Originally Posted by mastaflymason
Do you need a vacuum hose from the manifold to the MAF sensor on the megasquirt if you are using the stock MAF? Or is it just for if you are using the GM AIT?
These three concepts are largely unrelated.
On MAP vs. MAF:
The Megasquirt was originally designed to operate based on MAP (Manifold Absolute Pressure), and this remains by far the most common configuration. The MS contains a MAP
sensor, not a MAF (Mass Air Flow) sensor.
MAP-based operation works by comparison the measured manifold pressure and the current RPM to a table of known air-mass-units for that operating condition.
MAF-based operation works by directly measuring airflow in units of mass (or volume) per unit time.
It is possible to operate based on MAF rather than MAP, retaining the stock MAF (or VAF) sensor from the vehicle. This method is not preferred, it was implemented only as a concession to people racing in series where the rules permit ECU replacement but require the use of all stock sensors.
In theory, MAF-based operation offers the advantage that alterations to the head / exhaust path / etc, which alter airflow relative to manifold pressure, are automatically compensated for, whereas in MAP-based operation, such alterations require re-scaling of the chart.
In practice, MAF-based operation has not historically been popular among the MS community. There's nothing wrong with MAF-based operation per se, but don't expect much support if you decide to use MAF rather than MAP. Running in MAP-based mode has its own set of advantages, particularly on turbocharged or supercharged vehicles, not the least of which is the elimination of a potentially restrictive element from the intake path.
On temperature sensors:
You must use an intake air temp sensor of some kind.
In the NA cars, the IAT sensor is physically integrated into the VAF / MAF sensor body. Thus, for a naturally aspirated car, you might choose to retain the VAF / MAF assembly solely for the use of its internal IAT sensor, even though you have run a vacuum line to the MS and are using MAP as the primary load input. (In such a configuration, the wire relevant to the temp output of the VAF / MAF assembly would be connected to the MS, but the wire related to airflow would not be.) In the NB cars, the stock IAT sensor is separate from the MAF sensor, so removal of one does not affect the other.
If you elect to physically remove the VAF / MAF assembly from the car, then you must install a different temp sensor, such as the GM IAT sensor.
Here is a picture of a cat (unrelated):