Variable Rates Of Interest

by Simon Hargreaves

Variable valve timing: meet the engine technology that just won’t go away

In the list of pointless engine gimmicks, you’d surely put variable valve timing at number one. Only three production bikes have automatic VVT: Honda’s VFR800 VTEC (crude, two-stage system), Kawasaki’s 1400GTR (more sophisticated but barely noticeable) and, between 1991 and 1998, Suzuki’s Japan-only R-version 250 and 400 Bandits (crude and unnoticeable). And all of them gimmicks.

So what is VVT, and why do we keep hearing about it?

When a four-stroke sucks in fuel/air mix, the inlet valves open and exhaust valves close. Next, the mixture is compressed and burns so all valves are shut. Then the spent exhaust gas is expelled, so the exhaust valves open.

You’ll notice, in the ‘suck, squeeze, bang, blow’, the open valve strokes are next to each other – suck with inlets open, blow with exhaust valves open. You’d think, in a high-performance, high-revving engine, getting them open and shut quickly enough would be a problem. And it is, to an extent. You only get 0.0046 of a second at 13,000rpm to fill, compress, burn and empty a cylinder.
But it helps if valves are partially open at the same time (called overlap), because incoming mixture pushes out previous exhaust gas and the outgoing exhaust gas helps draw fresh mixture in (called scavenging).

So we now have valve timing. The actual timing of the timing – precisely when the valves open and close – is controlled by cam lobe shape. It’s the most critical design aspect of an engine. Imagine, as the piston rushes back down the bore pulling in fresh charge, it needs the inlet valves wide open. But as it hits bottom dead centre and stars to climb, compressing the mixture, the inlet valves need to be shut.

At low rpm, the earlier in the stroke the inlets are shut, the more mixture is trapped in the chamber – basically, good for bottom end torque. But at high rpm the later the inlet valves close, the more time for mixture to fill the chamber – basically, good for top end power.

And there’s the rub: you want different, or ‘variable’, valve timing for preferential characteristics at different rpm. Not just in engine torque or power, but also in fuel economy and emissions, which are at least as important.

The manufacturers know how important VVT is. As reported by MCN, most are working on systems. In fact most are working on more than one. BMW have altered the design of the 2013 R1200GS valvetrain to incorporate VVT – they said so themselves, and a recent spy shot of a GS with an elaborate cylinder head confirms it. Honda, Yamaha and Kawasaki have all taken out alternative VVT patents in recent years, and Suzuki seem to have a new VVT idea every other year. So why aren’t they on sale already?

Because, up until now, bike engines haven’t needed it to make them perform better: when was the last time you rode a gutless, modern 1000cc bike? But, as with many other aspects of bike design, emissions and fuel economy needs are supplanting all others. And VVT will soon become a necessity, and a gimmick no more.

pittsy's picture
Joined: 06/08/2011

Another good article Simon. It can't be easy to cover a topic like that with so few words. I'm losing touch with developments as I no longer buy any mags and have cancelled my MCN subscription.

On the subject of time, I may be wrong here, but the following seems wrong? Quote: "You only get 0.0046 of a second at 13,000rpm to fill, compress, burn and empty a cylinder."

The fraction of time you are describing seems to be for one rev. Shouldn't it be for two? Twice as long? Still a flippin small number though! Apologies if I'm wrong there.

Keep up the good work.

Navy Boy
Joined: 12/02/2009

This article touches on a topic that's becoming more and more important in the bike buying market. That of fuel economy and emissions.

Thus far us bike buyers have paid very little real attention to these areas because most people don't buy bikes with these factors upmost in their mind.

Thank you Simon for putting into relatively few words what VVT is and how it works. An area to keep one's eye on in the coming future I feel.