I've logged cars a lot that are tuned and played with fuels and how they impact timing advance and potential correction/retard/knock. On my Atlas for example, it has the 3.6 VR6. This engine used to call for premium/91+....now min. recommended is 87. Here are the logs. WOT 2/3 gear pulls, similar conditions, road, etc. to keep variables under control. Yes, I get less correction on 93 vs. 87 which the graphs below clearly show (all the colored lines are cylinder knock retard/timing correction). I can't tell the difference at all driving it normally and that impact is only at the higher RPMs/WOT. Interesting that it also sees some correction on 93 likely due to winter blend fuel. Both of these feel the same to me. Tehre is not any audible "knock" or detonation, most of this based on how the VW ECUs work is pre-emptive based on the ECU thinking knock events are coming based on lots of parameters. If I was towing, I would for sure run 93 but for around town, 87 and saving what can be $1/gal is well worth the small power drop at v. high demand. I never saw any meaningful difference in mpgs. I usually disregard folks saying "they can feel it" or "my XYZ runs like crap on 87" b/c to me, unless you have a blind test where you had to pick the fuel, I'm not sp sure most of this isn't just placebo from wanting that 91/93 to run better based on programming over many years. My 2 cents.
Octane vs mpg again...
- Thread starter AZjeff
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I linked an article here a few years back that showed the energy density of gasoline varies up to 4% even at the same gas station and irrespective of winter/summer blends or EtOH content. It showed how this variation made it impossible to determine “real world” fuel economy without at least using standardized test gasoline. There are many, many variables in the real world which impact fuel economy and none are controlled.
How can there be efficiency differences between brands?
All octane does is resist detonation by slowing down the combustion process. It's not some magical thing that will give you more power or efficiency, unless you need it to slow the combustion process down in order to eliminate KR.
If higher octane fuels produce more efficiency, why stop at 91/93 octane.....why not run 110 racing fuel so one gets even more fuel economy.
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A higher octane rating resists pre-ignition. It does not resist combustion.
And as I noted above the real advantage to using a higher octane fuel is the ability to increase the compression ratio which directly increases efficiency. Without doing that you’re limited to advancing the timing which is limited in what it can do.
sorry, combined two statements. It resists detonation by slowing down the combustion process.
It also allows for more aggressive timing and fueling while still keeping detonation in check.
As you noted, there can be energy differences between gasoline even at the same station. That being said, in this area at one time there was a strong correlation between gas bought at certain retailers and their sources. So much so, that one brand worked with the American Cancer Society to conduct product testing showing how much cleaner their gas was compared to competitors. And it clearly showed the gas was from different sources and composed differently - though all meeting the requirements to be considered fungible gasoline.
FWIW, at the time, it was Holiday who sourced their gasoline in the Twin Cities from the Flint Hills refinery in Rosemount, MN, compared to Superamerica who got their gasoline from their refinery in St. Paul Park, MN, and Amoco/BP who sourced their gasoline from Mandan, ND and to a lesser extent Whiting, IN. Since that time, the St. Paul Park refinery and Superamerica have changed hands multiple times, so the refinery is now Marathon and the gas stations are Speedway (but owned by 7/11), the Mandan ND refinery is now also a Marathon Refinery, and terminal was sold off and is now owned by NuStar and supplied by the same sources, etc...
Obviously this will vary wildly depending on the local market. Some places, gas is gas. Others, it can be traced back to an origin associated with a brand...
It’s not detonating when it knocks as there is still a flame front to initiate combustion. But the ignition is before the spark plug fires and this causes a shock wave that travels upward causing the noise. But the shock wave is insufficient to cause detonation. Fuel can detonate in an engine but that usually causes much more damage than knock.
It’s still not burn speed it’s resisting unplanned ignition. Any differences in burn speed are minor.
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I guess I was mostly going off the word efficiency. Efficiency is a measure of the engine extracting heat from the reaction. Your example appears to illustrate that some fuel produces more or less heat, not that it is more efficient.
I don't disagree - more or less going along with your point that fuel can vary - and in some places it actually can vary by brand. That can lead to a perceived difference in efficiency by the user - ie: I get better fuel mileage on this brand versus that brand. (Again as perceived by the end user - not by an actual measurement of efficiency of the engine on the fuel)
Again, that will vary wildly depending on the local fuel distribution setup.
And it all just points to how unless a boatload of variables are controlled (like standardized test fuels), claims about one octane being better than another need to be taken with a grain of salt...
Again, that will vary wildly depending on the local fuel distribution setup.
And it all just points to how unless a boatload of variables are controlled (like standardized test fuels), claims about one octane being better than another need to be taken with a grain of salt...
I have always been confused on which is which...knock/pre-ig/det - to me, pre-ign is the one where the air/fuel combusts on it's own before the spark and the one that cause your pistons to have holes in them. Det/knock was the air fuel igniting unevently with multiple wave fronts...that's the one where the ECU retards the timing until it stops and the one where octane helps resist. Anyone here can clear that up for me? Clearly if you aren't optimizing combustion you are not as a efficient but how much that impacts things and whether you are even having that issue is something that requires logging.
Daily driving predictable routes over the course of a year assuming 10,000+ miles a year or so moves the needle into couple sigmas though .
AKA if I have a few years worth of my driving documented with temperatures going the same routes repeated On 87 then do the same for e15 I can indeed note a trend on my specific car since I literally only have 4 different trip destinations.
You need to note average speed and temperatures to provide a baseline for comparison to another identical car.
If you are using instrumentation and tracking economy to improve cost and fuel economy on your specific car that is very much doable
Ecomodder.com
I could care less about testing values (epa) when I have over a decade of driving data on the same stick shift that show I consistently beat epa, even on 88e15
AKA, I’m saving money over the epa baseline and can predict my economy on different fuels and temperatures given my rather bland repeatable driving style and destinations.
I also had access to my same car in both stock and auto trims, despite the epa being similar for both the auto consistently was half the winter fuel economy on my daily trip.
My pocketbook is more important than the epa test regimen which has known flaws especially in the manual transmission throttle/ shift point definitions.
That's a direct injection motor right? That was my one caveat. I challenged a guy on another forum a few years back and he proved me wrong with the 5.3 in his Chevrolet Tahoe. But he does see differences in fuel economy by brand, as I have.
How can there not? Different additive packs for starters. For more than ten years, I would always count on Shell's 93 to deliver 2 mpg less than Mobil's 93. Cumberland Farms was even worse.
I paid a lot of attention to brands back in the '80s and early '90s. Then settled in to just using 93 from Mobil or Chevron. A decade or so ago, I wanted to see if things changed. This forum and some Toyota Prius forum showed that they were getting 4 mpg more than Mobil with Chevron from 42 to 46 mpg. I was able to experience the similar results with my old Pontiac V6 and my GMC trucks.
In my neck of the woods, Chevron is tops, followed by Mobil and Sunoco. My daughter fuels the Sierra at Sam's Club and while her mpgs are less compared with Chevron, the cost per mile is better.
I used to run Sunoco's 104 unleaded in my Buicks. That increased fuel efficiency even more and the motor loved the stuff. It of course ran faster in the quarter mile, ran 4 degrees cooler and idled more smoothly as well. Great stuff! My truck on E85 behaves the same way except for the gas mileage!
Yes, 3.6 VR is DI.
104 is so much money though, as a daily you ran this?
Note my post above about what efficiency means.
And racing gasoline is formulated differently than consumer product.
I ran some 100 at the track when I ran out of E85 to blend with the 93...was getting too much KR/power cuts with the tune under those conditions. I had lead in my next UOA from it.
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