Real Variable Valve Timing and Valve Lift
If ever there was a demonstration of how far behind motorcycle engineering is compared to car technology, it's in the implementation of variable valve timing, which first appeared on the Alfa Romeo Spider back in the '80s – but didn't make it onto motorcycle until 2008 when Kawasaki came with the 1400GTR.Motorcycles are very far behind cars with regards to variable valve lift, despite the fact that Honda has used its 'Hyper-VTEC' badge on several motorcycle models over the years, the all has the similar 'REV' system as far back as 1983. Both REV and Hyper-VTEC don't really work as variable timing and lift systems, as on Honda's VTEC cars, but instead just close off two valves – one intake and one exhaust – at low revs, creating an engine that works as a two-valve-per-cylinder unit when it needs high torque but low power, and a four-valve-per-cylinder engine when big power outputs are asked for.
Honda's true VTEC system - used on its cars since 1989 – is a real variable timing and lift set-up, switching between two different cam profiles depending on engine load and revs. It was the first production engine to use variable valve lift.
Now, though, motorcycle manufacturers are finally looking towards true variable valve timing and lift, with both Honda and Kawasaki planning machines that will use such systems in the near future. If you want to know how soon, likely the next model of the Honda CBR1000RR and the Kawasaki ZX10R.
Changing valve timing and lift helps address one of the fundamental problems with all internal combustion engines, namely that they have to be designed as a compromise, with the ability to operate through a range of revs and throttle openings.
The perfect scenario of all variable valve timing and lift systems is the ability to independently control each valve, getting a computer to decide just when it should open and how far, but despite decades of experimentation with hydraulic and electro-magnetically operated valves, nobody has managed to create a reliable, cheap enough and fast-reacting take on the idea.
One has come close, though, and if others take up the concept it could open new doors in valve timing. It's Fiat's patented system – Multiair – which has become available in some of the company's cars.One of the most cleverly designed pieces of engine technology in years, it paves the way for computer-controlled valve timing and lift with a system that's so simple it can even be retro-fitted to older engine designs, needing only a mmodified cylinder head to allow the Multiair system to be mounted where the intake camshaft normally sits.
It works by using a single camshaft, mounted where the exhaust cam would normally be, to operate all the valves. The exhaust are opened normally while additional lobes are used to operate the inlet valves. The clever bit is that there are no pushrods or rockers – instead, hydraulics transmit the movement of the inlet cam lobe to the valves.
By simply adding a pressure-release valve to the hydraulic system, controlled by a solenoid and a computer map that's no more complex than an injection or ignition map, Fiat gains something close to complete control over the inlet valve's opening time, duration and lift.
With the release valve closed, the inlet valve follows the profile of the camshaft ( a high-lift, long-duration design, like a race cam). But by timing the valve to open and close at the right moment, the computer can reduce lift, alter the timing or reduce the duration to give something approximating the perfect cam design for every situation – at certain speeds it even opens and closes the inlet valve twice during a single intake stroke, giving a pulsed fuel-air mix intake.
