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My 2002 Honda Accord V6 has a Manifold Absolute Pressure sensor (MAP). I understand that this sensor gives out information in pressure units (psi or equivalent). After doing a bit of research, I realized that my car does not have a Mass Air Flow sensor (MAF). Based on this document (page 4), I know how to calculate MPG from data retrieved from the MAF sensor (which my car doesn't have).

Is there any way for me to calculate MPG from my MAP sensor? Or is there a way to convert between data received from the MAP sensor to data that an MAF sensor would have reported if it existed? My end goal is to be able to calculate MPG using my MAP sensor (I'm using an OBD2 device to get all the data). Thanks!

  • I'm not an expert (by any means) on tuning and such, but I'm thinking the answer to this is no. While these two devices help derive the amount of fuel to get delivered to the engine, they do it in entirely two different ways. Most engines have both sensors to get an even more precise measurement. Usually when an engine has just a MAP sensor, it is being run in speed density. This doesn't adjust for irregularities like a MAF allows the computer to do. It also doesn't have the big obstruction in the intake either ;-) I'm interested to hear if there is a more qualified answer than my comment. – Pᴀᴜʟsᴛᴇʀ2 May 4 '15 at 22:21
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    I actually found this: lightner.net/obd2guru/IMAP_AFcalc.html – NewGradDev May 5 '15 at 1:37
  • Very kewl. You should post that as the answer ... definitely proves the first sentence of my previous comment ... '-) – Pᴀᴜʟsᴛᴇʀ2 May 5 '15 at 2:57
  • Interesting. I was going to say you'd have a really difficult time unless you factored in gearing and gear selected to calculate distance travelled, but i guess you can more accurately calculate distance with gps anyway. – Sir Swears-a-lot May 5 '15 at 5:16
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This is the calculation you're looking for.

IMAP = RPM * MAP / IAT MAF = (IMAP/120)(VE/100)(ED)*(MM)/(R)

Where manifold absolute pressure (MAP) is in kPa, intake air temp (IAT) is in degrees Kelvin, R is 8.314 J/°K/mole and the average molecular mass of air (MM) is 28.97 g/mole. Note that, in the above formula, the volumetric efficiency of the (4-cycle!) engine is measured in percent and the engine displacement (ED) is in liters.

https://github.com/oesmith/obdgpslogger/blob/master/doc/mpg-calculation

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First you need to look at all parts and how they work. MAF- uses two different sensors to regulate how much fuel goes into the motor based on the TEMPERATURE and AMOUNT of air going to the motor. MAP- only measures pressure not the temperature. This is pulled from the IAT which measures intake air temperature. That being said how a MAP based system determines the fuel amount is a speed density program, which uses MAP IAT and O2 sensors.

All of this being said it could be done, just not real reason or easy way to do it

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You can't directly use a MAP sensor reading to calculate load

Not unless you can correlate the manifold pressure to a flow rate value.

elmerfud's answer shows an equation involving volumetric efficiency (VE), which essentially represents that correlation between flow rate and pressure. This correlation will vary between engines and the only way to obtain it is through experimental testing.

An old-school way to establish the relationship between pressure and flow rate is to install a flow meter in the duct work that provides flow rate measurements, which can then be used in conjunction with the air pressure measurements to establish the relationship.

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