Bovensiepen Spider: The BMW Z4 Reborn as a 429hp Roadster

Bovensiepen revives the Z4 with a serious performance upgrade

BMW ended production of the G29 Z4 earlier in 2026, but the story of the two-seater German roadster didn’t finish there. Family-owned Bovensiepen — the company that adopted the Alpina legacy after BMW acquired the Alpina name in 2022 — has unveiled the Bovensiepen Spider: a limited-run coachbuilt take on the Z4 M40i that blends bespoke design, added performance and high-end materials.

This isn’t merely a visual conversion. Bovensiepen took the M40i’s 3.0-liter turbocharged inline-six and pushed it well beyond factory figures, while reworking suspension, brakes and aero to suit the roadster’s new character.

Where Bovensiepen comes from

After BMW’s purchase of the Alpina trademark, the historic, family-operated outfit rebranded to Bovensiepen and continued crafting exclusive models. Earlier projects include a Zagato-bodied reinterpretation of the M4 Convertible and the 05 GT based on the M5 Touring. The Spider continues that tradition: a boutique reinterpretation of an established BMW platform for buyers who want rarity, luxury and sharper performance.

Powertrain and performance: bigger numbers, familiar sprint

The Bovensiepen Spider’s headline is its engine upgrade. Where the Z4 M40i delivered between roughly 335 hp and a refreshed 382 hp depending on spec, Bovensiepen’s tuning raises output to 429 bhp (435 PS / 320 kW) and 428 lb-ft (580 Nm) of torque. That represents an increase of about 47 hp and 59 lb-ft over the most powerful factory M40i tune.

How was it achieved? The upgrades are pragmatic: enlarged intake plumbing, a bigger intake silencer, optimized airflow and a bespoke sports exhaust. The result is a marked increase in peak power and torque while retaining the engine’s character.

Despite the power gain, Bovensiepen quotes the same 0–62 mph (0–100 km/h) sprint as the factory M40i: 4.2 seconds. That parity suggests traction, gearing or electronic limits are shared with the donor car, while the Spider’s higher top speed reflects the tuning: a standard top speed of 162 mph (260 km/h) rises to 183 mph (295 km/h) when fitted with the Performance Package.

Key performance highlights:

• Power: 429 bhp (435 PS / 320 kW)
• Torque: 428 lb-ft (580 Nm)
• 0–62 mph (0–100 km/h): 4.2 seconds
• Top speed: 162 mph (260 km/h); 183 mph (295 km/h) with Performance Package

Chassis: wheels, tires, brakes and suspension

Bovensiepen fitted staggered 20-inch wheels (9x20 front, 10.5x20 rear) wrapped in Michelin Pilot Sport 4 S rubber — sizes 255/35 front and 295/30 rear. Dampers have been recalibrated and the company says longer auxiliary rear springs improve the body-to-chassis connection for sharper response.

Braking is available as an upgraded option: four-piston fixed calipers up front clamping perforated discs sized at 395 mm (front) and 345 mm (rear), paired with dedicated pads for enhanced fade resistance under hard use.

Design language: Frank Stephenson collaboration

Bovensiepen collaborated with veteran designer Frank Stephenson on the Spider’s exterior bits. The package emphasizes a sportier, more elegant silhouette without radical departures from the Z4’s original lines. Distinctive elements include a twin-tip high-gloss black front splitter, enlarged wheel-arch vents, revised side skirts, a reworked rear diffuser and an integrated rear spoiler. Bovensiepen describes the result as “sporty, elegant and timeless,” aiming to strike a balance between performance aero and coachbuilt refinement.

Interior: materials and customization

Inside, the Spider receives upgraded trims that complement its bespoke nature. Standard specification includes Vernasca leather combined with Alcantara seat panels and a Lavallina-wrapped steering wheel. Customers can opt for full leather dashboards and Alcantara coverings for the A-pillars, wind deflector and rollbar cover. Bovensiepen offers a variety of personalization choices so each of the 99 cars can feel individual.

Exclusivity, colors and production

Production will be strictly limited to 99 units, reinforcing the Spider’s collector appeal. Bovensiepen has allocated the run by color as follows:

• Sapphire Black: 40 units
• Sanremo Green: 25 units
• Skyscraper Gray: 20 units
• Thundernight: 9 units
• Frozen Gray II: 5 units

Black Vernasca/Alcantara is standard inside; cars finished in Sapphire Black and Sanremo Green can be specified with cognac Vernasca/Alcantara for a contrasting interior theme. Additional colors are available on request.

Price, availability and market positioning

Orders are open now, with deliveries scheduled for late Q1 2027. In Germany the Spider is priced at €115,500 (including tax), roughly €39,300 more than the Z4 M40i’s entry price when new in market terms — the factory M40i used to start at around €76,200 (including tax). Converted to dollars, Bovensiepen lists the European price at approximately $134,260.

That premium positions the Spider as a boutique, coachbuilt product rather than a straight tuning-box upgrade. Buyers are paying for exclusivity, bespoke design touches, mechanical refinement and the Bovensiepen badge — attributes that often appeal to collectors and enthusiasts seeking a rare, hand-finished alternative to mainstream sports cars.

Quote: "We aimed to create a roadster that feels both modern and timeless — a more focused, personalized expression of the Z4 for connoisseurs," Bovensiepen said in its announcement.

Verdict: who is the Bovensiepen Spider for?

If you wanted a Z4 M from BMW, the Spider is the closest unofficial answer: more power, sharper dynamics and tailored styling in a limited series. For buyers who value craftsmanship, exclusivity and an elevated interior, the €115,500 price tag will make sense. For others, the factory Z4 M40i — when available used or in remaining stock — remains a more affordable sporting roadster.

Ultimately, the Bovensiepen Spider is a statement: a small-scale coachbuilt revival that transforms a recently discontinued model into a collectible, performance-minded roadster with bespoke design and a clear focus on driving pleasure.

Highlights:

• Limited to 99 units
• 429 bhp and 428 lb-ft of torque
• Bespoke aero by Frank Stephenson
• High-end leather/Alcantara cabin options
• €115,500 (incl. tax) price in Germany

For enthusiasts watching the niche coachbuilding and tuner scene, Bovensiepen’s Spider is another compelling example of how boutique firms can extend the life and desirability of discontinued mainstream models — and how the Z4’s platform still has plenty of potential in the hands of experienced specialists.