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MAF Limitations and Speed Density

The term speed density has been used several times in the tuning section of this guide. In speed density systems, the MAP sensor and intake air temperature sensor (IAT) are polled by the PCM to calculate the air density. The PCM will then do a lookup in a pre-programmed volumetric efficiency (VE) table and get the VE for that particular RPM and MAP. The PCM can then calculate the mass air flow using VE, RPM and engine displacement.

MAF systems use a sensor that measures the air density entering the engine rather that calculating it as with a speed density system. The MAF sensor has a heated wire that maintains a constant temperature. Air flowing through the MAF sensor cools the wire requiring more current through the wire to hold the constant temperature. The amount of current flowing through the wire is proportional to the air mass flowing through the MAF sensor.

MAF systems are used on many current production vehicles. On a high boost, big horsepower turbocharged engine however, speed density is a better way to go. One reason is because MAFs and engine computers a limit to how much of air flow they can read. Tuners can get around this limit by fudging tables, but at this level it is wise to switch to a speed density system.

2 and 3 BAR MAP and Speed Density

The MAP sensor on most stock engines is a 1 bar MAP sensor. The atmospheric pressure at standard sea level is 1.013 bar, therefore, it cannot sense pressures above atmospheric. In other words, it cannot sense boost pressure. A 2 bar MAP sensor will sense up to about 14.5 psig, and a 3 bar MAP will sense about 29 psig. Remember that 1 bar is atmospheric pressure or 0 psig.

When tuning for speed density, you will need to replace your stock MAP sensor with either a 2 or 3 bar MAP sensor, depending on how much boost you want to run. 2 and 3 bar MAP sensors can be purchased from Summit Racing and other speed shops. Tuning for 2 and 3 bar speed density can be done with HPTuners, EFI Live, Bigstuff3, FAST XFI, Accel DFI among others.