What is their usefulness? Also any good on pretty much stock setup with headers?

I am looking at getting new valve covers and trying to decide if one with a breather is worth it.

Thanks.

 

Pros and cons. It does help relieve some pressure off the motor. But can add unmetered air into the engine causing the pcm to adjust more. I have one on my passenger side, but the tb hose/valve cover has been removed.

 

If you do a search on this topic, Injuneer has posted some interesting info. The general consensus is that forced induction guys can see some benefit, but there is no gain for naturally aspirated engines from using a breather.