First Drive: CarBahn’s M4 Is a Tuner Car Done Right

It’s been more than 10 years since Steve Dinan sold his eponymous company—a shop that turned out sharpened, more powerful versions of some of our favorite BMWs. Now he’s back on the scene with CarBahn, his new venture that still reworks BMW M cars but has expanded to tweak AMG models from Mercedes, S and RS models from Audi, and Porsches, too. Our first foray into Dinan’s latest iteration of tuned BMWs came in the form of an M4 Competition xDrive fitted with CarBahn’s mid-level CB3 performance package.

As-is, the BMW M4 Competition is no slouch, with its twin-turbocharged 3.0-liter straight six producing 503 horsepower and 479 lb-ft of torque through a ZF eight-speed transmission that sends power to all four wheels with its xDrive all-wheel-drive system. CarBahn offers several levels of tuning to increase that output with additional boost pressure combined with a streamlined intake.

Our test model came equipped with the $20,599 CB3 package that matches a tune with a larger charge cooler, a carbon-fiber intake, and a new fascia that flows 40 percent more air to net an output of 735 horsepower and 668 lb-ft at the flywheel using premium pump gas. That package also adds new coilovers and anti-sway bars that result in a lower, firmer ride, along with a host of carbon fiber trim both on the exterior and under the hood.

The list includes carbon fiber dive planes, a carbon fiber strut tower brace, and a carbon fiber decklid spoiler. Those new additions match the factory BMW carbon fiber roof and looked great on the car’s dark finish. The restyled fascia is also a styling improvement over the polarizing factory grille and helped give the black car an understated yet sinister look.

Our test vehicle was equipped with extra-wide Michelin Cup 2R tires that provide an absurd level of grip—the amount that can only be done justice on a track with a skilled driver. However, they still offered confidence on mountain roads where they were nowhere near their limit. With all four tires clawing the pavement, the CarBahn M4 could rocket out of corners without a hint of tire slip. The main drawback to the wide, sticky tires was their propensity for tramlining over grooved pavement. Before we forget, there’s another bit of performance gear that CarBahn adds: stainless steel brake lines. Without a back-to-back drive to know better, we can’t say whether they improve clarity of feel through the pedal. However, the overall response is immediate, and the calipers clamp down on the car’s massive rotors assertively.

Specs: CarBahn M4 Competition xDrive

• Price: $127,744 ($98,145 M4 Competition + $20,599 CB3 package + $4,000 wheels, +$3,500 exhaust and $1,500 warranty)
• Powertrain: 3.0-liter twin-turbocharged inline-six, eight-speed automatic
• Output: 735 hp; 688 lb-ft
• Layout: Front-engine, all-wheel-drive two-door coupe
• Competitors: AMG GT, Audi RS5

While the engine’s peak numbers are impressive, like most modern boosted engines, the torque curve is broad and flat, so there’s power to be had throughout the engine’s range. The engine behaved consistently at low, part, and full-throttle, and felt like a well-polished factory powertrain. We also appreciated that CarBahn didn’t add any gratuitous pops or crackles to the tune. Even the suspension, which was lowered and more firm than even the M4 Competition’s factory settings, retained plenty of travel and was compliant over rough pavement.

Our gripes with the car, and they were mostly minor, weren’t CarBahn’s doing—they stemmed from BMW’s choices about the interior. The deeply bolstered seats (replete with their odd center bulge) aren’t the most accommodating for taller folks, and this six-foot-three-inch, 250-pound author found things rather tight. The seats provide excellent support during spirited driving, but the length and depth of the bolstering on the lower portion of the seat constrain the legs a bit too much made ingress and egress a chore. In comfort, we’d give the seatbacks an A and the seat bottoms a C-. Of course, looks are also subjective, but the seats do fit that suitably aggressive M image.

Our only other quibbles are also baked into the M4 and aren’t related to CarBahn. For example, while most of the headlight functions are located on the dash in front of the driver’s left knee, the automatic high-beam function is controlled by a button in the center console along with the controls for the gauge cluster rather than on the steering wheel or column. The buttons for the climate control also gave us fits. Located in the center of the dash, the tiny HVAC buttons required a long reach and required close attention to hit the right one. Again, these are minor gripes—an owner would probably adapt to them in no time.

Part of the fun of going to a tuner is the ability to make tweaks to your liking, and CarBahn will let you select the parts that add up to the well-balanced CB3 package independently if you so choose. Other bits like the optional forged wheels($4000), brakes, aforementioned carbon fiber, and the optional exhaust ($3500) do look good and helped shed 200 pounds from the standard M4—a substantial number on a 4000-lb car, but we’d likely omit them from our build sheet. The CB3 package comes with so many performance enhancements as-is, and the chassis is more than capable of handling the power, especially in street application. That said, we get the allure of gorgeous wheels and assertive exhaust, and figure that at this point plenty of buyers will tick those boxes, too.

All told, the CarBahn additions worked well to elevate the M4 and create a system that works seamlessly together. That Steve Dinan continues to offer packages and parts that add even more spice to some of Germany’s most exciting platforms is a good thing, indeed.

Highs: Great balance of sporty handling and ride quality; tremendous traction; piecemeal performance upgrades available.

Lows: The M4 still has some inherent design quirks.

Takeaway: The CarBahn M4 adds thoughtful suspension upgrades, unique looks, and a sizable bump in horsepower for a justifiable price. The car for the tuner who has grown up to realize the value of parts designed to work well together.