Found this which I think answers my questions: -
"I recently had an old K03-035 come my way. This particular turbo had come off an Audi TT of 180HP dating back to the late 1990s. What struck me most was that the compressor housing casting number on this old K03 was exactly the same as the brand-new K04-001 I had just received from Kinetic. What was clear was that 3K had machined the inlet differently to use the larger K04 wheel but the casting itself was identical.
That turbo had a compressor wheel with the following dimensions:
Inducer: 33.6mm
Exducer: 46mm
Tip height: 3.7mm
This was a tiny compressor. The turbo used a very aggressive WG actuator spring and a standard K03 turbine and still managed to be rated at 180HP on that 5V TT.
The K04-001 compressor looks like this:
Inducer: 35mm
Exducer: 50mm
Tip height: 3.7mm
The compressor is only slightly larger in terms of its inducer but it has 4mm larger exducer dimension. It also has a much nicer blade design (4/4) versus the K03-035s (6/6). The K04 wheel is also quite a bit lighter so it has a lower moment of inertia and better spool characteristics.
It's a better wheel and is slightly larger than the 180HP-rated K03-035 wheel. It can flow enough air to make some decent power.
The K03-052 (K03S) compressor is a bit larger still:
Inducer: 38mm
Exducer: 51mm
Tip height: 4.4mm
and so could flow more than the K04 wheel, in theory. The K03S also uses a different compressor housing to go along with this new wheel. The larger inducer diameter required a larger inlet. The shape of the snail is also quite different as is the TIP mounting bolt. This is why a K03S TIP won't fit a K04-001 turbo. At first glance, the differences between the K03S and K04 wheels are subtle. Side by side you'd be hard pressed to look and tell one from the other. When you start measuring though, you see the difference.
The K03S is saddled with a tiny turbine:
Inducer: 45mm
Exducer: 38mm
Tip height: 6.8mm
Blades: 11
compared to the K04 turbine:
Inducer: 50mm
Exducer: 42mm
Tip height: 6.4mm
Blades: 11
Even though the TIP height fell a bit on the K04-001, its still a much better flowing wheel than the K03S and this is really the big difference between the two turbos: it's not so much that the K04-001 has a better compressor (it doesn't) it's that its exhaust side flows better. In addition, the WG actuator on the K04 is much stiffer than the K03S, meaning it will stand more internal backpressure before the WG forces itself open. Those that crank wastegates will appreciate this. (FWIW, "E05" turbos will make use of a slightly larger turbine than the K04 and, I think, a K03S compressor side...)"
had to register to your forum as i cant find the answer to this anywhere. I just bought the exactly same turbo advertised to me a as genuene BW K04-001 but when it came it had a black badge with numbers 06A 145 704L, K03-004, CF5018866. And the inlet housing had 5034 101 5066 number. Now whats confusing me is why does this turbo have a 12 vane compressor? I was under the impression that K04-001 have 8 vanes?
