Target A/F Ratio
The exact value for the target air/fuel ratio depends on the type of fuel used as well as the application. The stoichiometric air/fuel ratio for gasoline is 14.7:1. An engine with an efficient cylinder head and a mild cam will prefer an idle air/fuel ratio slightly richer than stoichiometric, usually around 14.0:1 to 14.5:1 for non-blended pump gas. Keep in mind that most pump gas contains at least 10 percent ethanol, which means that the stoichiometric value of that blended gasoline is now closer to 14.1:1. This means that the idle air/fuel ratio will have to be a lower number for the same application, usually around 13.5:1 to 14.0:1. For part-throttle cruising, it’s common to see an air/fuel ratio close to stoichiometric. However, when you’re dealing with an engine that has a large cam and less efficient cylinder heads, this number will tend to be a little richer. Moreover, at WOT it’s common to see an air/fuel ratio of around 12.8:1 to 13.0:1 for naturally aspirated applications. For certain power-adder applications, the WOT mixture can be even richer than 11.0:1.
Fuel Energy Constants
Although gasoline is the fuel of choice for most hot rodders, alternative fuels like E85, methanol, and compressed natural gas are gaining popularity as well. Fortunately, XFI 2.0 can be easily programmed to run off these different types of fuels by simply entering a fuel energy constant (FEC) into the software’s fuel tables. This automatically adjusts the target air/fuel ratio in each cell, instead of having to modify each cell individually. For example, E85 has a stoichiometric air/fuel ratio of 10.0:1 opposed to 14.7:1 for gasoline. By entering an FEC value of 0.68 into the fuel tables, the FAST software automatically adjusts the air/fuel ratio by the proper amount, since 14.7 multiplied by 0.68 is 10.0:1. Furthermore, the FEC values for compressed natural gas, liquid propane, MTBE, ethanol, and methanol are 1.17, 1.07, 0.796, 0.612, and 0.439, respectively.
XFI 2.0’s new traction control feature is a powerful tool for racers. It requires installation of a driveshaft speed sensor, which is utilized to keep the driveshaft acceleration rate below a predetermined level. In actual track testing with one of our company test cars, the intelligent traction control (ITC) system allowed us to leave at WOT on the hottest day of the summer without any track prep. Best of all, ITC isn’t difficult to set up. There are two different modes of traction control in the XFI 2.0. The first is the heuristic mode, which does not limit the rate at which your car can accelerate. By monitoring the change in driveshaft speed over time, the ECU can determine whether the tires are slipping, and then retard the timing to try and keep that acceleration rate within the acceptable range you program. The XFI can also reduce the boost level in turbo applications.
The second ITC mode is called PA. It differs from heuristic mode in that it uses actual driveshaft rpm as the base curve to calculate from. The farther and faster your driveshaft goes over the programmed curve, PA mode will begin cutting out cylinders sequentially, using both fuel and spark, until the driveshaft rpm comes back to even or below the programmed curve. This mode isn’t recommended for nitrous engines.