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Found it.

A blow-off valve is designed to prevent compressor surge on turbo-charged engines. Compressor Surge occurs when the engine's THROTTLE PLATE is suddenly closed, giving the Charge from the Turbo compressor, no where to go. So the blow-off is plumbed in somewhere between the Compressor's outlet and the Engine's Throttle body, allow unwanted Boost Pressure to be vented to Atmosphere or Intake stream before Compressor inlet. So, that being said, it should be obvious why most modern Turbo-charged, direct-injected Diesel Engines would have no need for a "blow off valve". A diesel engine is controlled by fuel, thus not requiring it to have a "intake air throttle plate". In a diesel engine, all mixing of FUEL/AIR occurs in the Combustion Chamber, by design the Diesel operates in EXCESS AIR situations, it can pump up to 600X times more air than it needs @ low idle. A Diesel builds peak power and Manifold Boost when UNDER-LOAD, so when you cut fuel to that engine while under load, the Manifold boost still has a place to go since the engine is still pumping air. The manifold boost tends to drop off quickly since it is being displaced by the cylinders, and since the turbine drive pressure will be less as well. This is why diesels provide higher levels of engine braking over petrol/gas engines as the amount of air to be compressed is still the same as if the engine was running at full speed and Jake brakes and other compression braking systems raise the engine braking performance even higher again.
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