We Rode Up A Swiss Hill Climb In One Of The Last 99 BMW Z4s

BMW has stopped building the Z4, and nobody in Munich is working on a replacement. Bovensiepen saw that coming. During the final four weeks of Z4 production, the company had 99 cars built and set aside. Those cars are now being converted into the Bovensiepen Spider, one at a time. We met company CEO Andreas Bovensiepen at the Bernina Gran Prix in Switzerland. He showed us around the car, then put us in the passenger seat for a run up the hill climb course.

The Spider is Bovensiepen’s third model, after the M4-based Zagato and the M5 Touring-based 05 GT. It’s also the simplest idea of the three: a light, rear-drive, two-seat roadster at a time when most carmakers are leaving the segment. Andreas brought up Jaguar and Porsche as two brands that have already walked away from cars like this.

A G29 Z4 Reshaped By Frank Stephenson Design

Bovensiepen again worked with Frank Stephenson Design, the studio that also did the 05 GT’s exterior parts. The result is closer to a heavy facelift than a new body. You can still tell it’s a G29 Z4, but almost every panel you’d look at first has changed.

The front fenders get new panels with functional air outlets that send air down the side of the car. Andreas described it as form following function, and the outlets also make the car look longer. The rear deck spoiler adds to that effect by visually extending the trunk.

The spoiler is functional too. Bovensiepen developed it in the wind tunnel, and adding downforce at the rear meant the car also needed the new front splitter to keep the aero balance right.

How The B58 Gets To 435 hp

The engine is the B58 3.0-liter inline-six, now making 435 hp compared to the 382 hp for the Z4 M40i (340 hp in Europe). Most of the gain comes from letting the engine breathe more easily on both the intake and exhaust sides.

On the intake side, a ram-air inlet is hidden in the grille. Air moving sideways in front of the cooling package gets pushed into a second intake tube, and the effect gets stronger as speed rises. That tube feeds a new, larger air filter housing with a wider diameter. It’s also branded, in case you open the hood and forget whose car it is.

On the exhaust side, Bovensiepen redesigned the whole system and removed the particulate filter. Andreas says the car still meets emissions rules without it. Less back pressure means more power, and he says throttle response is a little sharper as well.

The ZF 8-speed automatic carries over with paddle shifters. Andreas thought BMW’s calibration was good enough that Bovensiepen left it alone. There’s no manual option, which is the one thing we’d change. Bovensiepen has said elsewhere that a manual would have limited how much torque the engine could make.

Brakes, Cooling, And A 295 km/h Top Speed

The front brakes use 395 mm (15.6-inch) drilled discs with Brembo calipers. Andreas’s view is that the brakes now match the car’s output, which is more than you could say about many tuned cars.

Cooling needed less work than you might expect. Bovensiepen fitted a 1,000-watt electric fan, up from about 700 watts on the standard car, mainly to help at low speeds. The company ran hot- and cold-weather tests in southern France and Spain, driving the cars hard, and the rest of the cooling system held up.

The rear differential was the exception. At sustained high speeds on the autobahn, its temperature kept rising. So Bovensiepen developed an aluminum differential cover that holds more oil and has cooling fins. It comes with the optional Performance Package, which raises top speed from 260 km/h (162 mph) to 295 km/h (183 mph).

Firmer Where The Z4 Needed It

The suspension changes are specific. Bovensiepen stiffened the low-speed bound damping, which controls how the car handles dips taken at modest speed. The rubber auxiliary spring in the rear is also longer, which keeps the car more settled over bumps and compressions.

The Spider runs 255-section front tires and 295/30 rears, and it weighs 1,585 kg (3,494 lb). According to Andreas, it will still spin its rear wheels in first gear with DSC off. “You don’t need 500 horsepower,” he told us, and in a car this size with rear-wheel drive, we believe him.

Lavalina Leather And A Violet Console

Inside, the leather is a higher grade than BMW’s and covers more of the cabin than it does in a standard Z4. The steering wheel is wrapped in Lavalina leather with Bovensiepen’s traditional parallel stitching instead of a cross pattern. It’s softer to the touch, and it’s a detail Bovensiepen owners will recognize right away.

Our car had a two-tone center console in platinum and violet. The violet was the best part of the interior, and it worked well against the exterior color.

Up The Bernina Hill Climb

This was Andreas’s first time driving the Bernina hill climb event, though not his first visit to the pass. He said that after two or three kilometers, he trusted the car enough to get on the throttle hard and push up the hill.

Price And Availability

Only 99 Spiders will be built. In Germany, the price is €115,500 including 19 percent VAT, or about €97,000 before tax. That’s a lot of money for a car that’s still recognizably a Z4. But the upgrade list is long, and each car starts as a new, never-registered Z4 from the last weeks of production. Once those 99 cars are gone, there won’t be another batch. Whether that’s worth €115,500 depends on how much you wanted BMW to keep building the Z