When a bike is being developed by the manufacturer, powertrain engineers determine an ideal fueling and ignition scheme so your engine offers peerless performance, optimum power, and, for all we know, the ability to wash your dishes—if it had opposable thumbs. Unfortunately, that’s not all those engineers have on their plates. Emissions and noise regulations, which are becoming even more strict worldwide, force compromises in this programming that might make the bike run just a bit less optimally than it could.
Which is why we have products that allow you to retune the fuel and ignition systems to improve performance and accommodate changes such as a different exhaust or air filter. You can either change the coding inside the stock engine computer (ECU) or drop a control module over the stock system. The latter example is often called a piggyback setup, typified by Dynojet’s Power Commander.
All digital FI and ignition systems work with what’s called a base map. For any given set of variables, the ECU determines when to light the spark plugs and how long to hold the fuel injectors open. The map tells the hard parts what to do and when to do it, informed by a host of inputs that include engine speed, throttle position, actual throttle-plate position (on ride-by-wire bikes), gear position, exhaust flapper-valve position, ambient temperature, and barometric pressure. There’s even more on CAN-Bus bikes, where everything from brake-line pressure to turn-signal condition is in the data stream.
For metering fuel, it’s a matter of how long the injectors stay open. The longer they’re open, the more fuel goes into the engine. Injectors are either open (allowing fuel to pass) or closed. More fuel with a given throttle-plate opening makes the air/fuel mixture richer and tends to increase power at the expense of fuel economy. Leaner mixtures improve mpg and help give the catalytic converter less work to do burning unused hydrocarbons, which is why many stock bikes are mapped lean.
When you talk to a tuner, you often hear about the air-fuel ratio (AFR), which is simply the relationship of air and fuel delivered, by weight. Typically, four-stroke engines desire a ratio of 12.8:1 to 13.5:1 to produce best power. That’s a little more fuel than stoichiometric, a 14.7:1 ratio, which is the theoretical mixture that consumes all the fuel. (Leaner, and the fuel runs out before all the oxygen has been consumed, and vice versa.)
Engines don’t always run at an ideal AFR because they don’t need to—or, often, because the manufacturer needs another AFR for emissions compliance. When at partial throttle, for example, modern engines are quite happy to run leaner, often at 14:1 or more. That’s fine if you leave your bike totally stock, so the system remains in the balance anticipated by the engineers. But change something as simple as an air filter or as complex as the exhaust system and you might throw things out of whack. Your bike starts to run hot or surge at partial throttle—or worse.
That’s where aftermarket solutions come in. Since we’re dealing with digital signals to the injectors and coils, it’s a simple matter of reforming those signals to get the tuning we want. As such, these overlay boxes splice in between the stock ECU and the fuel injectors (as well as the ignition coils if that’s part of the kit) and may connect to other parts of the stock bike, including the throttle-position sensor. The piggyback module watches for the stock ECU to trigger the injectors or coils, intercepts the signals, tweaks them as necessary, and then sends the commands to the hard parts.
See? Simple. “Well, not so fast,” chuckles Ammar Bazzaz, head of Bazzaz Performance and at one time part of Mat Mladin’s epic Yoshimura superbike team. “There is actually a lot to it,” he says, regarding today’s more sophisticated bikes. “We have to be very careful to build piggyback systems that not only tune the bike the way we want but that won’t cause the stock ECU to throw codes. It’s not as easy as it sounds.”
Point taken, and it’s not a quick process either. It starts on a dyno with either a wide-band oxygen sensor or an exhaust-gas analyzer. The stock bike is run throughout the operating range to see what the effective AFR is. Then the piggyback box is loaded with skew values, which add or subtract fuel at given combinations of throttle position and rpm. The bike is retested to see where the AFR ends up. These data points are correlated with the torque reading from the dyno to optimize power. When you look at a finished map, you might see a reduction in fuel at, say, 20-percent throttle and 3,500 rpm but added fuel just a few cells away (say, 40-percent throttle and 4,500 rpm) because the overlay map is trying to smooth out the stock programming.
