helps a TON, but doesn't feel good in the gut.

Somehow there's a HELL of a lot of pressure in the oil pan for a motor where its "isolated" from combustion. At one point I had my OMP nozzles vented to the extra inlet on the filler neck (which is connected to the filtered catch can, and it caused a new motor to smoke... alleviated when they were filtered elsewhere. I've heard of people vending the pan from the other side as well, from the rear oil drain, and the thinking would be that air pressure could escape out the other side during cornering instead of pushing a bunch of oil up the neck. From what I've heard, it just produces MORE oil.

I've seen two posts—one from somebody overseas that had a scheme for "equalizing" the pressure somehow, but I couldn't understand it even with their diagram. And finally, I've heard of people tapping and venting the front cover and that working well, but I don't know precisely where. My guess though, is that it works because it is higher up, and therefore above the oil level—similar to my running the oil level down. The route of the problem seems to be that the oil is too high, and the vent too low. Does that make any sense?

zli944

iTrader: (2)

The problem is specific to the BNR TTs because of the dynamic seals I believe. Similar to the BNR Stage 4 issues with FCs back in the early 2000s. I'm not 100% clear on the technical details but essentially the seals seem to function on a pressure differential, and if the turbo oil drain is positively pressured and does not drain quick enough, then the seals can essentially overflow, causing the leakage.

My original setup with the BNRs that did not leak was a closed loop small catch can plumbed to the OEM oil filler neck and the primary turbo inlet.
With this, my only issue was oil leaking from the OEM filler neck due to hard cornering which I tried to resolve with a "baffled" metal filler neck. I think this is where the issues all started, where the half moon baffle typical in the oil filler necks somehow restricts the pressure build up in the oil system enough to prevent proper drainage from the BNRs.

I ran an open catch can off the -10AN on the filler neck but that was a no-go, oil leakage from the turbos.
I converted this to a closed loop system, -10AN to the can, then a vacuum from the primary turbo inlet. Still no go, the vacuum doesn't seem to be strong enough to solve the pressure differential.

Some people have mentioned that the center iron also has a capped vent nipple (mostly for FCs and rarely on some FDs) but unfortunately mine does not have one.

I think part of the wailing I'm hearing is due to a very slight opening of the single throttle plate. I've got idle adjusted with mostly the idle screw so I closed the plate up fully, will test for sound.