Source: Bike Bound
A Warhorse Reborn: GPz1100F from AC Sanctuary…
In 1981, Kawasaki launched the GPz1100 to recapture the Superbike throne. It would be the most powerful Z1-based machine ever, offering 109 horsepower from the fuel-injected, air-cooled 1089cc engine.
“The company set out to make this the quickest and fastest street bike on the market, counting on measures like the 10 extra horsepower and the wider rear tire to get the bike deep into the 10-sec. bracket at the dragstrip.” –Cycle World, 1983
While the GPz1100 would soon be superseded by a new generation of 16-valve, liquid-cooled sport bikes, there’s something inherently sexy and alluring about this wind-cooled warhorse — the last king of a bygone breed of air-cooled superbikes from the 1970s…bikes with far more horsepower than the tire, chassis, and suspension technology of the era could hope to wrestle into form.
If there’s one man in the world to make this old king more competitive with modern sportbikes, it’s Hiroyuki Nakamura of AC Sanctuary — a workshop world-renowned for their RCM (Radical Custom Manufacture) restomod builds. Nakamura-san and his team focus more often on the Zephyr 1100, which has the same rubber-mounted engine design. That gave them plenty of expertise to apply to the GPz…though they rarely come across GPz donors to transform.
“The air-cooled GPz1100 boasts the highest output among Kawasaki’s air-cooled 2-valve engine machines. Used prices are soaring now. This model is really difficult to obtain.”
For Nakamura and his team, the first issue to address would be reinforcement of the engine mounts. On the GPz, the front engine mount is a rubber-isolated, while the rear is hard-mounted. This often leads to bent bolts in the rear and excessive vibration.
“This is especially noticeable in the case of the GPz, which boasts the strongest output of the air-cooled Z series.”
So the team did “extensive processing” on the frame side to reinforce the rear mount, which Nakamura has found to be the best solution for reducing vibration and the risk of a mount failure.
Meanwhile, their friends at DINKS performed a thorough interior processing of the engine along with boring the cylinders for larger pistons.
“Using the latest electronically controlled honing machine, there is no variation in cylinder clearance between the four cylinders. Valve guide and the coaxiality of the seat ring is also highly accurate, and the engine runs extremely well due to the precision of the internal combustion engine processing.”
The inline four was punched up to 1170cc with 75mm Wossner pistons, and the engine now breathes through Yoshimura Mikuni TMR MJN carburetors and a Nitro Racing titanium exhaust. To keep those extra horses running at a healthy temperature, an 11-inch 13-stage oil cooler has been added, and the ignition is an AS Uotani SPII.
Of course, no AC Sanctuary RCM is complete without an extensive upgrade to the undercarriage. While most RCM owners opt for 17-inch wheels, this project is rolling on 18-inchers (2.75-18 front / 4.00-18 rear) with Dunlop GT tires.
The forks are Öhlins, with a Sculpture CNC swingarm and custom-made Nitron rear shock. Brakes are Brembo with Sunstar discs and the chainline was offset from the original 87mm to 104mm to compensate for the wider wheels and swingarm.
All in all, this is one incredible “GPz1170” — an old warhorse reborn, which could surely surprise some of the newer thoroughbreds on the strip and street.
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More AC Sanctuary Restomods…
Air-Cooled King: Kawasaki “GPz1170” Restomod – A Warhorse Reborn: GPz1100F from AC Sanctuary… In 1981, Kawasaki launched the GPz1100 to recapture the Superbike throne. It would be the most powerful Z1-based machine ever, offering 109 horsepower from the fuel-injected, air-cooled 1089cc […] Radicalized: Suzuki Katana 1100 Restomod – Final Edition Katana, Radicalized by AC Sanctuary… In 1980, the original Suzuki Katana 1100 rocked the motorcycling world with its angular, space-age design. It was the vision of legendary German designer Hans Muth, who’d been […] Ninja Type-R: Kawasaki GPZ900R Restomod – AC Sanctuary gives an original Ninja the Type-R treatment… Introduced in late 1983, the Kawasaki GPZ900R — aka the Ninja ZX900 — was the earliest member of the Ninja series, and it changed sport biking […] Ice Cool R: Kawasaki Z1-R Restomod – Kawasaki Z1-R from AC Sanctuary… As everyone knows, the arrival of the Honda CB750 in 1969 kicked off the superbike era. Over the next decade, a two-wheeled arms race between the big four Japanese manufacturers […] Restomod Reborn: Kawasaki KZ1000MK-II – 1105cc Kawasaki KZ1000MK-II by AC Sanctuary… The Kawasaki Z1000MK-II appeared in 1978, largely as a response to the Suzuki GS1000 and Honda CB900F. Known as the KZ1000MK-II in some markets, the machine had beautiful new […] Mark II Missile: Kawasaki KZ1000 MKII Restomod – AC Sanctuary’s MKII Missile… In 1979, Kawasaki introduced the KZ1000 MKII, which offered new angular styling and a slight power boost out of the 1015cc DOHC inline four engine, now making 93 bhp. The crank […] Déjà Vroom: Kawasaki “KZ1165” Restomod – RCM-554: 1165cc Zed from AC Sanctuary… The Kawasaki KZ900 was the direct successor of the mighty Z1. At the time of its introduction in 1972, the original 903cc Zed was the largest, most powerful four-cylinder […] Resto-Monster: Honda CB1100F Restomod – 1123cc Honda Restomod from AC Sanctuary… In 1983, the Honda CB1100F arrived as the company’s king sport bike, designed to battle head-on with the 1100-class superbikes from Kawasaki, Suzuki, and Yamaha — bikes that were […] Monster Zed: Kawasaki Z1-R Restomod – AC Sanctuary’s “Radical Construction Manufacture” Z1-R… In the world of high-powered Japanese restomods, one name has long stood apart: AC Sanctuary. Founder Hiroyuki Nakamura opened his shop in the 1990s. Since then, they’ve built a […]
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