Instant MPG

B

Be2lose

In title only.
I pulled this from another thread that had got of topic, that's strange!


J said:
Did you guys ever notice how you're MPG on the message center doesn't change in different gears? As long as the throttle position stays the same, the MPG stays the same no matter what gear you're in. Strange, haha.

Go test it...turn on your instant MPG and drive in o/d, hold the throttle steady and turn off o/d, your instant won't change. The load isn't really different, per say, but you'd think that with the higher RPMs(expecially with your guy's IMRC theory) that your instant MPG would drop. More air more fuel, but I guess the computer doesn't read the MAF perameters. Huh...
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I was following a car down a hill and instead of riding the brakes i tried turning off the overdrive. With the overdrive off my instant mpg was reading around 65. I was still going too fast so I down shifted into second- RPM's way up and now the instant mpg has pegged at 99. The car in front of me pulls ahead so as I neared the bottom of the hill I put it back into overdrive. Car speeds up RPM's go down and the instant drops back to the 60's.

Very strange
 
Haha that is wierd! 99MPG+ is super good, but if you think about it...you were speeding up(going down hill) with your throttle at 0%, so NO throttle, NO load and going 65-70 mph your gas mileage is going to be super high.

With my other post, I was stating that no matter what the RPM's are, as long as the throttle position stays the same, the speed stays the same, the instant MPG stays the same. You can rev up to 4500 rpms in first, hold that speed/throttle position and than shift to second, as long as you don't change your pedal position, the mpg is the same. What I was pointing out is that you'd think with a higher RPM your gas mileage would go down, but it doesn't.

Plus I have a Gen1, you're instant MPG might use different readings/parameters.
 
J said:
Haha that is wierd! 99MPG+ is super good, but if you think about it...you were speeding up(going down hill) with your throttle at 0%, so NO throttle, NO load and going 65-70 mph your gas mileage is going to be super high.
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Yes, but both times in second with high RPM's and in overdrive with lower RPM's(traveling faster) I did not have my foot on the throttle. Second gear read higher MPG than overdrive.

After thought:

Could the computer know that I was using the engine as a brake and lower the fuel into the cylanders?
 
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Be2lose said:
Yes, but both times in second with high RPM's and in overdrive with lower RPM's(traveling faster) I did not have my foot on the throttle. Second gear read higher MPG than overdrive.
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Yeah, thats right.....so you got better MPG in second with the high RPM's than you did in O/D at higher speeds, both with 0% throttle, haha. Lets all drive in second from now on when going down hill! :D

Be2lose said:
After thought:

Could the computer know that I was using the engine as a brake and lower the fuel into the cylanders?
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The computer takes its readings from the TPS(throttle position sensor) and in both instances, you were NOT giving the car any gas so the amount of fuel given to the engine should have been the same, which would have been the same as idle or very close.
 
Be2lose said:
Could the computer know that I was using the engine as a brake and lower the fuel into the cylanders?
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Yep. In fact, if the engine is turning above a certain RPM (varies depending on the engine, but usually ~1800 rpm) and it figures out that you're engine braking based on throttle & speed, it'll completely shut off all the injectors. I used to have an '88 T-Bird; its instant fuel economy readout would quickly climb to ~255 and then show "----" when coasting. Ours just stop at 99, which isn't as cool looking. :)

There's a very long downhill stretch in my neck of the woods. I coast down it in second, and most of the time it'll be at 99 mpg. The computer kicks the injectors back on every so often (to keep the cats & O2s hot, I assume), at which point it drops to ~65mpg and the engine braking is reduced quite a bit.

The EEC-IV/EEC-V systems do a lot of cool stuff like that. On stickshift models they can even tell if you're trying to push-start the car, and they'll adjust the timing & fuel to make it easier.
 
segxr7 said:
The computer kicks the injectors back on every so often (to keep the cats & O2s hot, I assume), at which point it drops to ~65mpg and the engine braking is reduced quite a bit.
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Is this what that wierd rpm surge/drop/change is when going down hill for a long period of time? Its extremely noticeable with my 3200 stall t/c and even more noticeable with loud exhaust. It almost feels like its going in/out of lock-up but its not because you can tap the brake and you feel it come out of lock-up but its still doing the surging. The rpms go up/down quite a bit, and it doesn't matter if its o/d and you take it out, or if its in 3rd already. Super strange.....could this be because of the engine braking?
 
