Gizmo: When you see a GET GPA
dial, think traction control.
NO-GO OR GO-GO Dear MXA, It seems to me that every factory bike has some form of illegal traction control, especially those bikes with GET Electronics’ GPA dials on them. Their sole purpose is traction control. Doesn’t the AMA enforce the “no traction control” rule? Let’s be serious. The AMA, AMA Pro Sports and MX Sports do not enforce very many of their equipment rules. They rarely sound test and almost never fuel test (probably out of embarrass- ment over the Ricky Carmichael buy-out deal). Additionally, they are incapable of catching cheaters who cheat through the black box. Honda had traction control in the 1997 Honda CR250 two-stroke, and even though they touted it in their original sales brochure, the AMA did nothing 16 years ago and hasn’t done anything since. Critics of the traction control ban point out how poorly written the AMA rule is, claiming that it doesn’t actually ban anything. They note that the rule says that “electronic devices specifically designed for traction control are prohibited.” Does that mean that an ECU that monitors fuel and the ignition maps but also has traction control in it would be legal? After all, it isn’t “specifically designed” for traction control? It is best to assume that rules that aren’t enforced are going to be broken.
JANUARY 2014 / MOTOCROSS ACTION 155
HOW DOES IT KNOW? Dear MXA, I’ve asked a lot of people how the KX250F’s dual injectors work, but their answers don’t make a lot of sense. Can you explain how Kawasaki’s dual fuel injectors work? On each intake cycle, an engine draws a breath of air from the intake tract, which can be viewed as a long rubber tube of air. At low rpm, the engine is taking quick little breaths and only using a fraction of that long column of air. A single fuel-injector nozzle must be placed next to the intake port to deliver fuel at low rpm. At high rpm, however, when the engine is drawing in the whole long column of air, a secondary injector can take advantage of the added volume of the long column of air. Atomization is what counts, and dual injectors can keep up with the needs of both low and high rpm running. Spraying fuel with the injector nozzle as far away from the intake port as possible gives the fuel time to atomize while being tumbled in the natural turbulence of the intake tract. It is also claimed that the upstream injector cools down the intake charge to make the mixture denser with oxygen, which translates into more power. This is the reverse of Honda’s Dual-Timing injection. The Honda system does not use two injector nozzles, nor does it meter the amount of fuel between separate systems. Instead, Honda’s Dual-Timing separates the spray from its single injector into two charges. The first charge is a short-duration spray that is used to cool the back of the intake valves’ heads for more oxygen. Both Kawasaki injector nozzles have the same output capacity. In
the KX250F system, only the first injector is running until 7000 rpm. At 7000 rpm, the upstream injector kicks in and gradually increases fuel delivery, while the first injector gradually tapers off. When the bike is running wide open, only the upstream injector is working, because the rpm is high and the demand for air is great, so the longer column provides the extra volume of air. Dual injectors make CPU programming more critical. The two injectors must work together seamlessly to optimize fuel delivery. When several non-Kawasaki race teams tried using a secondary injector simultaneously with a downstream injector, it didn’t work because they didn’t have the correct mapping. The secondary injector needs to come into the action when the engine rpm is high enough to utilize it. The extra time it takes for the upstream injector’s fuel to reach the combustion chamber must be calculated into the equation. The amount of fuel delivered by current motocross EFI systems depends on how long the injector is turned on, which is called “pulse width.” Obviously, two injectors add more dimension to the pulse-width equation. There is a certain amount of lag time from when the CPU sends the signal to turn the injector on and when it actually turns on. This lag can be a millisecond or more. When the pulse width is only 2 milliseconds at best, a delay of a millisecond is substantial. On a single injector engine, the engineers must figure out an average lag time and increase the pulse width accordingly to cover all the bases. With two simultaneously operating injectors, the pickup delay can easily be covered up by the difference between fuel traveling a short distance and fuel coming from farther away. The KX250F has one injector nozzle located close to the combustion chamber for low rpm running (when the need for air volume is less) and a second injector that is upstream so that it can deliver a greater volume of air down the intake tract.
Dual factor: Kawasaki’s second
injector, hidden in the rubber
airboot, provides the atomized
fuel necessary for high rpm