Premium Fuel?

[06mistreSS]

I've seen others with the ECO version running premium and getting significantly improved fuel economy. That said, I was curious about trying either midgrade or premium to see if it improved fuel economy in the 1.4L Ecotec Turbo. The manual states to "use regular unleaded gasoline with a posted octane rating of 87 or higher". It does mention running higher if there is knocking, which is not my case; I'm interested purely for any gains in fuel economy. It does not mention anything about not running a higher octane, so is there any reason I shouldn't?

Anybody have experience with running a higher octane in the 1.4L and, if so, what were the improvements, if any?

*I run premium (93 octane) in my Monte Carlo but it's recommended for the LS4 V8; stated in the manual.​

 

[70AARCUDA]

Do you have a ScanGauge-II™ or something similar (OBD-II bus reader)? If you do, then you can "test" your hypothesis yourself by checking ignition advance degree readings on each octane while driving over the same roadway.

If you find the ECM consistently applys more advance on the 93 octane fuel than you saw on the 87 octane, that's a good indication you're getting more POWER. Fuel economy should improve too, but octane is not the same as energy content (BTU's), which varies little, except for the summer vs. winter blend differences.

Unfortunately, these "tests" will be impossible to do back-to-back (ie: A-B-A) because you have to empty the tank each time. However, you can shorten the turn-around cycle by running the tank down almost to empty (let the lamp come on) on 87 octane--do your test--then fill-up with 93 octane and repeat the test...but that'll only be half of the desired scientific A-B-A test methodology. Also, all the old fuel already in the fuel system plumbing must be used up before the new fuel effects will become detectable.

A mixture of 2 gallons of 87 octane combined with 13.6 gallons of 93 octane is a mixture ratio of 87%...but should be 'close enuf.'

 

[06mistreSS]

Do you have a ScanGauge-II™ or something similar?

I do not have one that I'm aware of.

I'm thinking, sometime in the next three months, of trying at least three tanks back-to-back of premium (93 octane in Michigan). Running at least three should account for the remaining regular that is left in the tank/fuel system on the first tank of premium, like you mentioned, as well as give me a good average. The only way I would stick with running the premium is if I find that it's cost effective; i.e. the mileage increase, if any, makes up for the additional ~$2.20 spent on premium over regular. If not I can always try midgrade.

I'll keep everyone posted.

 

[06mistreSS]

Another thing to add.

Switching from running regular to premium in the Monte Carlo didn't show any noticeable gains in fuel economy, although I'm running a high performance air intake and exhaust system in that car so the engine is pulling in and exhausting more air. More air means the system wants to throw more gas into the engine to keep the air/fuel ratio the same. Although no gains in economy, the premium did slightly increase performance.

 

[70AARCUDA]

...speaking of "...more air...", remember, the turbo actually 'dumps' any excess air that the ECM decides isn't necessary...so without tweeking the ECM's programming (ie: a "tune" job) the 1.4LT could--likewise--be considered "maxed out."

 

[06mistreSS]

I'm still hovering right around 27.5 mpg on regular 87 octane right now and that's at about 65-70% highway driving.

 

[BIRDMAN]

I find that the difference in MPG when using higher octane becomes more apparent with more displacement. The larger American V8's I find will gain 10-15 HP with higher octane fuel. That extra power through the RPM range technically could allow the engine to operate more efficiently if you don't need to get in to the pedal as much to achieve your desired speed.

However, a .5mpg gain when you're getting 27mpg is negligible vs. a large V8 getting 12-15mpg it's pretty significant and (maybe?) worth the extra coin.

 

[06mistreSS]

I do agree, the higher the mileage the less a .5mpg savings is going to show at each fill up. Although, I am still a bit curious with the Cruze.

I could feel it a tiny improvement in performance going to premium in the Monte Carlo. The engine (5.3L LS4 V8) just runs a bit better overall. I'm usually in the pedal half the time with that car, so that and the fact that I'm moving more air through the engine with my current setup doesn't show any real mileage gains. I average about 17-18mpg with that car and that's mostly (70%) city driving.

 

[70AARCUDA]

The REAL difference between the 5.3L LS4 V8 and the 1.4LT is their respective 'sensitivities' to knock (the CAUSE) and its attendent amount of 'knock retard' (the EFFECT).

The V8 is a constant compression engine but the turbocharger makes the 1.4 a "variable compression" engine. Hence, the V8 ECM typically (depending upon fuel quality, ie: octane value) isn't constantly 'fighting' between optimum spark-advance and too much advance, which is exactly what happens when the turbo spools up on the 1.4LT and starts pumping air into the engine--effectively pushing the static CR of 9.5:1 up to as much as 22:1. And, that increased CR needs a higher octane level or the engine begins knocking (mild) or dentonating (extreme) instead of smoothly "burning" its A/F-mixture.

This is why using higher octane fuel in a turbocharged engine almost always produces both more POWER and better fuel ECONOMY, because the engine ECM can advance the timing to extract MORE power from the fuel under load without damaging the engine. Conversely, using a lower octane fuel won't *hurt* the engine because the engine ECM will constantly "pull" timing to keep knocking from occuring, but in doing so, both the power (significantly) and fuel economy (slightly) are reduced.

 

[06mistreSS]

CUDA, that's a great explanation.

I do have an update. I DID refuel with premium recently and went through my first tank on it. That first fill-up with premium was 11.25 gallons, so about 3/4 of the tank after refueling was premium.

