Engine Breathers

[fridayman]

I've been modifying my engine breather, which got me thinking... why does it have a breather? Where is the extra pressure coming from? Just everything getting hot inside the engine? I ask because I have a roughly 12mm ID hose coming from the cam cover to the oil catch tank, and thought that the 6mm OD airline that I have would be OK from the catch tank to the breather manifold. However it looks like the oil catch tank has been pressurising suggesting that their is a fair bit of pressure coming from the engine breather. I've ordered larger hose and fittings, but still wondering how the engine produces that much pressure??

 

[fridayman]

Short answer is blow-by. I've been reading up on this and I am very surprised at just how much blow-by there is in an engine - under no load it can be 100l/min+. No wonder my catch tank was pressurising with only a 6mm O/D outlet hose.

 

[Jon Wildsmith]

yes, piston rings don't give a perfect seal and what leaks past has to go somewhere You may spring a leak at an oil seal if you let it pressurize by using a too small pipe. The sizes of pipe used for diff etc breathers are usually much too small for an engine breather.