CFM Performance 1-0702-1 Billet Valve Cover Breather Oil Cap 2008-2015 Lancer Evolution X 10 & Ralliart (Flat Black) CFM Performance valve cover breather is the ONLY one in the market which incorporates a check ball to keep unmetered air from coming through while the crankcase is under vacuum Acts just like a factory oil cap under normal driving conditions and when you get into boost it will release any crankcase pressure present