The last time the Dinan name graced the pages of Autoblog, Michael Harley was waxing poetic about the S3-R BMW 1M Coupe, a car that still stands in his ranks as one of the best cars he's ever driven.

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, ...