What diameter up-pipe do I need?
 

I have Gruppe-S headers (original) with VF34 turbo. The inside diameter of the header outlet is 50mm and the turbo inlet is 45mm.
So I guess an up-pipe tapered from 50mm to 45mm would be ideal. This would gradually accelerate gas flow towards turbo entry and eradicate any steps in diameter.
Anyone know where I could get one?

If, as I suspect, I have to fit a non-tapered one what size should it be, 50mm or 45mm or in between? Any advice appreciated.

I originally had a Gruppe-S up-pipe fitted (48mm dia) but this died so I have refitted the standard one (did not measure size but smaller than 45mm). This resulted in much better pick-up at low revs but I guess not so much top-end.

Engine is a UK MY99 with Tec 3 set to 1.2bar and full decat with quiet Prodrive back-box.

Does anyone know the inside diameter of the standard classic up-pipe? Is it smaller than 45mm?

it should be between 40 and 45mm i think. Most after market ones should increase the flow of a standard pipe anyway, but as i have found out recently, matching the bore of the pipe to your needs (which is what you are trying to do here) is fairly key to how the car can perform.