The differences between the US and Euro BMW M3
There is a quite significant difference between M3's in the USA/Canada and M3's elsewhere (Europe/Asia etc.).
The "authentic" M3 (often referred to as the "Euro Spec." M3) has the real M-engine:
S50 B30 in the 3.0 Liter Euro M3 (built from 1993 to 1995)
S50 B32 in the 3.2 Liter Euro M3 (built from 1995 to present)
The S50 B30 develops 286 hp at 7000 rpm with 320 NM, the S50 B32 develops 321 hp at 7400 rpm and with 350 NM.
The following are the engines of the US M3:
S50us B30 in the 3.0 Liter US M3 (built from 1995 to 1996)
S50us B32 in the 3.2 Liter US M3 (built from 1996 to present)
The S50us B30 and S50us B32 both develop 240 hp at 6000 rpm and
The US M3 engine is - from a constructors viewpoint - a tuned standard 3-series 6-cylinder motor: The S50us engine is a bored and stroked M50/M52 engine which was/is used in the 325i(s)/328i(s).
The Euro Spec. M3 engine is a hand-built engine which has Double-VANOS, 6 single throttles (one individual throttle body per cylinder) and has the characteristics of a racing engine: High engine speed red line almost at 8000 rpm, High specific power output of over 100hp/liter. The single throttles are a typical feature of all M-engines from the late Seventies M1 to the allnew E39 M5 8-cylinder. They all had/have single throttles.
Beside this essential difference, the US M3 also has a softer suspension, other headlights, wheels of different materials and other more or less minor differences. Also the US M3 is available with an automatic transmission beside the 5-speed manual gear box. The Euro Spec. M3 is only available with a 6-speed manual or a sequential gearbox (called "SMG"), which either works like a manual gear box (but without a clutch) or automatically. In 1995 the 3.2 Liter Euro M3 also got the 1994 M5 3.8 high performance brakes with compound discs, which made it one of the best braking cars on the German market (even competitive to the Porsche 993 TT for example).
As for the ability to get one over to the U.S. it costs a large sum of money because there is strict emission requiriments for cars in the U.S. and you have to do quite a few mods before it is legal.