EFI Tuning - How It Works
Fuel Air Spark Technology unlocks the secrets of EFI tuning, and offers insight into its new XFI 2.0 system
From the September, 2012 issue of Chevy High Performance
By Stephen Kim
At its core, electronic fuel injection is nothing more than a central computer interacting with a bunch of sensors to keep an engine happy. Obviously, this is much easier said than done, and getting it right involves a synergy of hardware, software, and tuning expertise. For nearly two decades, Fuel Air Spark Technology has had the first two elements of that three-part equation thoroughly covered. The biggest wild card has always been the skill and experience of the tuner, but fortunately, FAST has some great advice for walking novices through the process of tuning an aftermarket EFI system. To get to the bottom of it all, we threw an onslaught of elementary tuning questions at FAST’s Kevin Winstead, and he was more than happy to share his expertise. Just some of the topics he shared with us include how to establish a baseline tune, the benefits of tuning on the dyno versus tuning at the track, how to set the fuel and spark maps at various engine loads, dialing in the air/fuel ratio for different types of fuel, and how to tune for boost and nitrous.
Furthermore, FAST’s existing XFI stand-alone system underwent an extensive update last year. The new XFI 2.0 system is a firmware upgrade that unlocks a new realm of performance potential. New features include a boost controller, internal data logging, a four-stage nitrous controller, a self-learning fuel map, an intelligent traction control system, transbrake control, and the ability to switch among multiple tunes by flipping a switch. The good news is that older XFI stand-alone systems can be upgraded to the version 2.0 software with an upgrade CD, as it is completely compatible with the existing XFI hardware. Naturally, we had Winstead explain all the benefits of XFI 2.0 in far greater detail to supplement his valuable tuning advice. Here are his words of wisdom.
There are newer stand-alone EFI systems on the market, but the FAST XFI system is still getting the job done. Based on the feedback we heard from racers at the track, we added a new firmware upgrade to the existing system last year to create XFI 2.0. FAST’s XFI 2.0 is now more versatile than ever. With the new Qwik-Tune technology integrated into the 2.0, you can toggle among four different tunes at the flick of a switch, eliminating the need for a laptop. For example, you can have one tune for daily driving, a race tune for running nitrous at the track, and another tune to maximize fuel economy. Likewise, the XFI 2.0 features a very robust NTK wideband O2 sensor as standard equipment, which is one of the most dependable sensors on the market. A new self-learning table allows for quick and easy fuel mapping automatically, and two new exciting options are intelligent traction control and internal data logging. For power-adder applications, XFI 2.0 has a standard boost controller, and it can also manage up to four stages of nitrous. Other perks include the option to choose between sequential or bank-to-bank injector firing, and compatibility with flex fuel sensors for use with E85.
For someone just getting started with EFI, the process of tuning an engine can be intimidating, but it’s definitely feasible. All it takes is some patience and some basic knowledge of what tuning changes your engine requires. It certainly helps to have a good baseline tune as well as intuitive software. Fortunately, XFI 2.0 includes many baseline tune files that allow users to get started quickly. Likewise, the self-learning fuel tables make it much easier to dial in the fuel maps. FAST also offers XFI tuning classes that provide a wealth of information for all aspiring tuners. Check out FAST’s website to get an up-to-date schedule.
After installing the XFI 2.0 system, following these five simple steps are all that is required in order to fire an engine up for the first time. Step one is picking out a baseline tune that is comparable to your application from our library of files. Next, the optional parameters must be set. This includes selecting between bank-to-bank or sequential injector firing, and setting up the ignition strategy and crank reference angle. Afterward, it’s time to dial in the fuel calculation parameters, which involves entering in the engine displacement, injector size, and the fuel energy constant. The final two steps are entering the engine’s firing order, and performing a sensor calibration. As long as you’ve entered the correct baseline tune, the engine is ready to fire up at this point.
Some tuners prefer dialing in an engine on the dyno, while others prefer doing it on the street or at the track. Where you choose to tune a car depends on which part of the tune you’re trying to optimize. The best way to initially tune the wide-open throttle fuel and spark maps is on the dyno. The controlled environment of a dyno cell is perfect for making small changes and optimizing the cylinder-to-cylinder fuel distribution and timing effects on each cylinder. However, there are also advantages of tuning at the racetrack, as it gives you an opportunity to make changes on shift transitions and the opportunity to optimize the tune for specific track conditions. The track also loads the motor differently than the dyno, so track tuning can offer a more precise calibration for race day. As far as the idle and part-throttle tuning, that is best achieved when you’re actually driving the car down the road.
