Steve Dinan Built a Better BMW M4: Review

Okay, so what the hell is a CarBahn? And what does it have to do with this $135,000, 735-horsepower BMW M4 Competition?

Only the most down-bad of BMW nerds will know: CarBahn is Steve Dinan’s newest venture after selling off his previous company, Dinan. The mission is the same: It takes new BMWs and thoroughly optimizes them for balance and function without going for ridiculous, internet-breaking power numbers or lap times. The car operates fundamentally as it did from the factory, but just better.

That said, this particular CarBahn M4 (which happens to be Dinan’s personal car) does have Hellcat levels of horsepower. But it also has extraordinarily wide Michelin Cup 2R tires, custom springs and sway bars personally tuned by Dinan himself, and a host of other upgrades meant to support that immense power.

The end result is spectacular. This is no ordinary tuner car. This is a tuner car that achieves what the factory should’ve. It’s precise, brutal, and fast. It’s communicative and balanced, and truly lovely to drive every day. This, folks, is what the BMW M4 CS should’ve been.

Photo by: CarBahn

The story goes much deeper than simple tuning parts. To be frank, the CarBahn CB3 treatment on paper does not necessarily warrant its $20,599 price tag, save for the abundance of carbon fiber aero parts. For your money, you get the CarBahn Stage 2 power package, which nets the aforementioned 735 horsepower and 668 pound-feet of torque through an intake, exhaust, upgraded primary heat exchanger, and a tune to support it all.

Then you get a host of suspension updates built around Dinan’s IMSA-winning magic. Using the stock adaptive dampers, Dinan retuned everything to meet his handling expectations and the stickier tires. In his words, he “better matched the spring energy required” of the stock dampers in Sport+ mode, and spent eight months carefully iterating the thickness of both sway bars and rates of the springs. There is one trick hidden: Bump stops. Dinan spent considerable time tuning the bump stop characteristics to finely control every inch of suspension travel.

Dinan explained, “We spent months figuring out the best packer for the car, as it’s essentially another tunable spring rate.” He makes it clear that a nice bump stop engagement makes a meaningful difference in ride and handling. Where the M4’s normal springs have a linear rate—stiffness remains the same throughout suspension travel—bump stops have a progressive rate, meaning that stiffness increases as they compress.

Photo by: CarBahn

Pros: Stupidly Fast, Lovely Suspension, Works & Feels Like Factory

If tuned correctly, bump stops can give the suspension a nice end-of-travel landing pad that helps further control the body over large bumps. If tuned expertly, they can even act as a supplemental spring to aid handling—which is what Dinan achieved.

To complete the effect, a set of spherical front bushings remove needless rubber from the front control arms, the rear toe links are converted to sealed spherical bushings of the Lemforder type, and a five-point carbon-fiber front strut-tower brace aids overall rigidity. While the CarBahn suspension is called “coilover,” it’s actually a height-adjustable spring on the stock M4 Competition damper. Meanwhile, both sway bars are four-position adjustable.

Photo by: CarBahn

Photo by: CarBahn

Photo by: CarBahn

Though it is rapid, accelerating as hard as 735 horsepower should, it is easily the least interesting thing about the CarBahn M4. What truly sent my coccyx and brain into another dimension was the handling; How this car conquered every situation I threw at it was core-shakingly impressive.

I took this brute on my roads of choice—the canyons surrounding Los Angeles—before even speaking to Dinan. I was convinced he used some highly bespoke and ludicrously expensive dampers, not a relatively simple retune of springs and sway bars built around factory dampers. What he achieved with stock dampers is exceptional.

Photo by: CarBahn

Cons: Expensive Kit For The Parts Used, Understated Looks, Could Use More Theather

The CarBahn M4 dispatches huge bumps with the suppleness of an S-Class minus the annoying secondary motion of an overly soft suspension. It has the transient response and direction changes of a much smaller sports car, all while maintaining pinpoint-perfect mid-corner attitude control and balance.

The BMW steering, which is normally numb, is weightier and more precise than stock thanks to the wide rubber—295mm front and 305mm rear Michelin Cup 2Rs—and it works better with the new chassis tuning. It’s easier to feel the wheel tighten up thean lighten with brake pressure and overall grip. Those looking for actual steering feel should still look elsewhere; there isn’t any extra granularity or texture through the wheel. But it still does the job well enough, and the front spherical bushings communicate road surface information through the car’s body.

If anything, the M4’s steering matches the effortless character of the CarBahn powertrain. The tune and throttle mapping are reassuringly progressive, while the linear pedal feels refreshingly intuitive.

Photo by: CarBahn

Photo by: CarBahn

What was loveliest about the CarBahn M4 was how organic it all felt—it wasn’t an exercise in defeating physics through electronics. The handoff between every stage of cornering and braking was beautifully controlled, showcasing Dinan’s talent, and the value of old-school chassis tuning.

All of CarBahn’s parts interfaced well with what already existed from BMW. The all-wheel-drive and adaptive dampers helped Dinan tune more rotation and aggression into the car, all while keeping it relaxed enough for everyday use. Surprisingly little else is changed; The gearbox tune for the ZF eight-speed is stock, as is the all-wheel drive torque distribution.

It’s a marriage of what’s so good about BMW, and what Dinan is exceptional at. It feels like a return to form that could’ve been from BMW. It’s a precision-guided sledgehammer, full of character that made old M cars so spectacular. CarBahn is doing what BMW M isn’t.