Dump valves allow the turbo(s) to carry on spinning in free air when the throttle is closed during gear changes. It helps to eliminate turbo lag. If you're a race driver you probably won't need 'em as you'll keep the throttle pedal flat down (against the rev limiter) during changes anyway.
Clearly this is more economical if your sponsor is paying for your tyres, clutches, and fuel.
Dump valves don't really have a place on the road with sophisticated engine management systems.
But they do make a good sound if you like that sort of thing. |