The open element is at atmospheric pressure, the crank spinning increases said pressure that the one (or two) open elements can't keep up with. The pcv can and will suck more pressure out of the motor as the crank speed increases. A standard off the intake/carb works great and the one being used off exhaust works as well just know that it can produce too much suction and pull in seals (seen it happen on a built 440 using the header evacuation system, ended up with oil all of the place after it sucked in an intake seal).
