[GSB]

Why another PCV/Oil-Catch-Can Topic? Because the majority of catch-cans don’t work very well, and the direct-injection engine is vulnerable to severe oil-fouling of the intake and valves. Especially since BMW makes the PCV system inaccessible.

Background:
At 120k miles, my N54 engine was spluttering and misfiring, shaking during idle, hesitating and jerking during sedate, low RPM driving, and smoking from the tailpipes. Downright embarrassing. I’d already installed an oil-catch-can on the high-side (boost) flapper, replaced the PCV valve, the valve cover, the spark plugs, the coils and the injectors (brand new index 12s), all to no avail. Walnut-shell blasting was next on my hitlist. As I dismantled my intake, I was appalled to find PUDDLES of oil throughout the intake system. Since the intercooler was swimming in oil too, I suspected the turbo seals, but the cause turned out to be the shockingly bad PCV system. Only 20k miles after walnut-shell blasting, my sparklingly-clean intake valves were again CAKED in oil and more puddles of oil were found throughout the intake system. I flipped my lid and began some research into proper oil-separation techniques.

The Weakness of the N54 PCV System:
This engine has a dual PCV system. The low-side (vacuum) PCV valve and its channels are built into the valve cover and are inaccessible to oil-catch-cans. The high-side (boost) flapper opens into the rear turbo intake. Sadly, if the low-side PCV system fails or becomes restrictive, the crankcase becomes pressurized and the vapors escape through the boost flapper, the turbo, the intercooler, the charge pipe, the diverter valves, the throttle body, the intake manifold and the ports, carrying loads of oil with it. To prevent oil-fouling, this oil needs to be extracted from the vapors on the low-side AND the high-side of the PCV system. The low-side is certainly the most critical.

The External PCV Solution:
An aftermarket “External PCV System” with a reliable, high-flowing PCV valve (like the one from “RB Turbo”), coupled with a good oil-catch-can, is a necessity to keep oil out of the intake ports. Note that certain “upgraded” aluminum PCV valves may be “reliable”, but very restrictive, resulting in oil-fouling through the turbo. Ask me… I know!

The Weakness of Oil-Catch-Cans:
When I tested a variety of catch-cans, I was pretty dismayed at the results. Most of them (even pricey ones) are pretty poor at extracting oil, water and fuel from the PCV vapors because they do not employ the best separation techniques, allowing much of the oil vapor to pass right through. To monitor the performance of these cans, I installed two compressed-air oil/water separators in series with each catch-can. On average, most catch-cans caught about half of the oil in the PCV vapors and allowed the rest to pass through. This is due to design flaws, guesses, or design compromises, such as space and cost constraints. These vapors are extremely hot, and they flow through the top of the can for a very short distance at high speed, leaving little opportunity to extract enough oil.
The ultimate goal is to CONDENSE liquids out of the hot vapor. It’s not that difficult! Distilleries do this all day long. Here’s how...

The Best Catch-Can Solutions Combine Four Methods of Oil/Fuel/Water Separation:

1. A cyclonic centrifuge, where incoming vapor enters tangentially to a cylindrical or funnel-shaped chamber. The vapors are forced to swirl around the walls of the chamber. Oil, fuel and water particulates, being heavier than air, fly into the walls, sticking, coagulating and condensing. (The BMW valve cover has some rudimentary cyclonic separators built in, but they’re just not effective enough).
2. A baffle chamber, which traps and condenses oil by forcing the vapors through an extremely convoluted path with countless condensing and coagulating surfaces. This chamber must not be restrictive to overall airflow.
3. An expansion chamber where vapor velocity slows suddenly, allowing gravitational separation. Here, any oil separated by the first two chambers can safely drip into the bottom of the container without being sucked back through the outlet by speeding air.
4. All 3 of these chambers benefit greatly from cooling. Condensation occurs on surfaces that are colder than the vapor. Mounting a metal oil-catch-can in the front of the engine bay, on the cooler, intake, side of the engine, helps with the condensation process.

Some consumer catch-cans rely solely on a large expansion chamber, others combine a rudimentary baffle, whereas others (like the Michimoto cans) employ a sintered brass baffle element. All of these work to some degree, but are still inadequate. For example, if a sintered brass baffle element is designed into the outlet side of the can, that baffle will condense oil, but high-speed air through the outlet will suck that condensing oil straight into the intake manifold, faster than it can drip into the can.

