The blowoff valve or dump valve is present in a turbocharged petrol engine to prevent damage to the throttle and turbocharger compressor and to reduce turbo lag.
Petrol engines have a throttle mechanism which restricts the flow of air between inlet (turbo-compressor) and the engine combustion chamber, necessary to keep the air-fuel mixture in the ignitable range while controlling the engine's power output. Both mass of air taken in and fuel added must 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 overpressure could deform a throttle butterfly, and will inevitably slow the turbo down due to back pressure (thus also putting more back pressure on the exhaust), leading to turbo lag when the throttle is opened again. The solution is to provide a pressure-actuated valve, which reacts to an excessive difference in pressure between the two sides of the throttle, opening and dumping the excess pressure to the ambient (dump valve) or back into the inlet (recirculating blowoff, which does a much better job by equalising the pressure on both sides of the turbo). Without this, throttles and turbos would have to be much more heavily engineered and throttle response would suffer.
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 side 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 run a 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 stroke. Gas flow around the compressor-combustion chamber-turbo impeller 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 impeller (exhaust) side of the turbine to prevent the turbine 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.
