The MAF kit is a nice upgrade for air flow meter engines. On smaller engines, gains of around 12 hp and 12 ft-lbs of torque are typical. On larger engines gains of 18 hp and 18 ft-lbs are common. You will feel the benefit with quicker throttle response, smoother acceleration and a more free-reving top end.
E30 M3 engine with stock AFM
E30 M3 with MAF conversion
The main benefit of the MAF conversion comes from the wide open cross section of the MAF sensor. This removes a major restriction from the intake resulting in less pressure drop, better cylinder fill and more power.
A second benefit of running the MAF conversion is the ability to retune the engine for other mods. The MAF kits come with a calibrator that is used to match the curve of the MAF sensor to the curve of the original air flow meter. The calibrator is also capable of fine tuning the fuel curve to compensate for things like a cold air intake, cams, headers and a free flowing exhaust.
There are two types of MAF kits. The A kit uses the front panel adjustable ARC2-A calibrator. It is on-the-fly adjustable from the driver seat. The P kit uses the laptop programmable PSC1-004. It requires a laptop to tune and is not real-time programmable. The PSC1 will require a few hours to tune vs. 15 minutes for the ARC2-A. However after the PSC1 is tuned you will have a far more precise tune and better performance.
Front panel adjustable ARC2-A used in A kits
Laptop programmable PSC1-004 used in P kits
Click here to view the AFM to MAF Conversion Tech Note.