Better MPG on 87 vs 89 gas?

I'm no chemist, but it was my understanding that higher compression engines require higher octane fuel. Our trucks simply weren't designed to us the higher octane. They're fairly middle of the road if I'm not mistaken. Combine this with the intake valve being held open during the Atkinson cycle... (does that lower compression?) and I think you'll start to understand that the trucks just weren't tuned to run higher octane.

 

Ok, quick prelude: I'm a recently graduated mechanical engineer. I took multiple classes focusing on the thermodynamics and performance realities of internal combustion engines. I've rebuilt a couple motorcycle engines, and am in the middle of installing a turbo on my buddies car, a 2012 Kia Forte Koup, which has never been offered with a turbo. I also just about only read about either firearms, engines, or suspension in my spare time. So, I'm not pulling all of this squarely from my ass.

Our engines have quite high compression for a truck at 11.8:1. At low speed and throttle conditions, the high compression ratio allows for better thermal efficiency and therefore better fuel economy without any real detriments other than harder starting. In the past, this would have required high octane fuel to achieve the kind of performance we see in our trucks with regular fuel under high load/power conditions. There are two main technologies that allow this: combustion chamber design, and direct injection of fuel. The combination of these two things allows us to run lower octane for the same compression ratio, by resisting knock through in-chamber dynamics like squish and swirl reducing the effects of hot spots, and the very finely atomized fuel from the direct injection reducing chamber temperatures via evaporation. These two things allow our engines to survive at peak power production on low octane fuels.

Note I said survive: The compression ratio is still too high for standard octane fuels in our trucks for optimal power. How do we know this? The Lexus IS350 uses the SAME 2GR-FKS engine we have in our V6 trucks. Same compression ratio, components, everything, but it makes more power. How? When they rated our truck's output, the ignition timing of the engine was tuned for 87 octane, and that's what they used. For the IS350 they raised the rev limiter from 6100rpm to 6600 and recommend minimum 91 octane to advance timing further at higher RPM. By doing those two things, the Lexus is rated for an extra 33 horsepower! It's possible that with a tune we could unlock this potential in our trucks. Me personally, I'll stick to 87 on the stock tune until this truck becomes a toy.

 

for the past 8 months, i have been using non ethanol marine fuel at 90 octane. according to my built in average mpg meter, i have increased my mpg 3.7. currently sitting on 26.0 for 25,233 miles. (3.5 v-6 auto)

 

The non-ethanol part is a non-insignificant factor here: Ethanol has a lower specific energy content than gasoline.

 

ethanol has 1/2 the btu value as gasoline. therefore you need twice the volume to equal the energy developed. we race cars using methanol and have to have carb opened up to allow enough flow to get good power. point is adding ethanol to gas reduces the available power. try it out for a while and see the difference

 

I'm going to go with magic...

 

seems like way too small of a sample size to make an accurate judgement. I run 87 and haven't had issues, 18mpg in the winter 20 in the summer sure I wish I got better mileage but it's not a priority for me

 

My 2009 four cylinder didn't vary much on mpg, no matter what I ran. This weekend I drove to my father's; a trip I make 13 times a year. 215 miles of steep, Ozark, up and down with cruise set at 68 mph. The best I've ever done is 17.8 mpg. Friday night I got 21.5 with a whole lot less 3rd gear, 4500 rpm shifts. 87 octane with ethanol vs 91 w/o. I reset my cumulative mileage when I switched away from regular 87 to no ethanol and I went from a 4 month average of 16.5 to 21.4 in the first three weeks. Anomaly? Maybe. But I am a scientific researcher and this is worth pursuing, for me.

 

Um, a gallon of gasoline is a gallon of gasoline. Octane rating has no bearing on energy potential. Seriously. Octane rating is simply resistance to knock, or engine ping.

 

Pure coincidence friend. No difference in 87 or 89 except for knock.

 

Anecdotal, but i recently started putting in 91 and my MPG decreased. Gonna go back to 87 next tank.

 

lol.....


There is no 87 tank....

Its a mix of regular and premium... Usually a rip off...

Stick to one or the other.. With 89, you may be getting 87...

 

Yep, the 89 tank is a mixture of 87 and 91 octane. The higher the octane the higher the ignition temperature. If you run too low of octane in a motor tuned for a higher octane then the fuel will prematurely combust or ping, if you will. So what is happening? The ignition takes place as the piston is still compressing the fuel air mixture, power is lost and excess wear could be taking place. On the other hand, too high of octane and the ignition could be occurring too late and the result just might be a loss in efficiency. Now I am not an expert but my sister runs a Holiday Express...

 

Don't matter

 

Mazda 3 has a 13.0:1 Compression ration and is tuned for regular 87 gasoline. Lots of turbo vehicles these days also run on 87 just fine. Simply saying high compression ratio or Turbo is why premium exists isn't valid any longer. It's all about how a specific application is designed and tuned. I myself would never run anything over 87 as it's a waste of money, unless I noticed knock, which is unlikely at my altitude.

 

If you get a motor tuned and designed to run on a higher octane, turbo etc, then the higher octane is a good idea. It is just matching what the motor is tuned to run on.

 

I have taken it upon myself to compare three types of gas from three different stations, Murphy (Walmart, my regular supplier), Valero and Exxon. They are all 1/4 mile to 1/2 mile from my house. I do 5000 miles oil changes and every oil change put a bottle of Gumout complete fuel cleaner in gas tank. Before any of you put down Walmart, noticed I drove my 1999 Tacoma for 230,000 miles almost entirely with Walmart gas and the engine was running as new when I traded for my 2016 taco.
I drive the exact same route several times a week (24.3 miles) and try to drive exactly the same way, paying attention to the speed, when press the gas pedal harder for hills and when I sort of coast , the RPMs, and more or less the same stops for the 6 traffic lights I encounter.
After many, many trips and careful record keeping, my own unscientific conclusion are that probably the amount of water or gas decomposition from all three above are the main culprit. I consistently get better MPG (20.5 to 21.8) with mid-range (89) from Valero, then Walmart and last Exxon, no noticeable difference with the high grade (20.5 to 21.8) and lowest MPG (19.5 to 20.3) with low grade.

 

You do realize that several different vendors get their gasoline from the same tanks right?

Also, a gallon of gas has the same energy density as any other gallon of gas, unless it has more ethanol or no ethanol.

 

I know but, they are numerous variables right after the gas its actually refined, such as the tanks it is stored before shipping, the shipping container, and the station tanks , any intermediated storage from the refinery to the station, the length of time the gas is stored before it gets to your tank, the ambient humidity,the elevation, etc, etc, etc.
The gas may be within legal parameters (mostly) but the difference in MPG may be noticeable.

 

Understood, but, when the refined gasoline is in the tank at the tank farm, ready to be shipped to distributors, that same big tank will supply a dozen different smaller companies. Here ya go:
https://www.edmunds.com/car-care/is-cheap-gas-bad-for-your-car.html