|Top Secret, a tuning house in Japan, created this wild custom turbocharged V12 Supra. Unfortunately the standalone HKS EMS they used could only handle up to 8 cylinders. Their solution was to run two of the systems and treat the engine as it it were two inline sixes! Photo courtesy of SpeedHunters.com
Boosted applications are nearly impossible to tune with Alpha-N, because there is no input for mass airflow or manifold pressure. In other words, the computer has no idea how much air is really in the engine. This is a big problem on a forced-induction motor, because the current boost level is not directly related to throttle position or RPM; it is dependent on other factors such as what the motor was doing previously and the current load on it. In other words, there is a hysteresis effect associated with a forced-induction motor.
Nonetheless, Alpha-N is used successfully on some naturally-aspirated drag racing cars that run huge cams. The low idle vacuum makes air readings difficult, and the engine spends most of its time at full-throttle anyway. For the majority of applications, though, Alpha-N is a very bad choice for an EMS configuration.
Speed Density is the second widely used configuration, but it is becoming less popular. In a speed density configuration, the EMS has a Manifold Absolute Pressure (MAP) sensor that tells it the exact air pressure in the intake manifold. With this information, the computer can get a pretty good estimate of the amount of air in the engine based on the known values of the volumes of the cylinders and intake manifold. In other words, the computer knows the volumetric efficiency (VE) of the engine and can thus calculate the needed fuel and spark based on the amount of air currently in the engine.
Image courtesy of SpeedHunters.com. The 2JZGTE in the turbo Mark IV Supra uses a speed-density control system. Note the lack of a MAF sensor prior to the turbo.
Originally, speed density was typically used in factory turbocharged or supercharged applications due to the MAP sensor's ability to provide a precise boost level to the computer, making it very easy to tune. For instance, the Mark IV Toyota Supra used speed density in its turbo models but mass airflow (which I will discuss next) in its naturally-aspirated models.
The advantages of speed density stem from the fact that the EMS knows the precise air pressure inside the intake manifold at any given time. Because of this, what occurs before the throttle body is largely irrelevant. Changes in the intake tract, such as cool air intakes or intercoolers, do not matter as much to the computer, because it is taking its measurement directly from the intake manifold. (This assumes that any modifications made do not significantly alter the VE of the engine.)
The disadvantage of speed density is that, because VE is assumed to be constant, changing anything after the throttle body (such as cams, valves, intake manifold, or cylinder volume) requires significant retuning of the entire computer, because the VE of the engine has now changed. Many aftermarket fuel injection systems, such as MegaSquirt, use speed density because of how easy it is to plumb a vacuum line from the intake manifold to a MAP sensor.
Image courtesy of SpeedHunters.com. The RB26DETT was originally equipped with 2 turbos. Two turbos means two intakes and thus two MAF sensors. When converting to a single turbo, some tuners keep both MAF sensors and install a split intake going into the turbo.
Mass Airflow is the third configuration and has become the most common. In this configuration, the actual mass of the air entering the engine is measured. The Mass Airflow (MAF) sensor (located before the throttle body and usually immediately after the air filter element) usually utilizes a heated wire to create this measurement. Because the exact mass of air entering the engine is known, the EMS knows exactly what fuel and spark measurements to use.
The advantage of mass airflow is ease of tuning. The EMS always knows how much air is in the engine at any given time. This makes it easier to tune, because any change that affects the VE of the engine also affects the airflow entering the engine, which will thus register on the MAF sensor. It is also easier to compensate for varying atmospheric conditions, because lower ambient air pressure directly equates to a lower mass of air entering the engine. (A speed-density-based EMS requires a separate barometric sensor to measure actual atmospheric pressure and compensate accordingly.)
One of the chief disadvantages of mass airflow is the cost of the sensor, and that is one of the reasons it was originally uncommon. Additionally, the MAF sensor is calibrated to assume a certain intake tube diameter, so changing the size of the air intake requires either recalibrating the EMS or purchasing a new, larger (and more expensive) MAF sensor. Older MAF sensors only came in smaller sizes, making it difficult to use for higher-horsepower applications, because the MAF sensor itself became an intake restriction.
Another “disadvantage” of a MAF sensor is that it requires non-turbulent airflow in order to get a proper reading. Because of this, the air intake tract must be designed in such a way as to give the MAF sensor a straight air path leading up to it in order to prevent turbulence at the sensor.
Finally, because the MAF sensor measures air immediately after the air filter, it is very sensitive to vacuum leaks in the air intake tract. If air is allowed to enter (or leave) the air intake after the MAF sensor, the EMS is unable to account for it, because it was not able to measure it. This is why mass airflow systems require recirculating blow-off valves that vent the discharged air pre-compressor and post-MAF. The air that was compressed by the turbocharger (or centrifugal supercharger) was already measured by the MAF sensor; venting it to atmosphere causes the EMS to assume that all that air is still in the intake tract, potentially causing a rich condition. Similarly, a leak in the air intake tract after the MAF sensor will allow additional air not measured by the EMS into the engine, potentially causing a lean condition.
Pro-M makes this MAF sensor for the new Ford Coyote V8. If installed in a 4” intake tube, it can measure enough air to support 1,500 horsepower.
Of course, it is possible to combine both speed density and mass airflow. Ford/Shelby Cobra GT500s, which are supercharged from the factory, have both MAF and MAP sensors, but this is not the norm.
If you had to choose, mass airflow really is the best choice for most engines out there, as it allows the EMS to know the exact amount of air in the engine at any given time, whether the engine is idling or full-throttle under 40 pounds of boost. This leads to more consistent air fuel ratios and power delivery and easier tuning.
Now that you know the basics of how Engine Management Systems work. Go forth and tune!