Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post

sword_guy8 · 01-18-2015, 01:13 PM

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).

01-18-2015, 01:13 PM	#1 (permalink)
sword_guy8 EcoModding Lurker Join Date: Jan 2015 Location: CT Posts: 19 Thanks: 0 Thanked 0 Times in 0 Posts	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).