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Old 01-18-2015, 02:13 PM   #1 (permalink)
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MAP direct correlation to FE?

Hello! This is my first post. I have an '89 Chevy Corsica with a 3.1ltr and the lowest geared 4 speed I could find (that was a project!). My original thinking was that simply having the engine at a lower RPM would boost my FE. And it is true, at highway speeds on level ground or downhill, my FE is definitely better (having the engine running at 2100 RPM instead of 3800 is a big difference!).

HOWEVER...I have discovered something while monitoring ECM outputs. I am using ALDLdroid, which is an Android based app which will read the output of the ECM by using the appropriate adx file. Also, within the adx, one can create different values, so I made one for instantaneous MPG. The refresh rate is about 8hz, so it's definitely instantaneous! Also, I e-mailed the developer and he said he would be able to make it so one could average or total any value (so I'll be able to do average MPG for any given ride).

Moving on. What I discovered was NOT peak FE based on engine RPM. In fact, engine RPM has nothing to do with it. The only thing I've found which has a direct correlation to FE is manifold pressure. My peak FE pressure seems to be about 20 inHg. To test this, I went on the highway and set cruise for 70. When going up a hill, the throttle position would increase steadily until the MAP was at 24. I took note of my MPG when it stabilized, then I downshifted into 3rd. The MAP dropped to 20, the MPG dropped by a few, but then, when everything stabilized, it was actually higher by up to 2. On the same hill (it was a long hill), I upshifted back to 4th and watched my MPG jump up slightly only to decrease to below what it was in 3rd gear. As the throttle position increased, the MPG value would decrease as the MAP increased past 20.

The next test was on a level surface. MAP was about 14. I downshifted to 3rd and the map jumped down, as did the FE. This second test shows me that RPM has little to do with FE, since the RPM changes in both tests were the same.

To make sure that this was not only related to the specific speed I was driving, I also checked this at 50 and 40 MPH (50 still starts in 4th gear, but 40 starts in 3rd). I had the same results. MAP of 20 inHg yielded the highest MPG reading.

In conclusion, it is clear to me that it is engine load, irrespective of RPM, which is the key to maximum fuel economy. It also seems directly related to throttle position, but there is no specific throttle position which will yield the highest FE (generally, less is more).

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Old 01-19-2015, 05:01 PM   #2 (permalink)
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82 views and nobody's thrown in their $0.02? I'm surprised. I would like to hear some thoughts on this.
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Old 01-19-2015, 05:29 PM   #3 (permalink)
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what are you measuring mpg for comparative purposes with?

fyi, some obd formulas, MAF (not just MAP), and speed/vss are the big ones
http://www.mp3car.com/engine-managem...from-obd2.html
but they assume a "fixed" fuel ratio. IIRC ALDL might have duty cycle, try tracking that against the pump too as it might be more accurate.

Last edited by P-hack; 01-19-2015 at 05:36 PM..
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Old 01-19-2015, 05:40 PM   #4 (permalink)
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Quote:
Originally Posted by sword_guy8 View Post
82 views and nobody's thrown in their $0.02? I'm surprised. I would like to hear some thoughts on this.
I did not respond because I did not see any contribution by myself as being constructive, since I just can not grasp the thought of efficiency being not related to engine RPM. It's my belief that MAP AND RPM determine efficiency or the conclusion that efficiency is NOT a function of RPM would be correct, when every speck of evidence I have ever read contradicts that conclusion.

The only other factor outside of RPM and MAP is vehicle speed.

The combination of those 3 will give you accurate MPG readings.

regards
mech
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Old 01-19-2015, 05:47 PM   #5 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
The only other factor outside of RPM and MAP is vehicle speed.
I remember seeing some obd formulas that took fuel trim into account to0, don't know if that is available on aldl (not going to re-educate myself on a 20 year old system). Most accurate is mpguino style, otherwise you have to guess at MAF and hope for stoic (and do a lot of polling).

