Well like every one else pretty much every where i always believed that cars needed backpressure to run properly and that this was the reasons for power loss on small cars with big zorst (or big cars with huge zorst), hehe i was wrong though. Apparently the real reason for the loss of power is that while a bigger zorst dose reduce backpressure it also reduces the air flow "speed", you need a zorst that lowers the backpressure while still having a high flow rate since this fast flow of air tends to have a suction type effect drawing out the emissions from the valves, and of course better air quality when combustion takes place = more power
Basically it's the same as the old backpressure myth but it's always good to know the truth behind some thing. And of course this rule only applies to NA cars.