Yamaha Chip-Controlled Intake (YCC-I) is a technology used in Yamaha’s sports bikes, where the length of the intake tract can be varied with respect to engine speed. A long tract length provides torque at low and mid-range revs, whereas a short intake tract length helps develop power at high RPMs. This technology makes it possible by reducing the intake runner length at high revs, and increasing it at low RPMs.
How YCC-I works:
The length of the intake runner is shortened by moving its upper section away when the engine revs above a certain rpm range and a certain throttle opening angle. A motor moves the upper part of the runner by levers and a rod.
With the upper part of the intake runner moved away at high RPMs, the pressure wave coming from the intake valve reflects at the opening at the end of the short lower runner and returns to the intake valve. The intake runner is designed exactly so that the intake valve opens precisely when the reflected pressure wave arrives, in order to maximize cylinder filling efficiency. At low rpm, there is more time between closing and opening of the valves. Therefore at low revs, the runner has its maximum length, by connecting its upper and lower parts. This way the pressure wave takes a longer time to travel, to match its return with the opening timing of the intake valve.