A MAFia is an add-on to the OEM MAF sensor or a replacement altogether?
I thought they were necessary on builds bigger than the OEM pulley/tune combination.
The MAFia is on add on device that plugs in between the MAF and the ECU. Essentially you plug the MAF connector into the MAFia and then the pigtail on the MAFia plugs into the MAF sensor.
Like many engine sensors on the car, the MAF communicates to the ECU using a voltage range of 0-5v. The more voltage, the more air it is measuring coming into the engine. When you have a lot of boost - several psi over OEM, you risk the chance that the MAF sensor can send a signal greater than 5v to the ECU. When/if this happens the ECU determines that the MAF has lost its mind and is no longer reliable. The ECU will toss a CEL and then - because it no longer believes the credibility of the MAF, it will usually go to limp mode where it runs the engine off of very conservative (and reliable) internal tables.
A MAFia takes the voltage signal from the MAF and scales it down before passing it along to the ECU. So, at the higher end of boost pressures, the MAFia may pass a 5.2v signal but the MAFia will scale that back to less than 5v so the ECU remains happy. When tuners like Torrie develop their tunes, they account for the fact that they are getting scaled MAF counts from the MAFia and can build tunes around it. However, most pulley/tune configurations do not require a MAFia because they don't generate enough boost to cause the MAF to send a signal above 5v. Now, if you've got headers, cat delete, and a very small pulley, a MAFia *may* be needed. With the amount of air that the Whipples generate, a MAFia is typically needed.
In no circumstances should you either add or remove a MAFia with also changing the tune in the car. The tune files need to know whether they're getting scaled values or not.
Hope that helps...