How does SManager convert MAP volts to vac/pressure?

[41SMF]

Does anyone know how SManager converts MAP signal voltages into pressures? I am using a Hondata 4 bar MAP and I see the quick select input values of 838 mbar/volt with an offset of 31 mbar.

I take the voltage x 838 and add 31, which should give me the mbar of pressure, but it doesn't agree with the SManager reported vac/pressures.

Does anyone know how SManager does the conversion?

[Hondata]

That's how the ECU does the conversion. What voltage and pressure are you seeing?

[41SMF]

There's nothing I've found in the help file or elsewhere that says what the math is, so I tried to come up with something that I thought made sense.

Here's an example:

The SManager Sensor display (and the Graph) for a particular frame says I have 18.4 psi of boost. PA for that frame is 97.3 kPA.

From the Datalog, that frame says the MAP voltage is 2.281 volts. I have a 4 bar Hondata MAP and the parameters are 838 mbar/volt with a 31 mbar offset.

The math is 2.281 x 838 + 31 = 1942.5 mbar, which is absolute pressure.

Converting to relative pressure above atmospheric and converting to psi I get

(1942.5 - 973) * .014504 = 14.1 psi, which is way less than SManager says...

I don't know if SManager corrects for ambient atmospheric pressure or not, but if I assumed 1 atm = 1013 mbar (14.7 psi), that would make the difference bigger.

[Hondata]

What does SManager show the pressure as in kPa or mbar? You might have the SManager units set to absolute. Post the datalog.

[41SMF]

Here is the Datalog. Frame 1224 contains the data for the example I put in my post yesterday.

I have SManager set to display pressure as vac/boost. Relative inches of vacuum below atmospheric and psi of boost above it.

[41SMF]

FWIW, I have been able to convert Wideband voltages from the datalog into lambdas and get reasonable agreement (my calc vs what SManager reports). I'm using an AEM wideband (30-0300).

[Hondata]

What you are looking at is not voltage but pressure in bar (2.281 bar)

[41SMF]

Thanks for straightening that out. I was making it more complicated than it needed to be.

So, now the math is:

map (from datalog) - 1.013 = relative pressure. 1.013 is 1 standard atmosphere

If that amount is less than zero, multiply by 29.53 to get inches of vacuum.

If that amount is greater than or equal to zero, multiply by 14.504 to get psi of boost.

And with that, values for vac/boost that I calculate from the numbers in the datalog agree with what SManager displays.