Steve Dinan has been enhancing BMW models since 1979. But don't throw his company into the ring with the dozen or so other tuners who tweak, tinker and piggyback upgrades on the famed German marque.

With 735 horsepower and a Dinan-tuned suspension, the CarBahn BMW M4 is a precision-guided sledgehammer. Okay, so what the hell is a CarBahn? And what does it have to do with this $135,000, 735 ...

First, Harley's conclusion that Dinan builds products that feel 100 percent BMW-spec is absolutely true. And second, Steve and the gang haven't just created a tuned M5, they've built a better one.