From the Vault: Unreleased content of Kawasaki’s Rotary Engine Prototype

The X99 was a rotary engine prototype Kawasaki motorcycle. Mr. Takarabe X99 was project leader on the interesting challenge. Take a look at some of the challenges that designers faced working on this project including the energy crisis that ultimately ended development. Let the good times roll


Did you know that Kawasaki designed a rotary engine motorcycle? Development of the X-99 prototype started in 1974. In the midst of the 1975 oil crisis, development was put on hold. The 896.8 cm³ engine featured a liquid-cooled housing, 2 rotors, cross-port intake and a 5-speed transmission. When the project was officially halted, the engine had achieved a max power of 87.8 PS and a max torque of 10.78 kgf-m.

The engine’s intake method was a cross port arrangement (also known as a combination port arrangement) with a side port as the primary port, and a peripheral port as a secondary port. Using a side port ensured stability at idle and at low rpm. And at high rpm the peripheral port was opened (via throttle cable) to provided additional intake. Interestingly, the only rotary engine powered motorcycle that did go into production, the Suzuki RE-5, used peripheral ports for both its intake and exhaust.

A performance test in June, 1974 for the prototype engine yielded figures of 85.1 PS with 9.96 kgf-m of torque. After that, a trial run was held in Yatabe. The smoothness and decreased vibration versus a reciprocating engine were there, but the issues that arose were mechanical noise at low rpm, weak engine braking, and heat issues among other things. Additionally, fuel consumption was much worse than anticipated. Compounded by a sudden jump in the price of oil due to the oil crisis at the time, the X-99’s potential was re-evaluated after this test, with development work put on hold in 1975 as a result. The project is still frozen. Just before work stopped, the engine recorded a power of 87.8 PS with 10.78 kgf-m of torque.The chassis used technology inherited from the highly respected Z1.

Had the project continued to completion, the X-99 might have become the largest-displacement rotary engine powered torque-monster on the market.