Here is my sermon on diesel dump valves: submitted as the draft for a sticky thread, if the mods deem is suitable. Comments please - lets make this as accurate as we can
Diesel dump valves:
Petrol engines have a throttle mechanism which controls the flow of air between inlet (turbo-compressor) and the engine combustion chamber, so as to keep the air-fuel mixture in the ignitable range while throttling the engine's power output. The mass of air taken in and fuel added must both be carefully controlled.
When you close the throttle on such an engine the compressor is still pushing air into the inlet and it suddenly has nowhere to go. The back pressure will inevitably slow the turbo down, and cause turbo lag when the throttle is opened again.
The solution is to provide a pressure-actuated valve, which reacts to a difference in pressure between the two sides of the throttle. A dump valve opens to dump the excess pressure to the atmosphere. A recirculating blowoff valve diverts the air back into the inlet and does a much better job by equalising the pressure on both sides of the turbo. Without one of these, throttle response will be slower and throttles and turbos would have to be much more heavily engineered.
So dump valves are about throttle response at gearchange and other sudden transient events, not about performance in the leadfoot top-speed way of thinking.
The need for a dump valve is a weakness of turbocharged petrol engines, putting another contraption in the inlet which disturbs the gas flow and is a point of failure. I find it mildly astonishing that anyone should be proud of their car farting on the overrun. But then I llike my diesel so I'm probably disqualified from having an opinion
Diesel engines have no throttle. Power output is controlled by the fuel quantity injected at each combustion stroke. Mass flow around the compressor-combustion chamber-turbine loop is always uniform. This makes diesels much better candidates for forced induction.
A dump valve can only do harm to the performance of a diesel.
Turbo wastegates are not dump valves, they limit the pressure on the exhaust turbine to prevent the turbo from overspeeding when the engine's gas flow becomes too high i.e. at high rev's. The higher-performing VAG turbos use variable-geometry inlet vanes and have no wastegate.
There are "dump valves" for TDi's that are activated by the ECU, such as the Forge one. If fitted and adjusted properly they do not affect performance, they just make a noise. So you can fit them anywhere you like, wherever you want the noise to be heard best - say on the dashboard, or maybe on the roof.
There is style, and there is substance. A noisy dump valve is a style choice, it has no substance, no usefulness on any diesel engine.
Here endeth the first lesson