Under boost there is no vacuum to the catch can, this type of system routes another line to the intake where the crankcase is vented so the catch can can get all the blow by under boost also. And since it took the hole the crankcase was vent, it's design vents the crank case to atmosphere. I understand how it works but think there could be a better way. But it does work. I'm just waiting for the perfect system before upgrading my basic catch can set up.