Speed Density SD vs. Mass Air Flow MAF
Often we deal with the question, "Which is better speed density (SD) or mass air flow (MAF)". As with most things in life there are advantages and disadvantages for both. Its better to understand each strategy so as to make the most informed decision.
Mass Air Flow
Mass air flow sensor equipped cars fuel either directly from or by a blended model of both mass air flow and speed density calculations. The MAF sensor is located somewhere in the intake stream typically away from sharp bends and typically at least 8-10 inches away from the throttle body however these rules are not always followed. There are several different types of MAF sensors but for most GM applications it is a frequency biased "cold-wire" sensor that generates a signal between 0 and 15k Hz. MAF sensors read not just airspeed but the relative mass of the air which eliminates pressure and temperature as variables. They are usually positioned to sample from the middle of the intake tubing which should give the best average reading.
Since the MAF sensor samples from a specific spot in the inlet tubing changing the airflow in or around the sensor requires recalibration of the MAF sensor. Changing the airflow around a MAF sensor can be caused by a change in size of the intake plumbing, bends before or after the MAF sensor, and changes to the MAF housing.
Several applications that use MAF biased strategies also use Speed Density as a backup or for a blended fuel calculation. They also use Speed Density as a sanity check for MAF readings.
- Typically more forgiving and easier to tune
- If the MAF sensor has been calibrated typically fueling remains consistent as modifications change. It can be said that the MAF equipped cars are a little more forgiving to new modifications
- Since the MAF sensor measures air mass directly it can be said that a MAF equipped car might stay in calibration as the engine wears or in extreme environment changes (however this is not normally a concern for well tuned speed density applications).
- Sensitive to changes to the intake plumbing
- Can be fouled by becoming dirty. Once fouled MAF calibration is skewed
- Sensitive to cam reversion on large cammed cars. The MAF sensor isn't directional so air pulsing back and forth can affect repeatability and reliability.
- Limits on performance. Most systems can not read at higher horsepower levels.
Speed density systems calculate the density of the air first by measuring the temperature of the inlet air and manifold pressure. With the density of air known the engine controller then looks up how much air it expects to be moving at a specific engine speed and manifold pressure. This is done in the Volumetric Efficiency table or VE table. Traditionally the VE table is 3D and has two axes Engine Speed (RPM) and Manifold Pressure or % Load. A engine that is 100% efficient moves exactly its displacement every two rotations in 4-cycle engines.
- Less restriction in the intake tract
- More freedom in the intake tract
- Able to measure air consumption on very high horsepower builds where a MAF sensor might be limited.
- Able to read boost if equipped with a greater than 1 Bar map sensor
- Free from errors encountered in MAF fouling.
- Works better with large cams where reversion affects reliability.
- Is a little more difficult to tune
- Relies on a well tuned Volumetric Efficiency Table
- Less forgiving to new modifications to the engine
- Might have errors in large weather changes however a proper tune makes this minimal if any error.
There are several vehicles that come with MAF sensors from the factory and there are several vehicles that come speed density from the factory. Really which fueling strategy you should go with on a aftermarket application is up to your modifications goals and expectations. If you are not the one to be tuning your setup please consult with your tuner for what he or she recommends.