There's topics covering this, should search on the topic of dump valve and diesels.
But basically it's not good - it's bad it,s basically a leak in the system, tdi don't need a dump valve/bov like it's petrol counterparts. Fitting one in your case will definately mean a loss of performance/economy.
It also doesn't sound like a dump valve/bov fitted on petrol counter parts.
If you still want to fit one then, you can fit any dump valve/bov, I'll have my forge split-r for sale soon if you can wait a week.
Normal BOV's may cause your turbo to stall . Decatting would release a few horses but a straight through exhaust would just make it sound appalling unless it was a really fancy pants system, you'd also have to get the system diameter correct to optimise performance.
Glad you changed your mind, a decat is definately a must on a derv or atleast I would recommend it, it's cheaper than buying a replacement cat, it frees up a few horses and gives more torque, plus best bit of it all is you won't have any problems with emissions when it comes to MOT [Y]
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