The M10 engine

The M10 is generally considered lame by E30 folks, but it was the engine that brought BMW back from the brink, in the 60s, and has powered the classic 2002 and E21 models.

With a turbo, it produces vast amounts of power, and that’s what powered some VERY fast BMW race cars in the 1970s and 1980s.

there’s no timing belt that can break like in the M20. There’s a nice, long-lasting, chain.

I used to own an M10-powered E21 but that was with lots of smog crap on it, and K-Jetronic.  The E30 variant has L-Jetronic, which is to me a lot less scary.