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Discussion Starter · #1 ·
Is it even possible to calculate? If so, how would I go about calculating it? I've already got a OBDII scanner that I built from scratch, so building a MPG calculator shouldn't be too much more difficult, right?
 

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Originally posted by Nomadic_Dragon@Jun 13 2004, 01:28 AM
Is it even possible to calculate? If so, how would I go about calculating it? I've already got a OBDII scanner that I built from scratch, so building a MPG calculator shouldn't be too much more difficult, right?
You've built an OBDII scanner eh? Interesting. Is it just a generic code reader? (I'm assuming so) I'm curious, how'd you get the connector for the OBDII port? Or did you just ghetto rig something? :lol: I guess it is possible to create a MPG indicator kinda thing if you were to have it calculate MPG based on speed, throttle position, engine rpm and engine load. But I'd think you'd have to do some testing and benchmarking to come up with a table for comparison, but that seems like alot of work. Just figure it like this, HIGHER RPM = POOR FUEL ECONOMY, LOW RPM = GOOD FUEL ECONOMY. :lol:
 

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I bet americanmototrsport.com would know something about this. It maybe should be in the EEC forum where he would be more likely to see it. He has a lot of knowledge on OBD, and is a very smart guy (despite the little debate we were having recently).

Seems to me that you'd want to look at injector dwell times, engine rpm, and vehicle speed.

You may also need to pay attention to fuel pressure, but I don't think so. I think in the older bypass-type fuel pressure regulators (the ones that had a return line), the regulator maintained a certain value above whatever the intake manifold pressure is at. That should mean that even though fuel pressure is higher or lower, it's keeps an equal pressure in relation to the intake manifold, and therefore have consistant flow. In other words xxx injector dwell results in the same amount of fuel flow regardless of what the intake manifold pressure is.

I just recently found out, that Ford switched to returnless regulators, so I am not sure if that manifold to fuel pressure relationship is still there, but I think it is.

Another thing that may cause you grief is the injector response times. It doesn't open and close instantly, so you will probably have to figure out just how much you need to correct for this.

I'm pretty sure that my parents had a vehicle that could display instant mpg on the little trip calculator thingy. I believe it was a Probe, but I don't remember for sure.
 

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My mom's 02 volvo s60 can tell you instantaneous mpg, average mpg, how many miles till an empty tank, it can download messages from volvo, and it will give you an alert message if a head light or tail light is out, etc, etc, all on this little display screen on the dash. It's pretty cool, you scroll through it with something that looks like windshield washer controllers.

I have no idea how you'd make one for the taurus though.
 

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Originally posted by mikehawk@Jun 13 2004, 10:34 AM
My mom's 02 volvo s60 can tell you instantaeous mpg, average mpg, how many miles till an empty tank, it can download messages from volvo, and it will give you an alert message if a head light or tail light is out, etc, etc, all on this little display screen on the dash. It's pretty cool, you scroll through it with something that looks like windshield washer controllers.

I have no idea how you'd make one for the taurus though.
There you go. Sell Taurus. Buy Volvo.
 

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Dad's Rental S60 does the same thing as Mikehawk. Heck if you can find a wreck s60 maybe you can rip it outa the Volvo and fabricate it into the Taurus somewhere.
 

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Originally posted by eCarâ„¢+Jun 13 2004, 09:38 AM--><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (eCarâ„¢ @ Jun 13 2004, 09:38 AM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-mikehawk@Jun 13 2004, 10:34 AM
My mom's 02 volvo s60 can tell you instantaeous mpg, average mpg, how many miles till an empty tank, it can download messages from volvo, and it will give you an alert message if a head light or tail light is out, etc, etc, all on this little display screen on the dash.  It's pretty cool, you scroll through it with something that looks like windshield washer controllers.

