they do nothing for performance. They are for smog and such. It's supposed a hose from the valve cover to the intake to recycle any fumes not sent out through the exhaust. It just sends the fumes through the engine again to get burn up. Once you remove that hose, for turbo's mainly, you should put a little breather there to filter the fumes a little. Plus, they look cool. Other than that, it's nothing.