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Engine Breather filter

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I've just taken my breather pipe off, the pipe does not have an oil/air seperator and doesn't run back to the airbox, the pipe just goes down onto the underside engine tray with a filter on the bottom.

Thing is the filter is pretty much clogged now with oil and I can't be assed to clean it (at least I am honest).

Question is why exactly do I need a filter on the end of it - when the heck does the crank case become vacuumed enough to draw a particle up a 1" diameter hose by about 2 vertical feet ?

I mean that would need a serious vacuum, I really dont see the need for it. I can understand if the breather was vertically pointing upwards or sideways but mine is piped 180 degrees back down towards the I missing something here ?

My engine does chuck out a fair amount of gas, I might have a fair bit of leakage past the pistons but for a car running this level of boost I would have thought that was fairly normal - afteral it's not as if it is losing much horsepower.

Edited by Jonathan

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Great question Jon,

I found my breather filter clogged just the same as you describe and becuase of its location, pointng down and out through the bak of the engine bay I just took the filter off. After doing a little reading up on it, I came to the same conclusion and I've left the crank case breather pip open ended. Given there is the Green oil air seperator at the other end of the pipe with a metal gauze filter I was pretty confident the vacuum issue would be a problem.

As we discussed about 2 years ago, when we were looking into installing catch cans - we found that the pressure under load coming down the crank case breather was substantial and is clearly one of the reasons why such a large diameter pipe is attached. My catch can had a large'ish diameter filter on the catch can but this was still causing pressuration of the sump enough to blow the dip stick out and oil to be blown out through the engine vents and over the back of the car.

Be interesting to see what the other views are on this.

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Mines been off for a while now and no issues - I can't find any arguments for there being a filter on the thing at all (probably a means to make money on thise little filters).

Interestingly most the the discharge from this I have found is water vapour - so it evapourates anyways I've had mine dripping into a baked bean can for a while and there is very little oil in there.

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I fitted a catch can and shortly after my head gasket blew, can't attribute it directly but it was darn suspicious! This was with the catch can having a vented screw top.

Anyway, refitted the can but with another pipe coming out of the catch can going to atmosphere via a small filter so if the sump was being pressurised, it shouldn't be now.

Time will tell, if the HG blows again (I will be pissed off!) then I'll know it was the catch can...

Reason I fitted the can was because the dump valve was blowing oil all over the engine which was due to the recirculation pipe sending all the gunk into the air filter and through the plumbing. Made a right mess and can't have been doing much for fuel air charge. 910 wasn't designed to be an oil burner!!!!

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When I had a catch can the oil used to blow out of the oil level dipstick hole and shower the whole engine bay.

The reason was way too high crank pressure, never had it since I took the can off - now my engine tray is the catch tank hehe.

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Hi Jon,

I find it odd that more people with catch can's havent reported the same problems we found. I know Dermot and Kato had both fitted the catch cans with Air breath caps (as detailed on the LEW guide) but Im not aware of them reporting any issues. I am shocked that the Air breath cap is able to release enough pressure not to blow the dip stick. One thing I found to watch out for was if the dip stick had lifted out from its sealed position. Its obviously only happens under heavy engine load, that the crank case pressure gets high enough to properly blow the dip stick and launch oil every where as it didnt happen all the time while I had the catch can fitted and my can was always fitted with a breather filter on the second port.

Edited by CarlC
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Either that of we both have compression issues or other cars arn't driven as hard as ours :)

If you plumb the second port of a catch tank back into the airbox like a lot of poeple do you avoid the overpressure as the turbo 'sucks' the fumes back into the system reducing the pressure at the crank case.

Main reason why I didn't do for this is lo and behold it also sucks the oil back in esp at high boost and when the tank fills up (which is the whole reason for fitting a catch tank !!!).

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  • 2 weeks later...

crankcase breather is also a flame arrestor... as well as an oil trap to make the oil drop back down to the sump.

this is what is in there normally


Oil mist is pretty easy to light on fire!

Without it you might be blowing more out of the system, rather than it just dropping back into the system.


Vulcan Grey 89SE


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yeah a plain oil catch tank isnt a brilliant idea

install a demister inside it and no oil back through the intake

when plumbed for recirculation

a simple demister setup I installed in mine was to roll up to window screens

and place inside the catch can

so that the oil mist has to pass thru many layers of screen before exiting

and condenses/accumulates on the screen and stays in the can



just because I don't CARE doesn't mean I don't UNDERDSTAND

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