I have no idea how you'd make one for the taurus though.
There you go. Sell Taurus. Buy Volvo. [/b][/quote]
Trip computers have been around for a long while. Used to be that you could buy an aftermarket one and install it on any car, haven't had much luck finding one on Google though.

I had one of these in an '85 Chrylser Lebaron which was a 5 speed with the 2.2L turbo. It was kinda fun, when you had it in neutral and coasting, you'd see 99mpg and when you nailed it off a light, it would drop down to about 3-4mpg.

Oh the '04 Taurus does have it as an option, don't think it's that much either. Sell old Taurus, buy new Taurus...
 

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Discussion Starter · #10 ·
I might do that. Sell my '03 Vulcan for a '04 Duratec. :) But for at least a few more years, its the '03, so I want to get some fun stuff on it.

Here is a question...I know the air/fuel mixture should be 14.7 lbs air per 1 lb fuel. My scanner can read the mass air flow in lbs/min, so I should be able to figure out lbs of gas per minute, figure in vehicle velocity at that moment, to get instantanious MPG. Would this work, or are there other _significant_ variables?
 

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Originally posted by HenrySel@Jun 13 2004, 08:59 PM


Oh the '04 Taurus does have it as an option, don't think it's that much either. Sell old Taurus, buy new Taurus...
The trip computer is STANDARD on the new Taurus ;)
 

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Originally posted by Nomadic_Dragon@Jun 13 2004, 10:34 PM
I might do that. Sell my '03 Vulcan for a '04 Duratec. :) But for at least a few more years, its the '03, so I want to get some fun stuff on it.

Here is a question...I know the air/fuel mixture should be 14.7 lbs air per 1 lb fuel. My scanner can read the mass air flow in lbs/min, so I should be able to figure out lbs of gas per minute, figure in vehicle velocity at that moment, to get instantanious MPG. Would this work, or are there other _significant_ variables?
You'd be off anytime the a/f ratio goes rich or lean, which is a lot. WOT, decel, even pressing the accelerator down a bit causes the a/f ratio to go rich for a split second. It's simulating what an accelerator pump used to do for a carbeurator.
 

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Discussion Starter · #13 ·
Might need a good programmer's help, but the way I figure it, we'd just query the ECU for vehicle speed, mass air flow, and fuel trim, every second.

So if we get 60mph, a MAF of .17102, a 14.7 a/f ratio, and .000208 lbs of fuel per second, it should be about 13 mpg?

(MPH/seconds in hour)/(gas mass per second * amount of gas mass per gallon)
(60/3600)/(*0.000208*6.15)


Keep a tab on how much gas has been used, and you'll know how much until empty as well
 

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Refer to the bunny below.

I'm glad you programmers are around, because that's not my thing.
 

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Originally posted by Nomadic_Dragon@Jun 14 2004, 03:29 PM
Might need a good programmer's help, but the way I figure it, we'd just query the ECU for vehicle speed, mass air flow, and fuel trim, every second.
I'm not very familiar with "fuel trim". I thought it was kind of a learned correction factor to compensate for the fact that no two cars are going to be exactly the same (even if they are built with the "exact" same parts).

If you only use 14.7:1 as a constant, you will only be correct when you are running stoich. At WOT, you may be as rich as 12:1. During decceleration, >20:1 is normal.

I don't think that OBD will report which a/f ratio it is shooting for.

I would just read the injector dwell times. If it's a 19lb/hr injector, and is open 50% of the time, you are using ~9.5lb/hr.

Actually it may be higher/lower than 9.5 because the injector can't open and close immediately. But, if the open and close times are the same, maybe they cancel each other out.
 

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Discussion Starter · #16 ·
Shouldn't have to do all the tricky calculations, because the computer is doing it all anyway. It already takes intake air temp and air flow to come up with Mass Air Flow. Fuel trim is how many % the a/f is off of normal, it'd change at WOT, idle, etc, so the computer already has the a/f ratio known.
 

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The Ford NGS scanner has this feature through the OBDII port.
 
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