Air-Cooled King: Kawasaki “GPz1170” Restomod

Kawasaki GPz1100 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 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

Kawasaki GPz1100 Restomod

While the GPz1100 would soon be superseded by a new generation of 16-valve, liquid-cooled sport bikes, there’s something inherently sexy and alluring about this wind-cooled warhorse — the last king of a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.

Kawasaki GPz1100 Restomod

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.”

Kawasaki GPz1100 Restomod

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.”

Kawasaki GPz1100 Restomod

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.

Kawasaki GPz1100 Restomod

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.”

Kawasaki GPz1100 Restomod

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.

Kawasaki GPz1100 Restomod

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.

Kawasaki GPz1100 Restomod

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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3 Comments

  1. Roman JURIŠ

    Very nice bike, color combination with feeling, amazing exhaust pipes

  2. Great bike. I had one in white with red wheels. Stolen.

  3. I owned a 1983 GPZ 1100, never gave it much thought when I sold it until lately, WHY

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