Re: e36 6 cyl engine
The main reason for that is both are entirely different engines. Euro S50 are designed as a high performance engine for M3 but US versions are just M50/52 engines with larger capacity and slightly more aggressive cams.
Why is that so? Well, its because of marketing reasons due to poor E30 M3 sales in US.
If you want the US version to perform as good as the Euro version, yes its possible but its costly. You will need to port the heads, change the cams and stiffer springs and solid lifters, ITBs etc......Very expensive...
Instead most people simply go the FI route. More power and yet cheaper. US is bless with cast iron engines instead of aluminium ones (due to Nikasil issue). Thus they are very strong and can take lots of boost.