Data logging regular vs. premium

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Always lots of chatter about regular fuel versus premium and whether it makes a difference. The consensus usually lands somewhere between, "If the manufacturer doesn't recommend it, you're wasting your money," and "Modern engines have knocks sensors and will adjust timing accordingly, so you won't hurt anything." A link to Consumer Reports or Edmunds is usually thrown in there sometime too.

I decided to data log knock retard and get receipts.

The setup for this is my 2020 Chevrolet Silverado, 5.3L V8, 8-speed trans, and ~23k miles. Notably, GM only recommends premium for the 6.2L, not the 5.3L. Both examples I'm sharing were logged while towing my 5,500 lb. travel trailer, so the truck was certainly doing some work. Both logs were taken on the same stretch of 55mph two-lane road in similar ambient conditions. In the screen shots, knock retard is the graph with the single red trace.

Regular
Regular.JPG


In the regular fuel log, you can see the saw-tooth knock retard pattern. The ECM responds to knock by retarding timing (~3-5 degrees in most instances), the knock abates, the ECM reduces the knock retard, the knock returns, the ECM adds knock retard, and so on.

Premium
Premium.JPG


Again, same road and similar ambient conditions. This trace is much different than the previous. Knock is largely absent with premium and what few blips do occur are easily corrected with minimal (~2 degrees) knock retard.

So, does premium fuel make a difference? In the case of my truck, absolutely. This is an engine that the manufacturer doesn't specifically recommend premium too. Imagine this same scenario in a 6.2L that does recommend premium: I certainly wouldn't expect it to be better.

Because I know the statements and questions are coming:

Why did you do this?
On a whim, I filled with premium. The driveability was so much better. Throttle tip-in was crisper, shifts were less mushy, fuel economy was better, and even restarts after an auto-stop were less jarring. I wanted to know what was going on.

The difference is because it was hot when you logged the regular!
Nope. Neither one was logged when it was hot. They were both logged in early fall, about two weeks apart. It was actually warmer when I logged the premium, which you can see in the increased engine oil and transmission fluid temp. Ambient temp and IAT is part of the standard channels that I log, I just don't graph them.

It's because you were towing! I don't tow, so this doesn't apply to me.
I have dozens of logs over the past 6 months, regular and premium, with and without the trailer. Regular consistently has more instances of knock retard on any given trip, no matter the situation. I chose these two to compare because I knew they were on exactly the same road and in a very similar operating envelope (ambient conditions and load). Comparing these two in particular shows a stark different between the two and also shows that even with premium, there still may be instances where knock retard will pull timing.

You had a bad tank of 87!
Not likely. See the part above about similar results over 6 months and multiple tanks of fuel. Both fuels were pumped from a very high-volume Costco, so they're Top Tier as well. This is the same station I fill up my wife's car at too. Never an issue with her vehicle.

From here, do with this what you will. I really don't care whether you use premium or regular and I'm not making a recommendation. I'm just sharing data where often only exists opinion.

Do I fill up with premium?
Opinion incoming: Yes. It's pretty obvious that the ECM doesn't have to intervene as much because knock is simply less present. Yeah, the extra cost stinks, especially when getting 9.5mpg while towing. For me, the better driveability is worth the extra cost. Much of the benefit for me comes down to the shift characteristics. GM's 8-speed isn't the most refined or sophisticated transmission. I'd go so far to say that the shifting is often mushy. What does that have to do with the fuel in the tank? It's all about timing control. GM's torque reduction strategy largely centers around retarding timing during a shift to reduce input torque while the shift is completed. I feel that the premium fuel allows much more stable timing control when the ECM is retarding and advancing during shifts. It makes the whole shift feel better because the ECM can begin advancing timing more aggressively after the shift, leaving the throttle feeling crisper when exiting the shift and less mushy. Same thing with regular throttle tip-in and during auto-stop restarts: Most stable timing control.
 
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If you look at map it's also using more compression. I've noticed on my cars when I use premium I am operating at a higher Map at lower loads. Timing does not matter because it will always be looking for MBT. It will use more or less fuel and egr to get there.
 
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I should try logging this with my truck, Ram “recommends” 89 but states 87 is “acceptable” for the 5.7. Butt dyno says 89 “feels” better, less sluggish.
 
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We have a 2019 Q5 , owners manual say premium recommended but will run on 87. First year we had it on drive to Florida from NJ(about 1200 miles))we averaged 31.5 mpg. I then ran 87 the following year and averaged 31.1 mpg. I figured it was too close to call and not worth the additional cost. I run 87 all the time now
 
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This is an engine that the manufacturer doesn't specifically recommend premium too. Imagine this same scenario in a 6.2L that does recommend premium: I certainly wouldn't expect it to be better.
Yep. It's a more extreme example, but reminds me of a modded turbo 4-cylinder putting down something like 450whp on the dyno on 93 octane. Added race fuel, changed nothing else, and it gained 20whp.

