Stops the crap going back in your engine/intake, originally made this way due to the enviroment, but to hell with it, dump the **** in to the atmosphere, through out the worlds history it has always heated up and cooled down, can you tell I don't believe in all this global warming bollox
if you block them off you'll pressurise your head and cause alot of problems including oil passing through your valve stem seals and into your bores, crankcase will not ventilate properly bla bla bla. leave it the way it is unless you have a catch can.
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