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
View attachment 123151
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
View attachment 123152
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.
