The following is from KLRCARY on KLRWORLD:
This is an overview, and install of the PCV valve, or PerCiVal Mod. This has been well tested by The Patman, Marty, MADMAX, and many others. The PerCiVal designation is Marty's, I believe. The purpose is to reduce crankcase pressure, by using an automotive PCV valve in the crankcase breather hose. There are two main benifits to this.
First, there is a slight, but noticable increase in power. The engine feels "snappier", and revs more freely. The piston does not have to work as hard on the downstroke, fighting crankcase air.
Second, beacause the pressure is reduced, less oil is pushed past the rings. Oil consumption is reduced. Although a few have reported lower oil use at higher rpm's, most report only a reduction in sub 5000 rpm oil comsumption. Measured reduction in my case amounted to 20-25%, and many have reported similar results.
This comment is from NOBRAKES at KLRWORLD:
The PCV mod on the other hand, allows air to flow out through the PCV valve, but not back in. What this does is create a vacuum situation in the crankcase. This can be beneficial for performance also in that the air inside the case acts just like a spring - it is stretched as the piston moves up its stroke, and released as it moves down. While the stretching tends to resist the piston, the releasing tends to aid the motion of the piston. The net result is zero resistance due to the air pushing and pulling over one engine cycle. Contrast this with the crank vent mod in which only the opening is made larger - you still move a certain mass of air in and out, though it is a less restricted movement than stock and thus has better performance. With the PCV mod, no air is being moved in and out so in theory it should perform even better.
I think this pretty much explains it as best as I can understand. All I know is I could tell a difference. And am hoping to see a difference in my oil consumption. No it isn't a big amount, but any help is good.
Hope this helps.