Motorcycle specifications Sunbeam Porsche Special 1300 1970 Flatter English

From time to time, manufacturers release machines that are more or less extravagant, representing a bold move or a showcase of craftsmanship. But these rarely rival certain creations born in the depths of a garage—shaped by hours of determination and driven by the vision of an enthusiast.

Gordon Griffiths is one such person. He is the man behind this Sunbeam with a Porsche engine. Often mistakenly perceived as a hypothetical German-British factory prototype, this motorcycle is in fact a one-of-a-kind creation, an assembly of mismatched parts born of this English enthusiast’s imagination. While the machine has regularly attracted attention at prestigious international auctions, such as those organized by Bonhams, a critical look at its origins and technical design is warranted, beyond mere aesthetic wonder.

From the Sunbeam S8 to the Stuttgart Engine

Originally, the basis for this creation was a classic 1952 Sunbeam S8. Historically, the S8 was a luxurious British motorcycle equipped with a 487 cm³ inline-twin engine and a shaft drive. However, the architecture of its frame left a particularly generous amount of space around the engine block—a feature that would inspire certain tuners in search of more power.

Gordon, seeking to transcend the limited capabilities of his original vehicle, initially opted to install a 1,200 cm³ flat-four engine from a Volkswagen Beetle. Dissatisfied with the performance delivered by this engine, he took the concept to its extreme by replacing it with a Porsche powertrain: a 1,300 cm³ flat-four engine (a unit from the “A” series dating from the mid-1950s). This nearly doubled the machine’s power! The original twin-cylinder engine produced 24 ch, while the German flat-four delivered 44 chevaux.

A prestigious but unbalanced European patchwork

In the end, the only things the Sunbeam-Porsche has in common with a Sunbeam are its frame and part of its drivetrain. To adapt the chassis to this German-imported engine, Griffiths had to draw from the best European components available at the time. The original fork was replaced with a Norton “Roadholder” model; the fuel tank was taken from a BMW; and the second owner installed a dashboard featuring a speedometer borrowed from a Vincent Black Shadow— the true Holy Grail of the British motorcycle industry.

The visual result and the ingenuity of the build captivated the Anglo-Saxon motorcycle press, even earning the bike the “Special of the Year ” award (Modified Motorcycle of the Year), presented by the British magazine Motorcycle News in July 1970.

Mechanical Limitations: Too Much Heart

However, the well-deserved enthusiasm for this engineering feat must be tempered by a rigorous mechanical analysis, based on the technical data available at the time. The Sunbeam S8’s final drive shaft, which used a worm gear system machined from soft bronze, was notoriously the machine’s Achilles’ heel. This mechanism had already proven fragile and prone to failure even under the modest torque of the original 500 cm³ twin-cylinder engine.

Retaining and coupling this final drive to the significantly higher power curve and torque of a Porsche sports car engine is a mechanical contradiction. In fact, the reliability of the final drive during spirited road use is highly questionable, as the power of the German engine far exceeds the tolerances of the British metal.

The Sunbeam-Porsche Special remains a striking testament to the boundless creativity of postwar tuners. It embodies the individual desire to break free from the limits set by manufacturers. However, this machine must be viewed objectively: it is not a marvel of balanced and durable road engineering, but a pure mechanical fantasy.

M.Bi - Photos: Bonhams