Many of you out there have probably seen an Oil Catch Can for your car on the aftermarket, and most of us are often left scratching our heads – what is the benefit of an oil catch can, like a Mishimoto Baffled Oil Catch Can for N54?

I know, looks mysterious, right?

Why do you install an oil catch can? Well, if you have a turbo car, especially a later-than-2004 model – there are quite a few very good reasons why it’s something you should consider. If your car’s naturally aspirated, it’s not as much of an issue – so we’re going to be focusing on turbo cars here – like the BMW 335i/435i/135i/535i / BMW-anything-35i really, the N20, N26, N54/N55, S55 are all affected, as are Ford’s EcoBoost series of engines, in the Focus ST, Fiesta ST and Mustang EcoBoost, these are all Direct Injection Turbocharged engines, and as such they’re afflicted by all the same issues.

As we explained in our article talking about why BMW recommends Walnut Media Blasting Intake Valve cleaning Service be done every 40,000 miles (which isn’t covered under your maintenance plan BTW) – all turbocharged cars experience oil blowby when under boost. It’s just the fact of the matter – even brand new, your turbo car’s going to have oil blowby if you’re heavy on the throttle.

Mishimoto N54 Baffled Oil Catch Can Installed

Okay, so what’s oil blowby? Why is oil blowby bad?


Blow-by is caused when a combustion in the engine forces fuel, air, and moisture past the piston rings and into the crankcase. From there, the blow-by is released by the crank case vents and back into the intake pipe. This occurs on all engines, but is worse on turbo engines due to the stronger combustion. This is bad because if nothing is done, this excess “gunk” will build up in the intake manifold, coat the intake valves, coldside of the turbo, intercooler, and intercooler chargepipes.



Check out the above illustration of how the average turbocharged motor is laid out along with a diagram of the PCV flow of a turbocharged car. So the PCV valve allows these oil blow by vapors to enter the intake tract.

As we said in the N54 walnut blasting article, the fact that many turbocharged engines nowadays are direct injected – which causes the inside of the intake to get much dirtier than it did on older multipoint fuel injection systems that sprayed the fuel mist into the intake manifold, spraying down the intake runners and intake valves with a mist of gasoline, which used to keep the intake clean. Since Direct Injection style fuel injection moves the injector directly into the combustion chamber, there’s no spray of fuel on the intake valves to keep them clean, so crap accumulates on them.

walnut-blasting-n54-nick-pics-3 walnut-blasting-n54-nick-pics-10Actual BMW N54 Intake Valve – look at all the crap coating the intake valve shaft, and the “ring”of crud that’s accumulated there from the movement of the intake valve.

walnut-blasting-n54-nick-pics-7Check out the yellowish green coloration to the intake area – that’s all oil. For reference for how it SHOULD look, scope out the before/after of an N54 engine that we did the Walnut Media Blasting Service for BMW N54/N55 on.


As the oil vapor collects on the walls of your intake manifold and intake valves and collects, it traps dirt and other fine particulates creating an icky sludge coating. The way the crud builds up on the insides of your intake is a lot like how your arteries can become clogged with bad cholesterol if you have a poor diet.

Much like clogged arteries will cause your body to perform badly, intake valves coated with blow-by and carbon buildup can cause the intake valves to stick or not seal as well, can negatively affect VANOS / VVTI type systems, lowers the effective octane of your fuel and generally makes performance suck – and the problem is exacerbated by higher boost levels.

engine-with-oil-catch-canSo, besides doing your Walnut Media Blasting every 40k or so (20k or so if you’re tuned) – what can you do to keep your motor clean and do your part for preventive maintenance? That’s where the Oil Catch Can comes in.


The solution is as simple as adding something mid-stream in the PCV line to collect the crap and oil vapors and give it somewhere to condense, before it has the chance to enter the intake manifold.


That brings us to finishing the answer for our original question – what is the benefit of an oil catch can?
Now that we’ve explained why you’d need one, we can talk about what the catch can will actually DO.

By installing an Oil Catch Can on your turbo car, you’re preventing those oil vapors from collecting on the inside of your intake manifold, helping you keep the need for media blasting intake cleaning spaced as far apart as possible. While a catch can won’t totally eliminate the need to clean the intake forever, it’ll go a long way towards keeping the intake clean for a long time, thereby allowing your engine to perform at its best for the longest period of time.

Should you need to have the BMW N54/N55 Walnut Media Blasting Service done, we’re able to offer that service at the shop at ModAuto (click here to make an appointment) we’re the lowest rate in town for the service and the least expensive shop that actually uses the correct BMW specialty tools for the service.

Story Nick Gregson