The Search for the Best Torque Curve

Yamaha Power Valve System (YPVS) was introduced in the early '80s to tame their peaky RD two-strokes. The height of a two-stroke's exhaust port is the one thing to which they are most sensitive, so Yamaha devised a valve to lower and raise the port roof depending on revs. This allowed an exhaust designed for maximum power with the valve raised, while lowering the valve gave better torque and flexibility at lower revs. It made an enormous difference to usability.

Four-strokes aren't as sensitive to a single alteration , but using a computer – basic by today's standards – a team lead by Kiyotaka Yamebe and Hideaki Ueda worked out pressure and flow in an Yamaha FZR400's exhaust. They discovered a throttle valve located at the end of the collectors could be used to tailor the pressure waves, and the theory was supported by experiments on real motorcycles. Yamaha realized they could now effectively build a full race system then use the throttle valve, by now tantalizingly called EXUP (Exhaust Ultimate Powervalve), to tidy up any resulting dips or hollows in the shape of the torque curve.

First used on the Japanese-market FZR400, Yamaha made big claims when the FZR1000R EXUP arrived in 1989: 10% more peak power than an engine without EXUP; low and mid-range torque increased by 30 to 40%; a more stable tickover; and a quieter exhaust. Tests in California showed slightly increased CO2 emissions but significantly reduced hydrocarbons.
Noise reductions were due to the valve being active a lot of the time. A four-stroke's exhaust tune only really works at one speed, so EXUP operation wasn't an open and shut case. Literally. At around 3,000rpm the EXUP opened to around 30%, by 5,000rpm was open almost fully, but at 7,000rpm only opened between 40% and 60%. From 8,500rpm it progressively opened wide. Operation was by a servo-motor controlled by the motorcycle's ECU, with a sensor monitoring pulley position and sending information back.

The Future

As regulations get tighter, the exhaust valve's popularity can only grow. It's a simple, effective technology for meeting noise restrictions, with the added, if small, benefit of being able to improve an engine's torque curve.

New technologies to achieve targets for both power and emissions can be combined with an exhaust valve for the best result. Influencing intake pressures by playing with the intakes and airbox produces similar effects on the torque curve as exhaust design; it's why Yamaha fit variable-length inlets on the Yamaha YZF-R6 and YZF-R1.

Tuning the intake to work at the same rpm as the exhaust will give the highest peak output but at thecost fo deeper dips in the torque, but a slight mismatch gives a wider spread and helps fill the holes. So using two (or more) different systems on the same motorcycle doubles the benefits – it's why Honda's exhaust valve has always been linked to a flap in the airbox and Suzuki's system works in conjunction with the ECU-managed secondary inlet throttle butterflies.

But while developments like variable inlets are welcome, Yamaha's original EXUP remains the most effective and most simple innovation. More engine performance, reduced emission, less noise. Absolute genius...
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