Gerard asked: Are these interchangable? Both use a 3B engine. Gerard: I have run a US spec 1991 200 3b engine with both the 200 3B and the S2 3B ECUs, with no issues HTH -Peter 1991 200 20v Q Avant Titan Grey 1991 200 20v Q Avant Indigo Mica 1990 CQ silver (awaiting S2 transplant) 1985 4000s Q....black and peeling CC