purpose of a dump valve

[Ed Doe]

People have nearly explained the whole theory here.

To maximise throttle response, the throttle plate will be positioned on the end of the inlet manifold, after the turbo (ie. it is sucking air in and forcing it down the pipework towards the throttle plate). This is fine when you're on wide-open-throttle. But when you take your foot off the throttle, the throttle plate closes. The turbo will still be spinning and dorcing the same quantity of air down the inlet however. This air has no route through, and therefore pressure will build up between the turbo and the throttle plate. When the pressure becomes too great, it will overcome the force applied by the compressor, and will reverse direction, causing a rapid and unbalanced deceleration or stalling of the compressor, which creates this chatter. Not to be confused with compressor surge which is a totally different kettle of fish altogether!

A dumpvalve is used to vent the pressure to atmosphere, and is actuated (ie the valve is pulled open) by the vacuum in the inlet manifold when the throttle plate is closed (fyi petrols have negative pressure (vacuum) in the inlet manfiold when the throttle plate is shut.)

A recirculating valve is very similar to a dump valve, but instead of venting to atmosphere, it recirculates the air back round into the inlet pipe just before the turbo, so in effect recyles the air again.