I’ve only found one OCC design that is based on thorough research, design and simulation. Of course, it costs a fortune. My quest led me to fabricate a custom system that works incredibly well for very little money...

[GSB]

To begin with, the generic can shown below (WITHOUT breather) can be found on Amazon, eBay and many other places for around $20, and it makes an excellent starting point because it combines a few great features:

1. A tangential cyclonic centrifuge is milled into the solid aluminum top deck of the can at the inlet.
2. A small baffle chamber screws in below the centrifuge.
3. The main expansion/drip chamber is well-made and easy to service.
4. It has a selection of inlet/outlet fittings to work with different hose sizes. I use the smallest fittings (11mm OD) with cheap 3/8” ID rubber fuel hose for the low-side (vacuum) external PCV system. And I use a second can and the largest fittings (16mm OD) with cheap 1/2” ID rubber fuel hose for the high-side (boost) flapper/breather.

This can is not perfect, but a few simple mods can make it far more effective at oil separation...

1. The small baffle chamber can be greatly enhanced by stuffing course Scotch-Brite stainless-steel scrubber material into it. The baffle unscrews into two halves. Compress as much stainless material as you can comfortably fit into the baffle enclosure and screw it back together. Do NOT add stainless material to the first circular chamber, which is designed for centrifugal separation.
2. Unfortunately, like many other cans, the inlet and outlet are too close together. Vapors that exit the baffle chamber are sucked directly to the outlet, less than a 1/4 inch away. Any oil that is trapped and condensed in the baffle is supposed to drip into the bottom of the can, but vapor speeding over the rounded edges of the baffle to the outlet, can drag oil with it, and I observed a trail of oil making its way from the baffle to the outlet. Not good. Adding stainless scrubber material into the main expansion/drip chamber does NOT add value because the vapors will not be pulled through the material. In fact, it may just allow another path for oil to reach the outlet. Therefore…
3. Add a tubular extension to the baffle chamber. Research papers have shown that the closer the inlet pipe is brought to the bottom of the can, the more efficient it becomes at extracting oil. First, I used plumber's PTFE thread-tape to seal the threads of the baffle chamber (which otherwise leak oil). Second, I cut a two-inch long tube out of a compressed-air spray can and rolled it tightly over a 3/8" diameter dowel rod. This forms a springy tube that can be expanded over the baffle chamber, wrapping it tightly and remaining securely in place. That entire tube can be filled with stainless scrubber material to maximize condensation. With this modification, the vapor is forced further down into the can, passing through far more condensing material, then flowing over a sharp edge to return vertically at least 2 inches to the outlet. Gravity and slowing/expanding airflow then make it impossible for oil to find its way up to the outlet. Obviously, this modified can must be emptied more frequently to prevent the oil level from blocking the new tube.
4. The hose that comes in the kit cannot handle heat and vacuum without collapsing, so throw it out and buy some cheap fuel hose instead.

WARNING: Do not use steel wool, which can rust and allow tiny metal fragments to be sucked into the engine! When cutting and packing the coarse stainless scrubber, be gentle with it, don't stretch it, and be cautious not to create shards that might come loose and fall into the can.

[GSB]

The modified catch-can does a pretty admirable job of extracting oil. However, I went one step further to ensure that NO oil makes it to my valves. I built a condenser out of a cheap, single-walled stainless-steel water bottle with dimensions about 9.25” x 2.75”. (Do not use double-walled thermos-type bottles). These bottles cost about $15 new, or $4 used. I picked one up from Goodwill for $2.50, then drilled and tapped both ends for 3/8” stainless steel barbs. I PACKED the bottle with as much course stainless-steel scrubber material as I could cram in there. This bottle is not at all restrictive to PCV airflow, yet, when laid on its side, it forms the ultimate velocity-slowing, abundantly-baffled, oil separator. At LAST, I have an oil-free intake system, with no more oily valves, no more performance issues, and no more smoking! The spark-plugs don’t foul anymore either, so they last much longer.

