reed valve revolution
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Return To Cylinder Reeds
When Ski-Doo came out with its new cylinder-mounted reed design, they did so to allow a lower engine mounting position in the chassis. Many wondered if Ski-Doo was taking a step backwards and sacrificing the possible performance advantages of a case-reed. Closer examination of the Ski-Doo cylinder reveals large passages bypassing the cylinder directly into the crankcase. This permits the unrestricted flow of mixture into the crankcase while retaining the higher mounting position of the carbs.
Performance on the racetrack has shown the Ski-Doo design to be competitive with the case-reed engines.
New, unrestricted flow paths are not the only reason for the increase in performance with the new reed design. The reeds and the reed cages themselves have also seen considerable development. In this case, from the aftermarket companies. An early problem with reed designs in high revving engines centered on reed stiffness. If you made the reed stiff enough to work at high rpm, it was too stiff to open at low speeds, canceling the low-end performance it promised. Boyesen solved this problem by designing a two-stage double reed. A smaller, softer reed sits on top of a window in the main reed. The smaller reed opens at low speed and the main reed opens at higher speeds.
The Boyesen reed is famous for its smooth part-throttle and low-end performance. It also is known to provide better fuel calibration in transit areas where more radical reeds often have lean spots that may cause piston seizures. The reed cage volume also affects good low-end performance. If the flow from the carb exits into a large volume in the cage in front of the reeds, the air speed slows down and then has to be accelerated again through the reeds. This results in a low end flat spot and loss of throttle response. To solve this problem, cages are now designed to prevent any airflow slow down such as Boyesen's new RAD valves or the stuffers used inside the cage to speed up the flow.
High-speed race engines present a special problem regarding the use of reed valves. At the higher speeds, the reed must open and shut very quickly. The engine also needs a big flow area. If you make the flow area larger by opening the reeds further, they will not be able to travel the extra distance at the higher rpm and power is lost. Make the reed opening shorter so the reed can follow the rpm and you lose flow area.