Calculating MPG or how to find MAF
Thread Starter
JK Junkie
Joined: Aug 2007
Posts: 2,911
Likes: 0
From: Broken Arrow, OK
Calculating MPG or how to find MAF
Guys,
I recently bought an OBD-II to bluetooth adapter, and I have been very intrigued by the data available. I guess I am becoming interested in "hacking" my Jeep
So, one of the first things I tried to do was see if my Jeep would tell me it's MPG. You see, my Jeep lacks the dash display.... I know, I know, calculating by hand is more accurate (which I do), but this is a matter of curiosity, practicality left a long time ago.
The software I am using for my Mac claims to be able to tell me the MPG if my vehicle has a mass airflow sensor (MAF). An earlier thread of mine this morning (and some googling) confirms that the JK does NOT have an MAF sensor. It would appear that many (all?) Chrysler vehicle lack this sensor, even if they have an on-dash MPG reading.
The normal way (or from what I read) of calculating MPG from OBD data is this equation:
MPG = 710.7 * VSS / MAF
Where 710.7 is some crazy constant which I haven't scrutinized yet, and VSS is the vehicle's speed in kilometers per hour.
I have seen talks (elsewhere on the Internet) of estimating MAF from other sensor data, namely the IAT, RPM, and MAP... but this feels like I am barking up the wrong tree.
Is this the correct path? Does anybody have experience with these types of equations? Is there, perhaps, a different equation that doesn't require an MAF value?
Any pointers would be greatly appreciated, my curiosity won't rest until I figure this out
I recently bought an OBD-II to bluetooth adapter, and I have been very intrigued by the data available. I guess I am becoming interested in "hacking" my Jeep
So, one of the first things I tried to do was see if my Jeep would tell me it's MPG. You see, my Jeep lacks the dash display.... I know, I know, calculating by hand is more accurate (which I do), but this is a matter of curiosity, practicality left a long time ago.
The software I am using for my Mac claims to be able to tell me the MPG if my vehicle has a mass airflow sensor (MAF). An earlier thread of mine this morning (and some googling) confirms that the JK does NOT have an MAF sensor. It would appear that many (all?) Chrysler vehicle lack this sensor, even if they have an on-dash MPG reading.
The normal way (or from what I read) of calculating MPG from OBD data is this equation:
MPG = 710.7 * VSS / MAF
Where 710.7 is some crazy constant which I haven't scrutinized yet, and VSS is the vehicle's speed in kilometers per hour.
I have seen talks (elsewhere on the Internet) of estimating MAF from other sensor data, namely the IAT, RPM, and MAP... but this feels like I am barking up the wrong tree.
Is this the correct path? Does anybody have experience with these types of equations? Is there, perhaps, a different equation that doesn't require an MAF value?
Any pointers would be greatly appreciated, my curiosity won't rest until I figure this out
Thread Starter
JK Junkie
Joined: Aug 2007
Posts: 2,911
Likes: 0
From: Broken Arrow, OK
Here is the OBD data that I think is relevant to this equation, I'm still sorting through all of this. These values come from a recent drive down the road.
Engine RPM: 2289
Vehicle Speed: 45 MPH
Intake air temperate: 50.0 F
Intake manifold pressure (absolute): 57.0 kPa
So, going back to the equation above, and substituting in vehicle speed:
MPG = 710.7 * VSS / MAF
MPG = 710.7 * 72.42048 Km/H / MAF
MPG = 51,469.2351 / MAF
Still no closer, but I figured I'd fill in some of the gaps for anyone reading...
Engine RPM: 2289
Vehicle Speed: 45 MPH
Intake air temperate: 50.0 F
Intake manifold pressure (absolute): 57.0 kPa
So, going back to the equation above, and substituting in vehicle speed:
MPG = 710.7 * VSS / MAF
MPG = 710.7 * 72.42048 Km/H / MAF
MPG = 51,469.2351 / MAF
Still no closer, but I figured I'd fill in some of the gaps for anyone reading...
JK Enthusiast
Joined: Jan 2011
Posts: 307
Likes: 0
From: Ticonderoga, NY
I know my previous Jeep, an '03 Liberty, used the "speed density" method of calculating air/fuel mixture. The JK is probably similar.
I'm not sure if that helps you cause or not. I'm not too familiar with the JK's set up since I've only owned one for two months.
Speed Density
Speed Density systems accept input from sensors that measure engine speed (in rpm) and load (manifold vacuum in kPa), then the computer calculates airflow requirements by referring to a much larger (in comparison to an N Alpha system) preprogrammed lookup table, a map of thousands of values that equates to the engine’s volumetric efficiency (VE) under varying conditions of throttle position and engine speed. Engine rpm is provided via a tach signal, while vacuum is transmitted via an intake manifold-mounted Manifold Air Pressure (MAP) sensor. Since air density changes with air temperature, an intake manifold-mounted sensor is also used.
Production-based Speed Density computers also utilize an oxygen (O2) sensor mounted in the exhaust tract. The computer looks at the air/fuel ratio from the O2 sensor and corrects the fuel delivery for any errors. This helps compensate for wear and tear and production variables. Other sensors on a typical Speed Density system usually include an idle-air control motor to help regulate idle speed, a throttle-position sensor that transmits the percentage of throttle opening, a coolant-temperature sensor, and a knock sensor as a final fail-safe that hears detonation so the computer can retard timing as needed.
CarCraft
Speed Density systems accept input from sensors that measure engine speed (in rpm) and load (manifold vacuum in kPa), then the computer calculates airflow requirements by referring to a much larger (in comparison to an N Alpha system) preprogrammed lookup table, a map of thousands of values that equates to the engine’s volumetric efficiency (VE) under varying conditions of throttle position and engine speed. Engine rpm is provided via a tach signal, while vacuum is transmitted via an intake manifold-mounted Manifold Air Pressure (MAP) sensor. Since air density changes with air temperature, an intake manifold-mounted sensor is also used.
Production-based Speed Density computers also utilize an oxygen (O2) sensor mounted in the exhaust tract. The computer looks at the air/fuel ratio from the O2 sensor and corrects the fuel delivery for any errors. This helps compensate for wear and tear and production variables. Other sensors on a typical Speed Density system usually include an idle-air control motor to help regulate idle speed, a throttle-position sensor that transmits the percentage of throttle opening, a coolant-temperature sensor, and a knock sensor as a final fail-safe that hears detonation so the computer can retard timing as needed.
CarCraft
Thread Starter
JK Junkie
Joined: Aug 2007
Posts: 2,911
Likes: 0
From: Broken Arrow, OK
Of all the cool info that you can read from OBD2, why would you go with something so simple? We used to have scanners in high school and we would check the different ambient air temps, o2 sensor voltages, and all the other ridiculous parameters in there. Then if my o2 sensor ever went I could hardwire in a signal to supply voltage and the computer would never know the difference 
If this is so simple for you, I openly invite you to actually answer my question. Otherwise I'll just assume you have no clue what you're talking about and are just trolling.