How do Harrop and Edelbrock SC kits not cause air metering problems?
 

Edit: Oops... I meant to create this thread in the Forced Induction sub-forum. Can a mod move it for me? Thanks!

I'm having some issues with the tune on my Harrop kit and I'm trying to track down the issue. While going over everything with a fine-tooth comb something occurred to me... Both of these kits re-route the front crank vent to the back of the airbox, before the MAF. The stock configuration runs this vent AFTER the MAF. The PCV line is still run from the manifold itself so it is under vacuum. But these two lines are both connected to the crankcase. That means that the PCV line is able to draw un-metered air directly from the back of the airbox, bypassing the MAF under heavy vacuum conditions. In both kits the vacuum lines are never exposed to boost so the un-metered is technically available to be drawn into the SC inlet through the same route.

Clearly this system works somehow, and I DON'T suspect this is the cause of my lean condition under WOT... but I just can't figure out how they get away with such a big vacuum leak designed right into the system.

-Matt

You are correct about unmetered air. The PCV valve restricts how much air can flow through under high vacuum. On full throttle it's unlikely there will be any vacuum to pull the air through. My guess is the volume of air that does get through unmetered is small enough to be tuned around.

they're not (primarily) MAF tuned.