The more interesting thing to me is the gas mileage. Some people's vehicles run fine on less than premium, but they use premium because they get better gas mileage from it, so they use premium since it kind of pays for itself anyway.

Though I remember a Grand National owner saying he used 87 octane while towing his boat or something cross country because the mpg was the same and it was a noticeable cost savings for such a long trip. But he made sure to not allow the turbo boost to go above 5psi during the trip.
 
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Our 2017 2.3 EcoBoost Explorer runs fine on 87 octane, per the manual using 91 or higher gives more power. Yes, it does increase the power, we used premium on some trips but at over $1.00/gallon for premium fuel it's not worth it for some extra get up and go. At 80,000+ miles there has been no noticeable loss of performance from any Intake valve buildup, if there is any.
 
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The more interesting thing to me is the gas mileage. Some people's vehicles run fine on less than premium, but they use premium because they get better gas mileage from it, so they use premium since it kind of pays for itself anyway.

The 2013 Chevy Volt recommends premium. At the time that car was made the cost difference between regular and premium was 30 cents, and the additional gas mileage that premium gave was worth the additional cost.

The cost difference is now more than a dollar.

Running regular it gets about [email protected] Running premium it gets about [email protected]

My math says the extra dollar per gallon for premium isn't made up by the extra 3MPG it gets you.
 
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The 2013 Chevy Volt recommends premium. At the time that car was made the cost difference between regular and premium was 30 cents, and the additional gas mileage that premium gave was worth the additional cost.

The cost difference is now more than a dollar.

Running regular it gets about [email protected] Running premium it gets about [email protected]

My math says the extra dollar per gallon for premium isn't made up by the extra 3MPG it gets you.
Chevy says to use 91 minimum for the Volt in the owners manual, not recommend.

66AA0EA2-123F-4A33-8DC8-0E8906DBF82E.jpg
 
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I'll let you know if the engine in my Volt ever breaks because I'm running regular in it.

Hint: It has a knock sensor.
Recommending a certain octane is one thing, requiring it is something else. I wouldn’t be driving around relying on my knock sensors to keep my engine in one piece because I want to save $10 at most a tank 🤷🏻‍♂️
 
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Recommending a certain octane is one thing, requiring it is something else. I wouldn’t be driving around relying on my knock sensors to keep my engine in one piece because I want to save $10 at most a tank 🤷🏻‍♂️

Folks who know much more about how the Volt works than I have said that 87 octane is fine in it.
 

AZjeff

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The KR trace isn't surprising, it's exactly what Octane in gas is supposed to do - suppress knock. What's surprising (to me) is the extent that mid-grade 89 octane gas reduced the KR function. The snapshot in time data is all fairly close but it doesn't seem to represent the same place on the road if trying to use similarities in the TPS or load trace. Curious that an identical TPS% shows 58 mph on the 87 graph and 55 mph on the 89 graph with the RPMs tracking the same 5% difference but the other values are much closer.

If this trace is definitive proof of something relating to octane what does the higher speed and rpm with 87 at the same TPS setting mean?

From Wiki:
"Octane" is colloquially used as a short form of "octane rating," particularly in the expression "high octane". "Octane rating" is an index of a fuel's ability to resist engine knock in engines having different compression ratios, which is a characteristic of octane's branched-chain isomers, especially iso-octane. The octane rating of gasoline is not directly related to the power output of an engine. Using gasoline of a higher octane than an engine is designed for cannot increase power output.

The octane rating was originally determined by mixing fuels from only normal heptane and iso-octane (2,2,4-trimethylpentane, a highly branched octane), and assigning anti-knock ratings of zero for normal heptane and 100 for pure iso octane. The anti-knock rating of this mixture would be the same as the percentage of iso octane in the mix. Different isomers of octane can contribute to a lower or higher octane rating. For example, n-octane (the straight chain of 8 carbon atoms with no branching) has a -20 (negative) Research Octane Rating, whereas pure iso-octane has an RON rating of 100. Some fuels have an octane rating higher than 100, notably those containing methanol or ethanol.

(Not being a petroleum engineer I have to find simple explanations for this stuff.)
 

OVERKILL

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I'll let you know if the engine in my Volt ever breaks because I'm running regular in it.

Hint: It has a knock sensor.
So does my SRT, but it was rattling like crazy when I got a bad tank of 91 (it was regular I assume) to the point that I thought it was going to break something. I had to dilute it with 94 to get it to behave.
 
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Great detective work and share!

OP; Just curious, your data log section for the higher-octane test shows 89 PON. Was it really 89 or 92-93 PON premium.
 
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MrHorsepower, Thanks! As you can see from my signature line below, I love data. It shines light on subjects, where otherwise, we may only have opinion. Of course, many here at BITOG hate data, as it often conflicts with their postulation. :giggle:
 
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