The flat six-cylinder engine has become almost inseparable from the Porsche brand. For the hallowed German automaker, this unique take on the six-cylinder engine has found its way into the cores of the German brand's most iconic models, both past and present.Porsche has also offered several versions of the flat-six engine, with some naturally aspirated, some with 4.0 liters of displacement, and some featuring a pair of turbochargers. However, for as many versions of the flat-six engine there have been in Porsche's stable, the company has never built a supercharged flat-six engine, but Theon Design did.This article focuses on the specifications and performance of the supercharged flat-six engine built by Theon Design and used in its 911 restomods. We also check out some of Porsche's many flat-six engine variants and Theon Design's history. A Brief Recap Of Theon Design Theon Design It seems as though there is a new company creating restomods based on old 911s every day. These retro-modern builds have experienced a significant uptick in interest. The Porsche 911, more often than not the 964 generation, is the most popular choice for restomod companies. As such, this niche market is full of companies putting their own spin on the air-cooled 911s of yesteryear, and Theon Design is one of them.Theon Design is a British restomod company founded in 2016 and relies on the 911 as the core of the catalog. It started as a family business, with Adam Hawley modifying his Porsche 911 before selling that example to drum up funding for the company. Today, with a sizable catalog to its name, Theon Design operates out of a facility in Oxfordshire and, unlike other 911 restomod companies, the business intends to create cars considered OEM+, rather than 911 restomods with exaggerated proportions and billboard-sized rear wings.That's just their design ethos. Mechanically, these restomods are built to the tightest tolerances, with parts 3D-modeled to ensure precision and fit. The end goal of each build, which can be seemingly endlessly personalized to the owner's desires, is for Theon Design's cars to be lightweight, have far greater performance than the 911s they're based on, and have far greater driving dynamism.Theon Design While the Oxfordshire-based restomod company prides itself on its relatively conservative design ethos, the powertrain isn't as conservative. Customers can choose a naturally aspirated flat-six in three sizes: 3.6-, 3.8-, and 4.0-liters. If that isn't enough, Theon Design also offers forced-induction options to eke out more power, and a few years ago, we saw the first engine from this company with a supercharger. The BEL001 Theon Design The first time we saw a supercharger paired with a flat-six engine in one of Theon Design's 911 restomods, it was the most powerful engine the company had built up to that point. It was dubbed "BEL001," a name derived from the fact that the client who commissioned the car was Belgian. Not only did it rely on the most powerful engine Theon Design had created at the time, but it was also distinctive for its advanced suspension system featuring active damping, a first for Theon Design.As for the rest of the commission, it was finished in an Aquamarine color, a nod to a similar hue used on Porsche 356s from the 1950s, and juxtaposed by caramel-colored Porsche-Recaro grand touring seats that were custom-fitted for the Belgian client. The level of detail throughout the BEL001 was impressive, from the black accent stitching to the metal pedals and flooring. However, the centerpiece of the BEL001 was undoubtedly its powertrain.Theon DesignThe innovations on the BEL001, both in terms of its engine and suspension, reflect what Theon Design is all about. We're driven by a determination to hone and sympathetically enhance the classic 911 recipe, using design-led innovation, dynamic focus, and close partnerships with expert suppliers to create the ultimate iteration of the world's most iconic sports car.- Adam Hawley, founder of Theon Design. Beneath Its Carbon Fiber Skin Theon Design On average, each flat-six engine built by Theon Design requires between 150 and 250 hours of labor. The engine in the BEL001 was improved by Theon Design by porting the engine cylinder head, a lighter bottom end, new Mahle pistons, stronger connecting rods, and a custom camshaft, to name a few. While Theon Design offers three engine sizes to choose from, the BEL001 relied on the smallest-displacement option. At the core of the BEL011 was a 3.6-liter flat-six engine, which was coaxed to higher power levels by a Rotrex supercharger.The supercharger was mounted on the side of the flat-six engine, with its mounting points significantly reinforced. The result of roughly half a year of work was a supercharged flat-six engine that produced 400 horsepower and 367 pound-feet of torque, in a carbon-fiber-laden car that weighed only 2,789 pounds, which resulted in an unsurprisingly exciting restomod.We wanted to deliver additional performance but also maintain the seamless delivery of a Porsche naturally-aspirated engine, so chose a Rotrex centrifugal supercharger. Effectively a belt-driven turbocharger, it gives a wonderful, linear power delivery and instant throttle response with no delay or lag. It was crucial to us that the Porsche flat-six sound was undiluted, and we’re very pleased with the results: the supercharger has a very subtle ‘whine,’ so the flat-six is still front and centre, with some wonderful added character.Theon Design To balance the weight of a supercharger hanging on the side of the flat-six engine, Theon Design opted for two charge coolers, an interesting choice considering the accepted standard is intercoolers. This decision was supported by the implementation of a water-methanol injection system, which rendered a traditional intercooler redundant.This setup means we can avoid adding air intake apertures to the bodywork, [and] retain the super-clean, distinctly 'Theon' look that's so important to us, both in the engine bay and externally.Theon Design While the BEL001 produced more power than any Theon Design 911 that came before, it wasn't enough to outpower the turbocharged flat-six engines we saw in models like the 911 Turbo S from that period and many of the lower-tier model variants. The BEL011 was lighter, commanding a greater power-to-weight ratio than models like the Porsche 911 Carrera. It's unclear how much this example of Theon Design's work costs, but on average, these cars were valued at roughly $400,000. Theon Design's Latest Restomod Bucks Forced Induction Theon DesignThere weren't as many examples of supercharged restomods from Theon Design as there were of builds with naturally aspirated flat-six engines. Since the debut of the BEL011 commission, Theon Design has expanded its offerings, with the latest product from its catalog being a naturally aspirated wonder with a better power-to-weight ratio than the modern Porsche 911 GT3 RS. Theon Design's take on the ultimate air-cooled 911 road car is dubbed the Theon R.Theon Design This carbon-fiber-laden features a 4.0-liter flat-six engine capable of revving to over 9,000 rpm and producing more than 500 hp, with a soundtrack akin to the RSR race cars of the 1970s. Paired with this vocal flat-six is a short-throw six-speed transaxle gearbox and Wavetrac limited-slip differential. Its power and weight give the Theon R a better power-to-weight ratio than any production 911.