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There's no official definition of the stages but the general consensus is:
Stage 1 - Air filter and freer flowing exhaust. Originally launched by Morgan's Aero Racing and subsequently systems offered by Krazy Horse, Garage 56 and some others. This transforms the drivability of the car and does not require any modification to the ECU. A budget option is the so called "Bafflectomy" where you drill out one (or more) of the stock exhaust baffles. Cheap but can give a loud, harsh exhaust note.
Stage 2 - 569 cams which require a remapped ECU. Originally MMC were able to provide unlock codes for the stock ECU so the map could be changed but this is no longer available. So nowadays you require a replacement unlocked ECU and suitable map. S&S unlocked ecu's are like hens teeth but the Emerald ECU is becoming a viable alternative.
Stage 3 - Anything else, predominantly a big bore kit, higher lift cams and larger throttle bodies.
Personally I see Stage 1 as essential and having had both the Aero Racing and Krazy Horse versions I feel the Krazy Horse is better. Garage 56 are still available and quite popular with a more mellow exhaust note but are somewhat larger diameter silencers. For info the main benefit of Stage 1 comes from the freer flowing exhaust, the air filter doesn't make much difference.
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