Every engine has some sort of crankcase breathing system. The internal combustion process creates pressure, some of which makes its way past the piston rings - along with fuel vapors - and into the rest of the block. Without proper ventilation, seals can burst and oil can leak, and that's of the block doesn't implode on itself first. Oil can also become contaminated from excessive fuel vapors being introduced. A typical Honda crankcase ventilation system uses engine vacuum to draw contaminants out of the crankcase and reintroduce it back into the intake manifold for recycling. It's known as the positive crankcase ventilation sysem (PCV), but it isn't the most effective thing in the world.

A PCV system takes into account performance and emissions, but with a big emphasis on emissions. It's supposed to reduce pressure. In a perfect world, we'd like to see 0 psi in the crankcase but this just isn't the case with stock Honda engines. All Honda engines exhibit some sort of crankcase pressure and forced-induction applications are much worse, creating significant power-robbing pressure down below thanks to blow-by. Introducing vacuum into the crankcase reduces these windage losses, which can translate into better performance - ventilation equals power. This is the same principle that's behind dry sump systems, although on a much simple scale.

In order to comply with emissions standards, modern PCV systems operate in a closed loop. This means that the oil vapors evacuated from the crankcase are reintroduced back into the combustion process. This is good for emissions reduction but introducing such gases back into the intake tract not only decreases performance but increases the likelihood of detonation. To understand the negative affects a PCV system can have all you need to do is look at all the gunk built up inside your throttle body and on your valves. Small amounts of fuel seep past the pison rings during each compression stroke - this is just the nature of the internal combustion engine, but it can lead to thin and contaminated oil. The PCV system's job is to remove this unburnt mixture and keep it from further contaminating the oil. It's instead reintroduced into the intake stream long enough to contaminate things and spit out the exhaust. The specially designed one-way PCV valve allows crankcase evacuation to occur, although how well it works at wide-open throttle is arguably minimal. It's one solution, but not exactly the best one.