Yeah, making an M103 motor faster is no easy task. Perhaps the cheapest route is a cam and throttle body, but at that point you're dropping some serious do-re-me.

I looked into all the options, as my wagon's sort of a slug (dependable and a great car, but a slug) so I took the advice you've been given and got a 500E!

Good luck, and sorry not to be giving you better news.

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Best regards, Michael
'92 500E
'88 300TE