Doing some comparing, since I purchased the car, on seven tanks of regular I have run an average of 27.7mpg. My first tank on premium I got 29.8mpg, so about 2mpg better than my overall average on regular and 0.8mpg better than my previous best, which was also on regular. I'll run premium for a while so I can get a running average on it and go from there. So far it looks like it may pay off, but I'm not going to get ahead of myself. I do notice a slight performance improvement which is nice.

Just to note, my previous best was 29.0mpg (on regular) and that was on the very first tank I went through from the dealership. That drive home from the dealership was mostly higher speed main roads and highway, plus I was driving slow(er) because it was through a blizzard.​

 

[70AARCUDA]

CUDA, that's a great explanation.

I do have an update. I DID refuel with premium recently and went through my first tank on it. That first fill-up with premium was 11.25 gallons, so about 3/4 of the tank after refueling was premium.

Doing some comparing, since I purchased the car, on seven tanks of regular I have run an average of 27.7mpg. My first tank on premium I got 29.8mpg, so about 2mpg better than my overall average on regular and 0.8mpg better than my previous best, which was also on regular. I'll run premium for a while so I can get a running average on it and go from there. So far it looks like it may pay off, but I'm not going to get ahead of myself. I do notice a slight performance improvement which is nice.

Just to note, my previous best was 29.0mpg (on regular) and that was on the very first tank I went through from the dealership. That drive home from the dealership was mostly higher speed main roads and highway, plus I was driving slow(er) because it was through a blizzard.​

Depending upon driving conditions, MANY turbo-Cruze owners find that using higher octane fuel (91 or 93) actually is less costly, but it ALL hinges on their driving conditions...and local price differences.

Also, something else to be aware of: MANY of the "high" octane fuels (93) do NOT contain alcohol, so they aren't a 'diluted' fuel, which means higher energy (BTU) content!

 

[06mistreSS]

So bit of an update. My last three tanks have all been above the 30mpg mark: 30.2, 32.8, and 32.4, all run on premium 93 octane. Those three tanks are the first I've ever run above 30mpg. These were mainly longer highway trips however; city driving was only about 15-20%.

Although, since I've been running premium, I am also seeing a noticeable increase in overall mpg for my typical, everyday driving (35-40% city): my average has gone from mid 27's (on regular 87) to about low 29's (on premium 93). That said, I'd say I'm getting a solid 1-1.5mpg increase running premium.

 

[70AARCUDA]

Now, the final question is: how many of those tanks were 100% gasoline and how many were E10-E15? If they were ALL 100% gasoline, you can almost 'bet' the MPG increase was due to the higher energy content of the 'pure' gasoline and NOT due to the engines ability to pull more power & efficiency from the higher octane alone by running more advance (less knock retard).

 

[06mistreSS]

Now, the final question is: how many of those tanks were 100% gasoline and how many were E10-E15?

Couldn't tell you right now, can't say I've ever looked for this. Is this something displayed at the pump, and is this something determined by gasoline company's for use in all their stations or by individual stations themselves?

 

[70AARCUDA]

Couldn't tell you right now, can't say I've ever looked for this. Is this something displayed at the pump, and is this something determined by gasoline company's for use in all their stations or by individual stations themselves?

Unfortunately, the answer is "no" to your last questions, as not ALL states require gas pumps 'state' how much ethanol 'dilution' there is in each grade of gasoline. That being said, the 'common' thought seems to be for gasoline (in USA):

• 87 octane = up to 15% ethanol is Federally mandated, so basically E10 or E15.
• 89 octane = sometimes *is*, sometimes *isn't* E10...here in AZ it *IS* E10.
• 91 octane = typically NO ethanol, so is usually G100.
• 93 octane = almost NEVER ethanol, so is G100 for sure.

 

[06mistreSS]

Just posted my best mpg in the Cruze this past weekend: 35.6 mpg (95% highway driving). I have continued to see an upward trend in mpg over the past six fill-ups.

 

[70AARCUDA]

Excellent, just wish GM would offer a "retro-fit" 3.53:1 final drive ratio 'swap out' package for us 2011 Cruze owners. The additional 2-3 mpg would eventually pay for itself for people who did a lot of highway driving.

 

[06mistreSS]

Just a bit of an update. I have gone back to running regular for the past 8+ fill-ups; just acquiring more data so I can better determine an average. Honestly I can't say if the premium is a benefit right now since the numbers seem to overlap quite a bit.

 

[Smitty]

I have seen a massive difference in my 2014 Cruze running 91 in the summer; especially up obnoxious hills. Being that this is my first turbocharged car and having just bought my Cruze this past summer, should I stay with 91 throughout the Upstate NY winter or will the engine be fine with 87 with the cold temperatures? Any experiences or advice? Thanks!

 

[70AARCUDA]

I have seen a massive difference in my 2014 Cruze running 91 in the summer; especially up obnoxious hills. Being that this is my first turbocharged car and having just bought my Cruze this past summer, should I stay with 91 throughout the Upstate NY winter or will the engine be fine with 87 with the cold temperatures? Any experiences or advice? Thanks!

IF fuel economy is more important than performance, then, yes, switch back to 87 octane during the colder winter months...the engine will simply "pull" ignition timing enough to ensure there's no "knocking" with the lower octane grade of gasoline. The Engine Control Module (ECM) computer "tests" the octane content of the gasoline after each MPG-reset and subsequent engine start-up, so it *knows* how much alcohol is in the gasoline even when you have half-and-half mixtures.