BlackMozes in the uk as long as the car wasn't made after jan 2015 (if i remember rightly) the vehicle doesn't need a cat so long as its a turbo diesel. just make sure it doesn't smoke loads
My 330d is also straight piped but my turbo whistle is so damn quiet! You can’t hear the turbo at all when the engine is idling. So how do I get as much turbo whistle as you??
Diesels with variable geometry turbos don't use a wastegate, so what you can hear is the so called "surge". As the compressed air is trying to flow back through the turbo's compressor blades, it makes a cool noise.
Pinyo 81 waste gates and VGT vains are on the exhaust side of the turbo, what you are talking about happens on the inducer side. The owl noise is the VGT vains closing and the exhaust gas being manipulated as it blows past them.
