I have a full Powervalve with their 200 cell cat. Madone is right, basically there's a valve at the rear silencer that is actuated by boost pressure at the inlet manifold. Anything under about 5 psi and the exhaust gasses go through the silencer, then as the boost builds the silencer is bypassed, it essentially becomes a straight-through rear section, and you get full on tone from the exhaust.The benefit of this is that the exhaust is quiet and subtle when you're just rolling around (albeit a deeper sound than the standard exhaust), but then when you plant your foot it really sings. And believe me, it's an incredible noise.
Other things I like about the exhaust - you get some lovely burbles and crackles on the overrun. It's 3" all the way. The quality of the welds is as good as you'll get. The sport cat is 'oversized' to maintain flow. The R32 style system with 'R' tips looks brilliant.
Google Milltek specs and pics for yourself and make your own comparisons - I've come across as a BCS fanboy in past threads, so I'm not going to directly compare them myself in this one...!
Performance - unquestionable. I could immediately feel the extra torque and flow as soon as it was fitted. In fact the clutch started slipping a few weeks after. That's on a Stage 1 R-Tech map. When I get round to having the map tweaked to account for the extra flow it'll produce even more power. As an example, @rashg60 on here gained 23bhp and 44lb/ft (if I remember correctly) going from Stage 1 R-Tech with the standard exhaust to Stage 2 R-Tech and the full Powervalve. That's with no other mods apart from a drop in panel filter that was in the car for the Stage 1 mapping anyway. And if you put the peak numbers to one side, the before and after graphs show how the torque now spreads much wider across the revs.
Best mod I've done.