“It’s funny,” Dynojet’s Dusty Schaller says. “We used to tune for maximum power. That’s it. But today’s riders are looking for better fuel mileage. Where we used to aim for a 13.2 to 13.5:1 AFR for best power, now we’re tuning closer to 14:1 to preserve mileage.” Bazzaz says his company starts at around 13:1 but will aim for different targets if best torque or drivability suggest it. In truth, any given engine’s mixture requirements change across loadings (how much power you’re asking it to make) and rpm, so there’s no one-size-fits all AFR.
Peak power is one thing, but what about rideability? “We’ve found that good, stable AFRs throughout the map will usually result in better throttle response,” Bazzaz says. “But we will sometimes make small changes to improve response. For ultimate refinement, we’ll run a self-mapping module on a closed course.” These self-mappers use an O2 sensor in the exhaust and read real-time AFRs as you ride. At the end of your test period, the software will suggest new skew values that you can put in the piggyback box.
The more sophisticated overlay boxes can be used to make very fine adjustments, but there are simpler overlay solutions that don’t require a computer or complex maps—these systems increase or decrease fuel over a broad range, which is often good enough for low-performance bikes.
Hacking vs. Piggyback
For a long time, the piggyback module was the way to change factory fuel injection and ignition systems. But in the last decade or so, enterprising software engineers have found a way to hack into the stock ECU to change the very map that it operates from. An advantage is that you don’t need extra hardware and that, done correctly, there are fewer chances of the ECU showing error codes. (But there are still opportunities.) Some software products also allow you to change throttle response—the way RBW reacts to your inputs—as well as a bunch of other things. For example, they can prevent the ECU from cutting fuel entirely on trailing throttle, an often-used emissions-reducing trick that hurts drivability.
Piggyback has the advantage that it can be removed when you sell the bike, and there’s no waiting for your ECU to go out to a programmer and then come back with new coding. The modern overlay systems also allow extensive user programming, which is not possible with a hacked box.
Open vs. Closed Loop
Oxygen sensors have arrived on modern bikes. They’re needed to meet ever-tighter emissions regulations and make possible something called a closed-loop system, basically this: Under certain riding conditions, the bike’s ECU monitors actual fuel/air ratios by measuring the amount of latent oxygen in the exhaust.
When the ECU enters the closed-loop mode, it will skew the injector duration to achieve a desired AFR. If you have added an aftermarket exhaust that influences how the engine runs, the closed-loop settings will help compensate. But only when the bike is in closed-loop mode. (In open loop, the bike runs on the base map only.)
The time spent in closed-loop varies by bike. Dynojet’s Dusty Schaller says that modern Harleys are in closed loop about 60 to 70 percent of the time. Honda’s CBR250 is in closed loop 80 percent. Other bikes do so in selected regions—only at steady state, below a certain rpm, and in certain gears.
Typically, an overlay fuel box will come with devices that take the O2 sensors off line, basically fooling the stock ECU into thinking they’re active when they’re not. The idea is to keep the stock ECU from moving the playing field. For example, if you add fuel with an O2 sensor in line, the stock ECU will trim back to achieve the desired AFR; you’ve then just un-done what the piggyback module was trying to accomplish. Closed loop becomes an open loop.
There’s a simpler solution to piggyback boxes that need a PC to program, sometimes called “electronic jet kits.” (That’s Cobra’s adaptive PowrPro tuner, right.) With these, you tune more or less by the seat of the pants—though it’s still possible to tune on a dyno—and make fueling changes with the press of a button or turn of a dial. No computer necessary. These devices are designed to shift fueling over an rpm range rather than by specific rpm/throttle combinations. Cobra’s system watches engine acceleration and changes fuel delivery to keep the engine’s responses smooth. Downsides? If there are quirks in the stock map within the shifted ranges, this kind of box cannot smooth them out without significant compromises.