The reason it's not on the passenger side is that some air is drawn into the crankcasevia the hose from the air intake, due to the negative crankcase pressure resulting from a properly functioning PCV valve. I wouldn't use breathers, because they are not as effective at eliminatingoil vapors and blowby gas, withresulting increased pressures, from the crankcaseas is the vacuum scavenging supplied from the intake manifold. The PCV system is more than just an emission control, it's a means of efficiently removing contaminants and pressure from the crankcase, good for the longevity of your engine and it's gaskets. The downside is that over time it does result in deposits on your intake manifold and piston tops, but that can be all but eliminated by using an in-line filter or catch can between the PCV valve and the intake mainfold orificeto which it connects.

 

+1

 

+2 that's what I've been told in my auto class.

 

I wouldn't run without one myself. That little bit of vacume can help increase hp. A lot of racers would take old smog pumps and connect them up for even more vacume, now they sell crank case pumps just for that reason.