A MAP measures absolute pressure (Manifold Absolute Pressure), not gauge pressure (which is relative to atmosphere), which means from absolute vacuum up to whatever the MAP's limit is... for a 1 bar sensor, that means absolute vacuum up to atmospheric (or approx 1 bar, which is 14.7psi)... so it will not measure boost, its limit is basically atmospheric pressure (zero psi boost). A naturally aspirated engine will always run at vacuum in the manifold, which is why many use a 1 bar sensor. For a 2 bar MAP, that means absolute vacuum up to 2 bar, or 29.4 psi, which is 14.7 psi of boost.
To convert absolute pressure to boost, or vacuum, yes - you have to subtract atmospheric pressure. So if the MAP is reading 29.4psi, that is actually 14.7psi of boost. So with a 2 or 3 bar MAP, you can measure 14.7 or 29.4psi of boost respectively. Given the boost on these SC's is 7 or 8psi, the 2 bar will easily cover it.
The Aeroforce in analog input allows you to set the slope & offset/intercept, so it will read in vacuum and boost from a 2 bar MAP. What I was interested in was whether it would do the same reading via the OBD port, given the engine management needs this info anyway, and the supercharger kit includes a 2 bar MAP which replaces the standard 1 bar MAP. From the reply below, looks like you can have both... the absolute measure (measured from absolute vacuum, in kPa), AND the vacuum/boost measure (which is corrected for atmospheric pressure).
You don't need a vacuum/boost gauge, you can get the info from the MAP and correct it.
So... a 2 bar MAP will give you vacuum & boost, by correcting the absolute pressure using the atmospheric/barometric pressure... and it seems the Aeroforce will display it. Happy days!
(if it didn't display via the OBD port, then you can always hook up the analog and add the slope & intercept, and then it will give you vacuum & boost from a 2 bar MAP)
Sorry the above turned out a bit tough to follow, hopefully it makes some sense. Another complication is the atmospheric pressure changes with altitude and even the weather, so the analog calibration you put in (the intercept specifically) will change, particularly at high altitude.
Hey man, thanks for the schooling on MAP sensors. MAP sensors have actually been around since the old carburetor days when they were basically just on off switches that tripped at a particular altitude. Old carbureted cars would use them while in closed loop. But who cares.
Its apparent that you misunderstand how MAP sensors actually work. First of all, they are fed a fixed +5vdc signal and a good ground. There is usually one output wire and this will range from 0 to +5vdc. The output does not convert 0 to 14.5psi, it is centered. Rather, it is -7.25psi to +7.25psi PSI. The -7.25psi can be converted to inches of mercury by multiplying the number by 2.03602. So a 1BAR MAP sensor would have a range of say 14.76Hg to 7.25psi. Similarly, a 2BAR MAP sensor would measure the range of 29.52Hg to 14.5psi. Though not practical for any significant amount of boost, the 1BAR MAP sensor does indeed measure positive boost.
Electronic boost gauges (that I am familiar with) are based on 2 and 3 BAR MAP sensors. I chose one based on a 2BAR to match the 2BAR Mopar MAP sensor that came with the blower kit. When I compare the output between my electronic boost gauge and the trinity they are within 0.20psi of each other and that is saying a lot because we are talking about two discrete MAP sensors and circuits. And the MAP sensor that came with the gauge appears to be a GM or some other brand.
The Trinity has a soft boost gauge built in, under the synthesized PIDs section. You can just add that to your gauge panel page. Even so, it will only read positive boost in the calculation. I wanted to read vacuum as well so I had to go to a real gauge. I am not sure any of the tuning devices or the aeroforce gauge can actually display both. It is being measured either way and fed to the PCM in terms of a 0-+5vdc signal.
The boost gauge does not calculate the boost real time like the above solutions do. It references barometric pressure when you first turn the key on and sets the zero point there. Because of this, you have to be careful to wire the gauge to a circuit that is switched on with the key but not interrupted during cranking/starting as most circuits are (except ignition and fuel, as examples). Interestingly, I think there is a situation where the gauge can become temporarily inaccurate. I live right at about sea level and live life under ~500. I have to drive 3 hours to the mountains (~7,500) to go wheeling. The gauge wont recalibrate the zero point until I stop for gas before trail head or when I am airing down, right at trail head.
I looked at the aeroforce gauges at first too but ultimately passed on them. I would have needed two for my purpose plus a third gauge for the water methanol injection. Compared to a Trinity or any other touch screen type gauges, the touch screen gauges can have 4 or more gauges on a given screen page. That and the gauges can be just plain old digits like the interceptor or they can be anything else. More importantly, you can do data logging directly with devices like the Trinity and pull in the analog signal directly in for your tune-by-email guy. The aeroforce gauges can have multiple functions but you have to cycle through them. I have two analog inputs being pulled in. AFR and WMI flow. I can log it all easily or watch it all real time. To do what I needed with the aeroforce gauge would have required two gauges plus a tuning device anyway. Two aeroforce gauges come to about what you can get a single tuning device for that also has the same functions and then some. One of my most used gauge panels consists of the following four gauges: KNK RETARD ST, KNK RETARD LT, AFR and IAT. Other handy gauge panels are a page (and a button click) away.
It sounds like the aeroforce is just giving you the raw MAP signal and calling that boost. You might was well just learn it in terms of the 0 to +5vdc output and just watch it with a simple volt meter. That is what the PCM is doing.