Per specs that are widely circulated online, dimensions of relevant turbos are;
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K03-005 aka K03:
Compressor: 36/50
Turbine: 36/45
K03-029 aka K03S Specs:
Compressor:36/50
Turbine: 40/45
K04-015 Specs:
Compressor: 38/51
Turbine: 42/46
elsewhere as;
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Ko3 - 2000
COMPRESSOR WHEEL
Inducer: 33.6mm
Exducer: 46mm
Tip height: 3.7mm
TURBINE WHEEL
Inducer: 45mm
Exducer: 38mm
Tip height: 6.8mm
Blades: 11
Ko3S - Late 01+
COMPRESSOR WHEEL
Inducer: 38mm
Exducer: 51mm
Tip height: 4.4mm
TURBINE WHEEL
Inducer: 45mm
Exducer: 38mm
Tip height: 6.8mm
Blades: 11
Ko4-001
COMPRESSOR WHEEL
Inducer: 35mm
Exducer: 50mm
Tip height: 3.7mm
TURBINE WHEEL
Inducer: 50mm
Exducer: 42mm
Tip height: 6.4mm
Blades: 11
Ko4-015
COMPRESSOR WHEEL
Inducer: 35mm
Exducer: 50mm
Tip height: 3.7mm
TURBINE WHEEL
Inducer: 50mm
Exducer: 42mm
Tip height: 6.4mm
Blades: 11
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Lastly, my own measurements I have personally made are;
KO3-029 aka K03S, OEM pulled from a 2003, measurements of the actual wheel diameter, not the housing ID:
Compressor:34.5/50
Turbine: 39.3/44
KO4-022, from a 2001 TT with 225hp code AMU:
Compressor: 41.8/56.0
Turbine: 42.0/50.0
What I thus observe is that there are several different conflicting measurement for the wheel size of the K03-029 (aka K03S) that came in B6. I am not sure if there were different size "KO3s" actually made (vw vs audi use?) or if these conflicting measurements contain errors, sloppy rounded measurements or if some of them were measuring the (larger) ID of the housing rather than OD of the wheel. I can confirm the last two for the K03-029 & K04-22 that I took myself with calipers from disassembled turbo wheels.
That said, it appears that the KO3-29 and the KO4-15 use the same cold side compressor paired with a larger hot side turbine on the KO4-15. Also not that not all "KO4" are the same, the OEM 225hp KO4-22 uses a substantially larger cold side compressor (41.8/56.0mm) as compared to the K04-001 & KO4-15. Dont expect to get OEM 225hp turbo performance by changing from a KO3-29 to a KO4-15, seems that the larger hot side compressor wheel of the KO4-15 might even give later/slower onset of boost when paired with the exact same (small) compressor of the KO3-29.
As codemode mentioned, the transverse application turbos will not bolt-up interchange onto a longitudinal 1.8T. If you had a mark IV GTI (KO4-001??), you might be able to interchange the compressor housing cover from a KO3-29 to get the correct (short, unbent) outlet/charge pipe needed for a longitudinal B6. The hot side turbine housing will not bolt-up, uses a different incompatible size/spacing bold pattern and even if you could get it to bolt up, the entire turbo would be oriented wrong for longitudinal use. If you had a free KO4 from a transverse application that you really wanted to bolt up to your audi 1.8t, either use the hot side turbine housing from a KO4-015 or modify your KO3s turbine housing to accept the larger K04 turbine wheel.
I had an otherwise unused KO4-022 laying around. I used a handheld die-grinder to slowly and carefully enlarged the throat of a KO3-029 turbine housing from 39.3mm up to 43.3 mm in order to fit it over the KO4-022 turbine wheel and bolt to the KO4-022 CRHA. I also had to do some grinding on the volute/scroll of the K03-29 turbine to open it up to better match the larger turbine wheel. The turbine A/R is a bit smaller than the original K04-022 housing and there is always threat of the housing cracking where I reduced the wall thickness but overall it has been a success and I have been driving it for 6+ months without problems. Onset of boost is no later than the OEM KO3-029, probably helps that the larger compressor wheel does not need to spin-up so fast to start making boost and then when it does get going the larger size has significantly more flow capacity. No problems experienced with choked wastegate boost creep from the hybrid hot side.
K04 Compatibility
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