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Discussion Starter · #1 ·
A quote from another thread regarding fuel used for EPA testing:

...when an tiny turbo engine is under boost it will need to run slightly richer than a much larger N/A engine would for a given power level, necessary to keep combustion temps in check. As long as the small turbo engine is kept below that range where the air/fuel ratio starts to richen up it should be more efficient than the larger engine at that power level. Start richening up the mixture under boost and then toss in a little knock retard and it's not hard to see why more agressive/faster drivers start seeing a rapid decline in economy. Eco MT drivers on 87 octane report drastic mileage reduction above 65-70 MPH and this could be why.
So I finally started data logging with HPTuners after buying the package two years ago and watching it collect dust ever since...

I went out on Saturday afternoon and did some consumption vs. speed testing. This is two way averages done on a level stretch of road in fairly calm contitions with the cruise control set. I data logged one of my 75 MPH runs with no AC, though it was ~90F outside and any sane person would probably use it at that temp.

For the record, my Eco has lowering springs, elevated tire pressures and .038" plug gaps running without plug resistors, but I doubt very much that any of those alterations has a significant affect on fuel economy beyond a stock Cruze Eco.

So at 75 MPH on level ground with no headwind and no AC at 90F (ideal conditions) on 87 octane in my '12 Eco MT, I was seeing a little bit of timing retard. Manifold pressure was hovering right around the boost/vacuum threshold, varying from around 14.7 psi absolute +/-0.5 psi (14.7 psi is 0 boost) and seeing up to a couple degrees of knock retard in the timing. The car was averaging 41.3 MPG (DIC reading of 44.2 MPG corrected at 7%) and the engine was turning ~2300 RPM. At lower speeds and lower engine loads I would expect the knock retard to go away completely.

With AC use, a bit of a headwind and undulating topography, I would fully expect the engine to have a constant load MUCH higher than this, which means more boost and more timing retard. This pretty much explains the mileage reduction phenomenon reported by Eco MT drivers who claim reduced mileage at speeds above 65-70 MPH.

If I get around to it I will try to post some data from the log file.

I did runs at 62 MPH with and without AC and the mileage dropped by 7.4%, from 53.8 MPG to 50.1 MPG, once again corrected at 7% from the DIC readings. The AC was set to fan speed 2. This demonstrates the significant additional load placed on the engine while using AC... so at that speed under these conditions, using AC for 15 gallons of fuel burned, one gallon of gas is being used just to power the AC. Real world use would be less than that, this is based on averaging 62 MPH and 50 MPG over 15 gallons of fuel used (one gallon is about 7% of 15 gallons).

I would assume that the other models of Cruze would have a slightly higher speed threshold where their mileage would start to fall off at a higher rate simply because they have shorter gearing and the %engine load for a given speed would be less, but I don't have other models of Cruze to test so that will be up to someone else. :)
 

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Good work. Something that would be really nice to see is a direct datalogging 87/89/91/93 octane comparison with AC use, MPG and knock retard observed using each. Although it would be hard to consistently duplicate similar conditions for each run. I know there was a good thread on knock retard with 89 vs 93, but no MPG analysis.
 

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Driving across Kansas last Friday I averaged 36 MPG. Cruze control set at 80, hard cross to slight headwind. Outside temps above 100 peaking at 109. Needless to say I was running A/C. Cold tire pressure was 45 and rolling pressure at 52 PSI. The car reported engine temps at 203F and 13.5 V all the way across Kansas. Once I got to Colorado without the wind and temperatures back below 90 but at the same speed my engine temp went back to 217-219 and the alternator dropped to 12.5 V. I'm going to see right around 40 MPG on the tank when I fill it later this week. The car was definitely working a lot harder in the heat and wind, so I'm not surprised my fuel economy dropped.

I did average 65 MPH from the Ohio/Indiana State line to about 20 miles west of Kansas City and the car reported 50.0 MPG which translates for me to about 46-47 MPG at the pump.

I run 91 or higher octane, if available, at all times. My plugs are at 0.035".

I discovered boosting the cold tire pressure from 40 to 45 PSI results in about a 2 MPG improvement in fuel economy at 65-70 MPH.
 

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So you're seeing knock retard on 87 even at cruising speed, eh? Makes sense with temps that hot I suppose.

I'd be interested to see what you see when temps are around 60-70 outside once summer is on its way out. Seat-of-the-pants, my car feels great on 87 in those temps, but trips all over the knock sensors once the temp starts going up outside in normal city driving.

Higher engine speed, more wind resistance, more rolling resistance, and higher engine load would ALSO account for the reduced MPG at higher speeds ;)
 

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Rolling resistance becomes less a determining factor as speed increases. Wind resistance really takes over the friction equations.
 

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Discussion Starter · #6 ·
Now that I'm actually using the logging software I'll try to get as much data as my spare time allows. I only had time to log that one run at 75 MPH, I would have liked to log again at 70 and 65 to see at which speed the knock retard went away. I would also like to see roughly how many MPH using the AC costs as well... right now based on FE alone it seems AC costs you about 6 MPH (ex. 62 MPH with AC = 68 MPH without).

Keep in mind that this is all done on level road as well. In the real world of ups and downs there's always going to be more or less load as things change.

Agreed j, at 70F the car feels really solid on 87 (with the wider plug gaps), and I'll hopefully find out exactly how strong with some hard data regarding KR soon. I've got about another six weeks of hot weather up here for testing and then September rolls in with usually cooler temps almost over night.
 

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This is interesting... here is my 2 cents.

I run a scanguage II in my Eco. One of the parameters that I monitor is HP output (how this is measured I do not know) along with boost pressure, ignition timing and instant MPG.

What I do see at idle is that 3.5hp is made without AC, it shows 4.5-4.7hp at idle with the AC on and fan speed at 1. So there is definitely a bump in idle speed and resultant HP (along with fuel consumption) with AC on. How this affects consumption at higher speeds I am unsure of- I'll leave that to the math whizzes on this site... :)

Let me know if anyone wants further data from my scanguage... its a neat toy.
 

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A quote from another thread regarding fuel used for EPA testing:

Manifold pressure was hovering right around the boost/vacuum threshold, varying from around 14.7 psi absolute +/-0.5 psi (14.7 psi is 0 boost)
14.7psi would only represent 0 boost if the ambient air pressure was also 101.325kpa (1 Atm / 1.01325 bar). You would have to be logging near mean sea level elevation (MSL) to obtain this. Otherwise, if the ambient air was for example ,94kpa (few thousand feet up), and you are reading 14.7psi, it would be ~+1.06psi positive pressure. This would mean this is NOT boost/vac threshold, but rather.. "Boost".
Same would apply if you were below MSL, and for example, you have 103kpa ambient air. Even If no turbo existed and you were running NA, you would have a value of ~14.94psi = 0 boost. 14.93 psi would be your 0 boost.
 

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Driving across Kansas last Friday I averaged 36 MPG. Cruze control set at 80, hard cross to slight headwind. Outside temps above 100 peaking at 109.
I did a very similar drive across Kansas last November with my 1LT auto, though the average temp was 15 degrees Fahrenheit, similar head/side wind, cruise set at 80mph. I only managed 27.8mpg, one of my worst fill ups.
 

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Discussion Starter · #10 ·
14.7psi would only represent 0 boost if the ambient air pressure was also 101.325kpa (1 Atm / 1.01325 bar). You would have to be logging near mean sea level elevation (MSL) to obtain this.
Roger that. Ottawa is only 230 ft above sea level. The weather can have a greater affect than our altitude.


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