MTM (Motoren Technik Mayer) offers ECU upgrades for the 2001 Audi B5 S4 that can significantly increase performance. These are typically software conversions applied to your existing engine control unit (ECU) or purchased as pre-flashed used ECUs. 

MTM offers several stages of ECU conversions for the 2.7T engine, which involve reprogramming the stock Bosch Motronic control unit. The performance gains are substantial, but achieving higher stages requires additional hardware modifications like upgraded air filters, intercoolers, and turbo kits