Why no YZR500i?

WE’VE become used to seeing fuel injection replace carburettors on bike after bike, and the electronic system is now nearly as common a sight on road bikes as disc brakes.

But if fuel injection is the way to go, why do the most exotic bikes on the planet, million pound 500 GP bikes, continue to use the old technology of carbs?

Surely bikes like the Yamaha YZR500 Max Biaggi rides, or any of the other GP exotica in top-class racing, could make the most of the advantages fuel injection offers? Precise computer-controlled fuelling, full engine management control and infinitely adjustable mixture mapping systems sound, on the fact of it, like useful tuning tools.

Most World Superbikes are already fuel-injected. It clearly works for them, so are GPs actually starting to lag behind in technology?

The common perception of fuel injection is that it’s a performance-enhancing feature. It costs more money, computers are involved and therefore there’s a natural assumption it is a route to more power.

In fact, it is rarely responsible for any significant increase in power. What it does do is allow an engine to produce the same power as a carburettor version, but with reduced emissions. To put this another way, where the power of an engine with carburettors might have to be artificially reduced in order to meet emissions regulations, fuel injection allows it to reach its full potential while remaining within the regulations, making it seem as if it’s responsible for extra power.

World Superbikes use it (except for the ageing Kawasaki ZX-7R) because the rules specify that their fuelling systems must be essentially the same as the road bikes they’re based on. And the road bikes have fuel injection because they need it to pass the increasingly strict exhaust emissions laws around the world.

Grand prix bikes have no such artificial limitations. Without them, the technical reasons not to have fuel injection take precedence. For a start, carburettors are extremely good at what they do.

The fuel they supply is determined by the air flow through them sucking it up through the various jets. And at times that air flow can display some very subtle and complex changes.

It can become so complex that even the most sophisticated fuel injection can’t keep up with the fluctuations in demand. A carburettor, on the other hand, deals with these tiny changes in air flow automatically, adjusting the amount of fuel sucked in to perfection.

Carburettors also have a happy habit of dumping extra fuel into the air flow when the throttle is snapped open. Which, by chance, is exactly what’s needed for a sharp but controllable throttle response. Fuel injection manufacturers and programmers still have difficulty mimicking this behaviour (or at least, doing so without ruining their clean emissions). This is so much the case that many fuel injection systems can feel a bit on-off. One moment nothing happens when you twist the throttle, the next, it all does. This can be unsettling when you crack open the throttle on the exit of a high-speed corner.

Honda experimented with fuel injection on its 500cc GP bikes in the early ’90s and it was this throttle response problem which finally caused them to give up and return to carbs.

There is little pressure for GP manufacturers to start experimenting again because the need for clean emissions is simply not a factor with racing bikes.

There’s no reason why a two-stroke can’t be fuel-injected in the same way as a four-stroke. But because the fuel-air mixture in a two-stroke has a much longer inlet path its atomisation (where the fuel droplets separate out for a better mix with the air, and therefore a better burn) is better than a four-stroke’s to start with. The need for the improved atomisation you get with fuel injection is therefore much less.

There are additional physical problems. A fuel injection system needs a high pressure pump and relatively large computer to operate, and these add weight to a machine compared with its carburettor-equipped equivalent. And on a GP bike every gram counts.

Fuel injection systems also have high electrical demands, typically of the order of four amps or so with higher peaks. GP bikes have no alternators and rely on total loss battery-powered electrical systems which aren’t capable of delivering such high currents. Either unacceptably bulky and heavy batteries would be needed, or the engines would have to be redesigned with alternators, which mean more weight and slower throttle response, as well as additional loads on already enormously stressed crankshafts.

In the light of all this, the only real surprise is that even Honda’s race boffins ever bothered to try to make fuel-injected GP two-strokes work.