Why do I need a filter on the crankcase breather?

[Sask_Lotus]

If I am just venting the crankcase to the atmosphere, why do I need a filter on the hose? Seems to me that the crankcase will never have negative pressure. Also, if i do vent to the atmosphere will pressure build up in the crankcase under high load?

[910Esprit]

I guess you may get negative pressure under throttle 'lift off' conditions if your rings or valve/guides were not sealing perfectly? (The opposite condition of worn engines blowing their dipstick out) Question 2 - No, but you will potentially get crap all over the place as the vapourised oil/steam etc condenses - Steve

[Mike6]

Have you read up on this on the LEW site. The perceived wisdom is that if you dangle the breather tube down under the shield it is highly unlikely that anything would get sucked up that far. Filters are no longer added since they can get clogged and then this restricts the fumes exit.

[USAndretti42]

I think the idea of the filter is to catch the oil as it comes out like the wire mesh in the oil filler cap on old Mini engines. If you have a dry sump car, the scavenge pumps could pull a vacuum (racing engines usually run a vacuum in the sump to reduce aerodynamic drag on the crank).

[silverfrost]

Hi,

Just add an oil catch tank, very cheap and simple, also means oil does not end up on the road and you can fit a filter aswell, best of both worlds.

[Vulcan Grey]

and with a catch tank, you can keep an eye on the amount of bloyby and general engine health.

[Sask_Lotus]

Thanks for all lthe replies. I think I'll intall the catch can, makes the most sense.