Back in Tune

CarBahn CB3 M4 HIGHS: A monster inline-six with an available warranty, massive grip and lateral stability, a better-looking face. LOWS: The exhaust needs an off switch, suspension links clatter, steering is still mute.

- David Beard

WE'RE AGING and, at times, forgetful. Deep within our lateral-g-scrambled heads filled with boost pressures and quarter-mile times are memories of when Steve Dinan's tuned-up BMWs roamed the earth. Dinan sold his company in 2013, and our recollections of screaming inline-sixes and V-8s started to fade. Enter CarBahn, Dinan's latest powertrain- and chassis-tuning adventure, here to reignite the days of his hopped-up BMWs.

Dinan's newest iterations won't completely overwrite our recollection of the fantastic massaged and high-revving E39 M5 and E90 M3 V-8s. Those cars were special, extra special. Now, in the era of turbocharging, adding horsepower is less labor-intensive. It's also a lot less expensive.

In 2010, it cost $31,234 to get an extra 113 horsepower and 112 pound-feet of torque from the M3's V-8. Today, dialing in a claimed 735 horsepower and 668 pound-feet from BMW's twinturbo 3.0-liter inline-six requires only CarBahn's Stage 1 Power package, which adds air filters, an extra heat exchanger, and 3.3 psi of boost for a total of 28.0 psi-all for $3385, plus installation. If you worry about the engine puking its internals out the bottom, CarBahn will match BMW's fouryear/50,000-mile warranty for $1500.

Exactly how much power and torque the kit adds depends on your starting point. BMW says the M3/M4 Competition xDrive models produce 503 horsepower and 479 pound-feet, numbers we've long believed to be conservative. CarBahn strapped the engine to a dyno and confirmed our suspicions: 591 horses and 575 pound-feet at the crank. Add the same kit to a 543-hp BMW M3 CS or any S58 inline-six, and the result is again 735 horses since the hardware is all the same.