What is “tunnel porting” ? Is this only a Ski-Doo thing ?
To be accurate, tunnel porting is performed on rotary valve intake engines, and since Ski-Doo is the only sled maker using this type of intake, it is a “Ski-Doo thing” . This procedure refers to the grinding out of the crankcase from the rotary valve inlet towards the transfer ports at the cylinder base. This procedure streamlines the flow from the rotary valve inlet to the cylinders. This modification really only benefits engines running above 8000 rpm. When installing larger carburetors, opening of the rotary valve cover and the crankcase openings may also be included in a tunnel porting job to match the new carburetor bore. However, when installing carburetors larger than 42mm, do not enlarge the opening at the valve side of the cover beyond 42 mm. Taper the opening smoothly from the carburetor flange down to 42 mm on the disc valve side. The opening in the crankcase should match it at 42 mm and “trumpet” out towards the transfer ports. Also, tunnel porting should only be done by accomplished engine modifiers.