With indisputable benefits and advanced engine management, every major motorcycle builder now employs throttle valves in their exhaust systems, and on all manner of machines – Kawasaki ZX-6T, Yamaha MT-01 and Suzuki M1800R all feature a widget up their chuff.
Designs have evolved. The original EXUP system used one valve running through all four side-by-side pipes at the end of the collector, rotated by a pulley and working like a guillotine. In 2000 Honda introduced H-TEV (Honda Titanium Exhaust Valve) on the FireBlade, still using one valve but with the four headers arranged in pairs on top of each other, in a square layout. The valve was located in the upper pipes, and when closed it opened up a hole through to the lower pair, restricting the total area and when closed it opened up a hole through to the lower pair, restricting the total area and making use of the bottom pipes' tuned length, as well as the new length of the upper pipes. The 2002 Yamaha YZF-R1 used a similar arrangement, but with a valve in each pair of pipes.
Today, valve location varies considerably. Kawasaki's ZX-6R and the current FireBlade hide them deep in silencers, while Triumph's Daytona 675 nestles one in the secondary pipe, between collector and end-can, just after the catalytic converter.
'We didn't want the weight and bulk of the valve and its actuator at the back of the motorcycle,' says Triumph's Product Manager. 'This location is the best place for keeping the Triumph Daytona 675 compact, putting mass where it will have the least impact on handling while still giving us the effect we wanted.'
And the desired effect has changed. With advanced injection and ignition systems, the role of the throttle valve is no longer about filling the midrange or chasing horsepower as you can see from the performance of the Triumph Daytona 675. 'On the Daytona 675 it has nothing to do with emissions or peak power,'continues the Triumph Product Manager. 'There's a small effect on torque at lower engine speeds and it can improve drivability in some conditions, but it really helps on noise – a valve in the secondary pipe helps to take the edge of exhaust noise without compromising power.' They're corks bunging up piper to keep them quiet.