The swap is entirely possible. It wont' work as well in the 93-95 models as well as it would in a pre-91 model however. The 93 uses a 12V V6 which is found also in the Audi 100. Different engines mounts/suspension etc. The best swap would be a 91 or older and put a 20V Turbo 5 from a 200q or an early S4. It's actually not that expensive either provided you find the engine. Another common swap is the turbo 10V 5 from a 5000CS/200 into a 4000. I have also seen 20V turbos put into coupe quattros.
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The S4 in my name is for urS4
Never Follow
The S4 Engine - 20v 5 valve comes standard in:
1990-1991 - Audi 90 Quattro / Audi Coupe Quattro
Not so. The UrS4/S6 engine is a 2.2L 20v 5cyl turbo (AAN engine code). The motor in the Audi 90 and Coupe Quattro is a 7A code non-turbo motor. The '91 Audi 200 came with a early variant of the UrS4/S6 engine, known as the 3b. They are a little cheaper, but hard to find nevertheless.