I regularly empty tablespoons of oil/fuel/water sludge out of the modified oil-catch-can, but nothing has ever dripped out of the stainless bottle. After a year or so, it certainly stank of oil, so I half-filled the bottle with gas and shook it for a few minutes. The gas came out almost black, indicating that it is indeed trapping plenty of oil.

The diagram illustrates how the incoming vapor (blue) slows and spreads as it hits the wide bottle diameter and extremely convoluted stainless baffle inside. Gravity pulls any condensing oil (red) to the bottom of the can, while the clean air exits at the top. As that stainless material becomes sticky with oil mist, it only becomes more effective at trapping oil.

[GSB]

Here’s how the system is placed in the engine bay. I haven’t painted the condenser bottle black yet because it makes people laugh…

[GSB]

Below are the hose connections for the RBTurbo External PCV adapter and the boost flapper…

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Below is the intake manifold, drilled and tapped, with a 3/8” barb for the RBTurbo External PCV System. This is the perfect spot for the PCV hose to clear the throttle body and to avoid stress-cracking of the plastic manifold…

[GSB]

One more thing to bear in mind… When the low-side (vacuum) PCV system is working correctly, little to no oil gets through the boost flapper and into the high-side oil catch can. Usually, there’s only a mist of oil at the bottom of this can, or at worst, a millimeter or two. If you suddenly notice oil puddling in the high-side catch can, something is probably failing on the low-side. The PCV valve may be becoming restrictive or the PCV vacuum may be leaking somewhere. Fix it immediately to prevent your intake system fouling through the turbo, AND to prevent your rear main crankshaft seal from popping and leaking all over the place.

[Phil325i]

Great post OP! I would be interested in your opinion on this OCC:

https://www.saikoumichi.com/internal_baffle_design.html

It seems to answer some your criticisms of most OCCs. I've used it on my DI 6 cylinder (N53) for many years and it seems to do a great job. My indy recently reported minimal carbon build-up on the intake valves (at 130K+miles). He said the valve stems were still shiny bright metal...

[GSB]

Originally Posted by Phil325i

Great post OP! I would be interested in your opinion on this OCC:

https://www.saikoumichi.com/internal_baffle_design.html

It seems to answer some your criticisms of most OCCs. I've used it on my DI 6 cylinder (N53) for many years and it seems to do a great job. My indy recently reported minimal carbon build-up on the intake valves (at 130K+miles). He said the valve stems were still shiny bright metal...

Fantastic to hear! That is actually a pretty good design. The inlet snorkel causes the vapor to hit the oil at the bottom, trapping more oil, and the mesh baffle does some condensation on the way out.

A white-paper addressed that very design to show how it could be further improved with a tangential outlet. The paper is entitled, "Design Optimization of an Oil-Air Catch Can Separation System" by Anuar Abilgaziyev, Nurbol Nogerbek, Luis Rojas-Solórzano (Department of Mechanical Engineering, School of Engineering, Nazarbayev University, Astana, Kazakhstan)

This image, adapted from their paper, shows how a tangential outlet pipe creates a vortex swirl inside the can, keeping the vapor in contact with the walls of the can for longer, therefore maximizing condensation even further.

[GSB]

An excellent reverse-variation on this design, is the tangential inlet system on the Improved Racing CCS High Efficiency, Quick Release Oil Catch Can

This design is well engineered, scientifically sound, and in this size, perhaps as good as it gets... If you can afford it.

[GSB]

By the way, the improved tangential outlet, referenced in the white paper above, only improved the can's efficiency to 70%, meaning that 30% of the oil in the vapor still gets through. Plus, the simulations were seemingly done without the filter in place. The filter would certainly add to the efficiency, but it would completely change the characteristics of the airflow inside, so we don't know for sure what the total efficiency would be.



Another technique used in distilleries, is the double-helix copper condensing coil (below). This makes an extremely effective high-speed centrifuge to force air-borne vapor particles into contact with the walls for much longer. Plus, copper is one of the most effective conductors of heat, so cooling capacity is a huge strength of this device. This is awesome for condensing oil/water/fuel out of crankcase vapors, and PCV hoses slide right on to the tube, but an expansion chamber with a catch can would still be required to slow the air enough to capture the condensed liquid.

I've tried one of these in series with a catch can, and the results were very impressive. However, the stainless condenser I built above still takes the cake for performance, cost, simplicity and looks.