The Whipple air bypass valve is the key to excellent fuel economy with a blower. It's plumbed between the supercharger and the throttle body and it opens and closes quickly to equalize the pressure in the system. When high vacuum is present (during idle and part-throttle cruising), the valve is open and no boost is made. As soon as the vacuum level drops under hard acceleration, the valve closes and the boost comes up instantly. We've seen as much as 19 mpg with our boosted 5.3L LS engine in this truck.