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I haven't see this posted, but seeing that GM actually confirms using 91 octane is better I thought I would link it.

2011 Chevrolet Cruze: Hot Weather MPG Part 2


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The original link is hard to follow. I followed it and found this after a few minutes. -Mike.

Link to Edmonds report. Note the fourth paragraph:
Instead, the hot weather led to a very noticeable lack of drive-away power and sub-par highway fuel economy. Subsequent discussions with GM powertrain bigwigs and our own Jay Kavanagh revealed that small turbo engines are especially octane sensitive, which means their computers may agressively dial back the engine calibration in order to ward off knock in high load situations or in very hot weather.
 

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Yeah, could have guessed that... I get noticably better mileage when it's 70F with the A/C on than I do when it's 95F with the AC on...

Mike
 

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...FYI, *all* EPA 50-state-certification tests are performed using 91 octane fuel, regardless of what the Owners Manual says.

...93 octane is used for non-CARB and non-50-state EPA tests.
 

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This really applies to all vehicles, not just our Cruze. Although, it impacts on a lesser of scale on N/A engines.

Think about it. The hotter the air, the less dense the air, the easier it is for the A/F mixture to detonate and the less power made. Add a Turbo/SC, and it compounds that effect.
 

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This really applies to all vehicles, not just our Cruze. Although, it impacts on a lesser of scale on N/A engines.

Think about it. The hotter the air, the less dense the air, the easier it is for the A/F mixture to detonate and the less power made. Add a Turbo/SC, and it compounds that effect.
Sort of...

If an engine has a low compression ratio and a set high-point for ignition advance that's in the 87 octane range, and almost never retards timing, then there's really nothing to gain from higher octane... The Trailblazer, for instance, gains absolutely nothing from anything over 87... It's because the compression ratio isn't really high, and the maximum advance on the timing is well within the 87 octane range...

The engine controls the fuel to go with the temperature/density of the air... It puts in less fuel when it needs less fuel...

Mike
 

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Interesting. Mine is going in tomorrow for maintenance and to see if they can figure out why it has absolutely no power at times when hot. It has been 100+ here for 2 weeks and the car will barely get out of its own way. If the dealer can't find anything, I may run a few tanks of 91-92 to see how it works. I drive 3000+ miles a month and keep track of everything. I'll be able to tell pretty quickly if it works.
 

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my gas mileage has dropped ever since the weather went to the 90's and humid. just installed trifecta so we will see plus i have to use premium fuel anyway. My car was hesitating badly when hot and humid.
 

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FINALLY!!!!!!!!!!!!!

We were seeing timing being pulled even in the early spring season with 87. 89 is pure minimum on the stock tune, 91 is even more ideal.
 

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Hot weather makes an incredible difference with this engine - far greater than my naturally aspirated truck. The A/C will also really hurt your mileage if you're not in recirculate mode.

I live in Houston and bought my car when the weather was still in the 50s and 60s. It's a major difference compared to now where it's 100. In fact it's a big difference between the 80's in morning and upper 90's in the afternoon. The biggest effect I've notice when the car's hot is that it feels like it has zero power until about 2000 rpm. In cool weather you can feel the power ramp up from much lower engine speeds.

At any rate though, this weekend I went to the beach, drove through 98 degree weather at 70-75 mph, and still got 39.5 mpg at the pump and 40.9 on the DIC.
 

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Ive been filling up at 91 since i got the car. Cleaner, better gas I say to myself lol.
 
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