Variable Valve Timing on Sportbikes

With Suzuki all set to launch their new GSX-R1000, a lot is being made of the fact that it uses variable valve technology taken from the firm’s MotoGP bike. While this is the first variable valve technology hs been seen on a 1000cc sportsbike, the technology itself is far from new. In fact, technically the ability to alter the valve’s opening times dates back to steam engines! But today we will simply concentrate on motorcycles…

The first motorcycle to have variable valve technology was the Honda CB400, which used a version of the firm’s car-derived Vtec system. While this motorcycle failed to make it on the global market, the same technology first came to most riders’ attention with the launch of the Honda VFR800 Vtec.

Plagued by an overly abrupt transition from two to four-valves, the Honda Vtec system wasn’t well received and despite the fact that Honda soldiered on with it, other manufacturers decided to hold off. Then, in 2010, Kawasaki unveiled its own take on variable valve timing when the 1400GTR gained a far more sophisticated form of the technology.

Ducati followed suit in 2015 with the XDiavel, once again improving on the system, and that leads us to the present day with Suzuki and its own SR-VVT system. But what is variable valve timing all about?

Variable valve timing is pretty self-explanatory – it is a mechanical system that automatically alters the point at which either the intake or exhaust valves, or both valves, open and shut. However to fully appreciate, you first need to understand exactly how an engine works.

Within the cylinder head of a double overhead cam (DOHC) engine there are two camshafts that are turned via either a cam chain, cam belt in case of most Ducati engines, or gears for fairly exotic machines such as Honda’s RC30 or SP-1 or 2. These cams have egg shaped lobes with each lobe acting on a sngle valve via the valve train. There are three general types of valve train – bucket and shim, finger followers or rockers and desmodromic – but ultimately they all achieve the same final result – which is opening and shutting the valves.

In a bucket and shim system, the valve’s stem is covered by a small metal bucket, When the cam turns the lobe pushes down on the bucket, opening the valve. Once the lobe has passed its widest point, the valve spring pushes the valve closed again but only as quickly as the lobe allows it. This type of valve train is cheap to produce and is also very reliable, which is why it is found on most modern motorcycles and earlier sportsbikes. However lately there has been the likes of the BMW S1000RR, Yamaha YZF-R1 and new Suzuki GSX-R1000 all using this kind of valve train.

A finger follower valve train does away with the bucket and instead the lobe activates a rocker, which pushes down directly on the valve. As with the bucket and shim system the valve is closed via a spring, but the benefits of a rocker system are a reduction in the weight of the components moving within the system and this means the engine is able to rev higher and with greater accuracy and reliability.

Finally we have the desmodromic valve train which sees the valve opened using a rocket in a similar fashion to the finger follower valve train, but instead of a spring closing the valve, another rocker with a kind of fork on its end physically grips the valve and shuts it. In theory, this mechanical system gives greater accuracy when shutting the valve and does away with the potential of valve float, which is when the valve bounces as a result of being shit by a spring. Okay, now that that is clear, just what is variable valve timing?

What is Variable Valve Timing?

The problem with any valve train is that it is a fixed system and once the engine is built, the point at which the cam opens and shuts the valves can’t be altered. Why is this an issue?

The performance of an engine varies throughout its entire rev range and what works at a lower rpm may not be as efficient at high revs, meaning that manufacturers have to compromise and set the valve timing for what they believe will be best in terms of power characteristics, efficiency and longevity of the engine. You often hear about race engines running re-profiled cams, well that is simply the cam lobes’ shaped being altered to change the point at which they open and shut the valves to suit a tuned engine built for top end performance. A variable valve system aims to add an element of controlled variation into the cam timing, effectively mechanically altering the point at which the valves open and shut without the need for changing the whole cam shaft. So how does it work?

What’s Going on Inside the Cylinder Head?

Each manufacturer uses a slightly different technique to achieve effectively the same goal, so we will describe a general variable valve system. The cam chain (or belt) drives each cam via a cam sprocket, which is located on the end of the cam shaft.

While the sprockets are often slotted so that they can be tweaked very slightly to alter the cam timing when the engine is being built, a variable valve timing system such as Suzuki’s VVT, Kawasaki’s VVT and Ducati’s DVT replaces the cam sprocket (Suzuki and Kawasaki only replaces the intake cam’s sprocket, the Ducati DVT replaces both the intake and exhaust cams’ sprockets) for a variable unit. What this means is that when the variable system is activated it physically turns the whole cam slightly on its sprocket, altering the point at which the lobe opens and shuts the valve without actually physically changing the lobe at all. So what’s the point?

The Effect of Valve Timing Variations

Ducati with its DVT engine, which is used in the Ducati XDiavel whose target is midrange grunt and not outright performance, Ducati talk about the system altering valve overlap. Valve overlap is the brief moment when the intake valve is starting to open but the exhaust valve hasn’t fully shut yet – meaning both valves are partially open.

This point can be both helpful and a hindrange to a engine as the pulses of gases exiting the exhaust and the resulting vacuum do help draw new air/fuel into the cylinder, however they can also create turbulence which negatively affects the efficiency of the burn. With its DVT system, Ducati alters the timing so that it can either increase or reduce valve overlap to match a set of predetermined figures, effectively tuning the engine while it is running to give it better low down power. However, where Ducati’s variable valve timing system is searching out midrange, Suzuki’s is all about top end power.

Interestingly, Suzuki’s VVT ony operates on the intake cam and not the exhaust cam and it comes into play at high rpm. What Suzuki has effectively done is tune the GSX-R1000’s engine for stacks of low end grunt and then used the VVT system to retard the intake valve’s operation to give it a high-rpm boost. Why have they done this? At low revs a ‘mild’ valve timing which is an earlier opening of the valve, creates low end performance and is less stress on the engine while an ‘aggressive’ timing opens the valve later.

Why the difference? Well, with a mild cam the air/fuel flows nice and smoothly into the engine, giving good low down power characteristics and a gentle throttle response. With an aggressive cam the air/fuel is forced far faster into the cylinder and while this is great for top end power, at low revs it gives the ‘lumpy’ engine feeling that is associated with a highly tuned engine. With their VVT, Suzuki move the cam to create an aggressive action right at the top of the rev range where you want it and where it matches the faster flow of fuel/air into the engine through the forced airbox, while ensuring it is nice and smooth at the low end. Kawasaki’s VVT does a similar job, but its focus is at the low end of the rev range and it varies the cam timing to increase midrange performance rather than top end.

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