J said:
Is this what that wierd rpm surge/drop/change is when going down hill for a long period of time? Its extremely noticeable with my 3200 stall t/c and even more noticeable with loud exhaust. It almost feels like its going in/out of lock-up but its not because you can tap the brake and you feel it come out of lock-up but its still doing the surging. The rpms go up/down quite a bit, and it doesn't matter if its o/d and you take it out, or if its in 3rd already. Super strange.....could this be because of the engine braking?
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Yep, same thing here in the hills of WV with 4.30s, 3800 stall and 'loud exhaust'. You can feel it, hear it and see it on the tach.
 
I don't follow as to why this is confusing. Think about it in more simple terms:
1) Same throttle position = same fuel flow, ie: gallons per hour
2) Same speed @ same throttle position = same fuel flow again
3) Time to travel 1 mile is identical

Therefore the mileage must be the same since the fuel flow and distance traveled are identical.

I think :confused:
 
Well Leo...the confusing part is that you'd think the parameters/fuel use would change driving the same speed in second at 6000 rpms vs. in O/D at 1500 rpms. Its just "wierd" no matter how you look at it.

In you're calculations you don't include RPMs, and in all of our situations, thats what we point out as the strange part.

driller said:
Yep, same thing here in the hills of WV with 4.30s, 3800 stall and 'loud exhaust'. You can feel it, hear it and see it on the tach.
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So......are you agreeing that this is the computer adjusting for "engine braking" or are you just saying your car does the same thing?
 
J said:
So......are you agreeing that this is the computer adjusting for "engine braking" or are you just saying your car does the same thing?
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Both. At first I thought it was the converter, but then I could unlock the converter and still notice it.
 
J said:
So......are you agreeing that this is the computer adjusting for "engine braking" or are you just saying your car does the same thing?
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Both. At first I thought it was the converter, but then I could unlock the converter and still notice it.
 
Ok....and same here, I thought it was the stall converter because I took a long road trip(the cars first in my care) and it was right after the install.

So....nothing to worry about then?
 
Sweet :D I remember Serge asked about this a while ago, but I didn't know it was caused buy the computer using "engine braking". Super cool :D nice we're driving big rigs with Jake Brakes, lol.
 
LeoC2 said:
I don't follow as to why this is confusing. Think about it in more simple terms:
1) Same throttle position = same fuel flow, ie: gallons per hour
2) Same speed @ same throttle position = same fuel flow again
3) Time to travel 1 mile is identical

Therefore the mileage must be the same since the fuel flow and distance traveled are identical.

I think :confused:
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Since the IMRCs open up at 3000, airflow resistance goes down, actually increasing airflow, and thereby increasing fuel usage at the same throttle level.

It's a great theory, but was obviously proven wrong; the throttle body is the point of greatest restriction thereby dictating air/fuel consumption.
 
Boy I could use some help on this subject. Since I installed all SC etc I no longer have the correct MPG and miles to empty feature working correctly and so far I haven't found a fix. Any thoughts here.
 
Roadboss said:
Boy I could use some help on this subject. Since I installed all SC etc I no longer have the correct MPG and miles to empty feature working correctly and so far I haven't found a fix. Any thoughts here.
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Exactly what is it doing or not doing? Does the fuel gauge work properly?
 
logres said:
Since the IMRCs open up at 3000, airflow resistance goes down, actually increasing airflow, and thereby increasing fuel usage at the same throttle level.

It's a great theory, but was obviously proven wrong; the throttle body is the point of greatest restriction thereby dictating air/fuel consumption.
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Which is true.....unless said "engine braking" "engages". :D The TB is the greatest point of restriction, but it doesn't dictate the air/fuel consumption, the PCM does that, and if you were at 0% throttle, the TB wouldn't even be open, the only air coming through would be through the IAC vavle, so the TB isn't even in the equation. The TB doesn't open UNLESS you do it manually. :)

Speaking of engine braking, I drove my stepdads 98 Chevy 1500 with a 5.7 and after being off the throttle for a couple of seconds, the engine braking would kick in when going down hill....VERY noticeable, to where it would feel like a brake is being applied. Its cool, I could actually hear the "tone" of the exhaust change when the ECU cut fuel....I never even paid attention before because I didn't think the ECU was capable of accomplishing this, hence the reason I thought my stall converter was causing my RPM surge.
 
Fuel gage works fine. My MPG figures now are way incorrect, reads way higher than actual. On my last trip on the highway I got a true 20.4 MPG while my computor said my average was 33 something. It did read correctly prior to the transformation.
 
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