Yamaha's Variable Porting

YAMAHA’S VARIABLE PORTING

Thanks to Yamaha's company magazine and to the quick work of photographer Mush Emmons, we now know how the works YZR500 gets its variable exhaust port timing.

The photo shows a barrel assembly with cables. The barrel mounts outboard of the exhaust ports. The cables run to the tachometer: as rpm increases the exhaust timing changes.

The sketch shows what’s inside. There’s a drum-like valve inside the housing, up against the cylinder wall. The passage through the drum is the exhaust port. At low revs the “port” is lowered, so the exhaust port is uncovered late in the cycle. At high rpm the port is raised and the exhaust opens sooner. The powerband is widened and the bike is easier to ride.