Tuning aftermarket EFI systems can seem confusing because there are so many different screens and tables to navigate. Fortunately, familiarizing yourself with the most commonly used tables can simplify the tuning process. In practice, the Base VE table and the base spark table are the two screens that are by far the most frequently used when tuning. These tables are the basis for fuel delivery and ignition timing, so learning to navigate them pays large dividends. There are also multiple tables that are used to set up the cranking fuel, acceleration enrichment fuel, and cold start fuel as well. However, if you choose the proper baseline calibration file, then these tables will need very little attention.
There’s a fine line between advancing the timing as much as possible for maximum power, and advancing it so much that the motor detonates. To walk this fine line, remember that as you approach the ignition advance for maximum torque you’ll see a point of diminishing returns. If you have a dyno that allows you to hold the engine speed at a steady state, you can easily establish the proper timing value for maximum torque at that engine speed. Some engineering books refer to this timing value as MBT, or maximum brake torque timing. Once you exceed this number, torque values will decrease. If you keep increasing the timing value you will eventually get into detonation. An experienced tuner will recognize the MBT value and stop advancing the timing before you reach the detonation point.
Fuel and Spark vs. Engine Load
Unlike a carbureted motor, EFI systems can monitor engine load as a means of dialing in the fuel and spark. This is a very useful feature, since the amount of fuel and ignition timing an engine requires varies as engine load changes. As a general rule of thumb, you can run maximum timing and relatively lean air/fuel ratios when an engine is operating at the low load areas of the table. This is when you’re just cruising and the engine is under very little load. As far as timing is concerned, when an engine is at very low rpm, such as at idle, you do not need much spark advance. Under these conditions, you will typically have a fairly lean air/fuel ratio that’s not too far from the stoichiometric ideal. Conversely, the air/fuel ratio will be the richest at WOT.
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.
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.
Forced-induction motors present extreme challenges in terms of power management, fuel consumption, and exhaust gas temperature. The good news is that XFI 2.0 offers a variety of features that deal with these challenges quite well. The system now has a three-port boost control solenoid for controlling the wastegates. It can be operated off either manifold pressure or compressed CO2. For flexibility, it can be plumbed to either add boost or to remove boost from the wastegate. Likewise, the XFI 2.0 software has both an open loop and even closed loop feature for boost control. Utilizing closed loop control, you can target the amount of boost that you want, and it will maintain that boost. If you utilize the open loop mode, then the solenoid pulse width changes over time. Since the XFI 2.0 software has the option to drive multiple injectors, it makes managing large quantities of fuel much easier. We have customers who run two or three injectors per cylinder on applications that require a lot of fuel delivery. XFI 2.0 also has an optional EGT system that allows monitoring exhaust temperature in each cylinder.
A great perk of the XFI 2.0 system is that it includes an internal data logging system. This eliminates the cost and complexity associated with installing a separate external data acquisition system. XFI 2.0 can monitor all of the engine parameters such as rpm, throttle position, coolant temperature, air temperature, and air/fuel ratio, just to name a few. With an external data acquisition system, additional sensors must be purchased to log these parameters. XFI 2.0 also features additional inputs that can be used for several more external sensors. By utilizing these additional inputs, it can monitor fuel pressure, oil pressure, wastegate pressure, backpressure, and driveshaft speed. The XFI system can also be combined with the FAST touch-screen dash that allows for even more inputs as well as hours of data-logging capacity.
Naturally aspirated and forced-induction motors call for vastly different tuning strategies. Much like a naturally aspirated combination, the air/fuel ratio of a forced-induction motor at idle and cruise speed is between 13.0:1 and 15.0:1. However, as boost increases, the fuel demands are much greater than that of a naturally aspirated combo. Between 14.5 and 29 psi of boost, an air/fuel ratio of 11.8:1 to 12.5:1 is common. At boost levels beyond that, the air/fuel ratio can get as rich as 10.0:1. As far as spark timing is concerned, ignition advance varies greatly depending on fuel type and inlet air temperature. In steady state cruising, anywhere from 25-35 degrees of timing is common. Between 14.5 to 29 pounds of boost, timing can range anywhere from 15-30 degrees. Beyond that level of boost, timing can drop as low as 12 degrees of advance. At any rate, forced-induction motors are very sensitive to changes in air/fuel ratio and spark advance, so it’s extremely important to get the tune dialed in right. CHP