Last edited by P-hack; 01-19-2015 at 05:52 PM..
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Old 01-19-2015, 06:04 PM   #6 (permalink)
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I'm glad you asked! Yes, RPM does correlate to MPG, BUT there is a caveat. If you load the engine too much, a lot more fuel goes in. More than if you just unloaded the engine a little bit by downshifting. So, if you constantly run the engine at lower RPM, you will see gains in fuel economy. I'm saying you'd see greater gains running the engine at a set MAP. Here is the formula for my MPG readings:

(mph)/((bpw*(1/3600000)*(16.71)*(6)*rpm*(60))/6.073)

Where BPW is the pulse width of the injector at any given instant, "1/3600000" converts the milliseconds pulse width to hours, 16.71 is injector rated flow rate in lbs/hr, 6 is the number of injector pulses per revolution (V6 engine), 60 converts RPM to RPH, and 6.073 is how many pounds of fuel per gallon. Everything from BPW to 60 gets fuel used per hour in pounds. Then it's divided by the number of pounds in a gallon. And all of that is divided from MPH. In the end, you have MPH/GPH. H cancels, and you get M/G.

Before anyone jumps on the fact that pounds per gallon changes with temperature, bear in mind that even if these readings were, in an absolute sense, incorrect, they would be correct relative to each other in a short period of time (such as driving up a hill). Also, based on my quick calculation at the fuel pump and the numbers I'm getting from the computer, my FE readings are correct, or at least incredibly close.
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Old 01-19-2015, 06:06 PM   #7 (permalink)
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I should point out that running the engine at lower RPM increases MAP to a better level in most cases (level, downhill, slight uphill) which is why you would see gains running at a generally lower RPM.
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Old 01-19-2015, 06:23 PM   #8 (permalink)
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not sure this is an instrument question. Worth noting that a car that old might just be a speed density system, only computes fuel based on rpm and throttle basically, which is somewhat analogous to map. Not sure how useful the map only approach is in general, "real" stoic needs to know the mass of the airflow.
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Old 01-20-2015, 12:01 AM   #9 (permalink)
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I'm not sure if this is an instrument question either, but I wasn't sure where to post it. If the moderators feel that it should go elsewhere, please feel free to move it.

I'm not sure what you mean by speed density system. Are you saying that the computer doesn't use the MAP reading to compute fuel? I'm pretty sure it does.

I'm not really concerned with MAP for my calculation though. It isn't incorporated. The calculation uses BPW, which is the length of time the fuel injector is actually open. Knowing the flow rate of the injector, I don't think you can actually get more accurate. The equation uses the actual amount of fuel that is flowing into the engine. It does not rely on MAP, MAF, or what the mixture ratio is. Also, I only said I see a direct correlation between MAP and MPG, not that the MAP was being used to determine MPG. I have not seen a direct correlation with any other reading.

Also, shouldn't the MAP be affected by the mass of the air since air with a lower mass has a slightly lower pressure? Or am I off base? Please correct me if I'm misunderstanding your post.
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Old 01-20-2015, 12:43 AM   #10 (permalink)
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there were a few progressions in fuel control, here's the main 3, I don't know who did what when.

Electronic Fuel Injection Mass Flow vs. Speed Density- Car Craft Magazine

But even mass airflow can be improved upon. But if your car doesn't use mass, then your reverse engineering of map vs mpg may be 1-1.

Quote:
Originally Posted by sword_guy8 View Post
The calculation uses BPW, which is the length of time the fuel injector is actually open. Knowing the flow rate of the injector, I don't think you can actually get more accurate.
Couple ways
increase polling rate OR monitor injector/vss events at very small time resolution.

factor in rpm (injector pulse frequency specifically) to sort-of account for the difference in injector opening time vs closing time. (done on every pulse in the event model)

consider if your fuel regulator is referenced to ambient or manifold pressure.

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