Originally Posted by servion

Many of you are confused.

The stock PCV system exists to take pressure and vapors out of the crankcase (both above and below the pistons) and suck it back into the intake manifold. The purpose is so that instead of venting it into the atmoshpere, you are sucking it into the motor with the air from your intake and combusting it (i.e., you burn it off and filter the exhaust with your cat instead of just venting it out into the air).

Stock, its setup for optimum emissions and worst performance. There are 2 general situations this stock PCV sytem can be in: Idle/crusing (vacuum system), and WOT (atmospheric pressurize/perhaps boost).

1) Like someone said previously, the intake manifold has a lower pressure. When you're idling, the vacuum in the IM is higher than that anywhere else in the system, so it draws the oil vapors from the crankcase (both the back of the block and the valve cover) into the intake manifold for combustion. You are sucking up nasty oil vapors and burning them (which premotes detonation). The only upside to having the stock pcv lines hooked into the IM and intake is that the vacuum they provide in the crankcase can actually "pull on" the rings to create a better ring seal. This only improves the ring seal while under vacuum conditions (it doesn't increase your compression or help your rings seal better when you're racing).

2) at WOT, the crankcase becomes pressurized. The valve cover vent and stock separation box (the black box on the back of the block) allow extra pressure from the crankcase to vent into the intake/intake manifold. This extra pressure pushes out the oil vapors into your intake air stream and combusts them, premoting detonation. If you're turbocharged its even worse because the boost locks the PCV valve shut. At this point, the only place oil vapors can escape the motor is the valve cover vent. If it is routed back into the intake (like before the turbo or on the intake tubing), you're increasing the chance of detonation.

The best way for your performance to handle the stock PCV system is to set it up so that the oil vapors are AT LEAST not going back into the intake or intake manifold. Removing the oil vapors from the intake air stream created by positive crankcase pressure will decrease your chance of detonation, and your mixture will just plain burn better.

The only reason you would want to run the tube to your intake from the valve cover or to your intake manifold from your stock oil separation box is if you're concerned about emissions (i.e., saving the bunny rabbits). If you're worried about saving the bunnies, don't modify your car.