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# GEMS MAP mBar options

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Hi guys,

Bit baffled by this.

I have one cal with MAP mBar m set to 610 and MAP mBar c set to -101 and another cal (which runs my Prodrive gpN car fine) with MAP mBar m set to 315 and MAP mBar c set to 787.

Reading what RickS says in a post on the support forum, the values in the cals I have do not seem to correlate with what he says is the formula used:

"The 'MAP mBar M' is a multiplier and 'MAP Mbar C' is an offset used to scale the MAP parameter. It is used thus....

Air Pressure = ('MAP raw' x 'MAP Mbar m') + MAP Mbar C."

Using the above equation and assuming units of millibars (mBar) NOT pascals, kPa, bar or some other unit. Using a syringe to apply vacuum and pressure (respectively) to the MAP sensor with a boost gauge T'd in; at a MAP Raw reading of 58 I'm at about -0.3 bar gauge, at atmospheric I'm at MAP Raw of 83 and at 0.4ish bar gauge I'm seeing MAP Raw of 120.

These values equal about 700 mBar absolute for the 58 MAP Raw reading, atmos (1000mBar)for the 83 MAP Raw reading and 1430 mBar for the 120 MAP Raw reading. The only way I can get these number to equate using the above formula is the a MAP mBar m valuse of 12 and MAP mBar c value of 0.

This seems a country mile from the values in the base map (Subaru 99 v2.30) on the website or either of the cals I have here. Using the values in the base map I calculate 'Air Pressure' (in millibars?) of 38287, 54812 and 79269 for the MAP Raw values 58, 83 & 120 respectively - clearly this cannot be correct.

Doesn't seem to correspond to any other units (such as pascals) either?

What actually are the MAP mBar m and MAP mBar c options used for, is it used for scaling the Y axis of the fuel and ignition maps to the max boost pressure you want to run? Surely MAP min and MAP max do this?

This is where the last post on the support forum thread by RickS is even more confusing:

"I will get some solid info from the software developers on this next week as they are all at the Autosport show at the NEC this week. Now, I maybe gravely mistaken but the way that I have derived these figures in the past (and it seems not to dis-similar to the cals that I can find in the archive) is that you take the absolute boost that you want to run and multiply it by that magical computer nimber 256. So for your 1.4 Bar of boost, add 1 Atm to get an absolute figure and multiply by 256. Now subtract 10% to give a little 'safety headroom' and you should end up with a figure of 553. This is your 'm' value. To obtain 'c' just enter a value which gives the correct atmospheric pressure reading when the sensor is open to the atmosphere pressure only. This will be trial and error as there is usually a little variation between sensors and the ambient pressure on the day that your calibration takes place. If you can't check the ambient pressure and don't have a barometer handy, then just assume a value of 1000 mBar; this will be near enough to get car running. In the meantime, if you/anyone knows of a more accurate/detailed process, please post it here."

Why would you use a value in bar when the option is mBar? And what does the maximum boost you intend to run have to do with what appears to me to be a sensor linearization calculation?

Do the 0-130 units in the Y axis of the maps correspond to MAP Raw values or are they (as I suspect) arbitrary 'GEMS' units?

If you have access to the vehicle, it's a good idea to use a vaccum/pressure tool like a MityVac MV8500 so you can manually change the pressure received by the MAP sensor. Watching how closely your current calibration matches the mechanical gauge can be very useful, and you can quickly adjust the sensor pressure to test calibration changes and watch how that affects calculated fuel or spark table lookups. I've used GEMS software once or twice and I think you're correct that the MAP scaling factor uses a different unit system than the MAP offset number.