Full disclosure, this Project Farm testing is all being done on small engines, both of which are carbureted but we’ll talk about why that might be important in a second. In this video, he’s testing four different grades of pump gasoline, 87 octane with ethanol, 91 octane with and without ethanol, and 93 octane with ethanol. All four types of gasoline are being tested in a generator with a fixed load to see how fuel efficient each one is compared to the other, and then all four are being tested in a small Honda engine that is powering a go kart pulling a load. All four fuels are being tested for ethanol content as well, and the results for all the testing are in the video.
Here is one thing to keep in mind during this testing. Both of the engines being used in these tests are carbureted. This will be brought up during testing. Remember, a carburetor doesn’t much care what fluid is in it, it is going to flow whatever the engine it is bolted to is demanding based on the vacuum that is pulling fluid through the carburetor. Unless fluids being tested have a different viscosity, which would change the available flow rate through small orifices, the carburetor will flow the same amount for any given load. This is important because fuel efficiency is one of the reasons that people often complain about fuels with ethanol in them and this just isn’t a thing in a carbureted engine if the carburetor is unchanged from ethanol fuel to no ethanol fuel. In a fuel-injected engine, the computer is compensating for a lean condition that it would see with ethanol and therefore will use more fuel than it would with gasoline that was ethanol free. In theory anyway.
But in your average carbureted engine, not a race engine, you can run ethanol levels in the 30% range with virtually no difference in performance. Why? Because the ethanol makes a colder intake charge and that helps with keeping detonation away even in leaner conditions, and the ethanol inherently is higher octane than the base gasoline anyway. So if the carburetor is going to meter the same amount of fuel, and the vehicle is driven the same, then the ethanol fuel will get the same mileage but will do so at lower cost which means your miles per dollar are better and that’s what we all really care about when talking about mileage. I don’t care if the car gets 100 mpg is the gallon costs me $100. I care that it gets the best mileage per dollar of fuel, so if the cheaper fuel gets the same mileage and doesn’t cause harm to my engine then that is the fuel for me.
In a fuel-injected engine, like the 5.3 LS that you’ll find in may mid 2000’s GM trucks, the ethanol content might have a slight effect on mileage, but the octane will definitely have an impact on power. This is because their knock sensors, that listen for and detect pre-ignition or detonation, will pull timing when detonation is detected and that will result in lower power. Higher octane fuel will keep that detonation from happening and allow maximum ignition timing and power. This is not something that will matter for you in your commute vehicle that isn’t under a big load. But if you are racing or towing, then going up a grade in gasoline octane can be a good thing. My 2005 Suburban saw small improvements in power on the chassis dyno with higher octane fuel under full load runs, but in normal driving and commuting you would see no difference.
Watch the video.
Had a 6.2 in a Yukon XL Denali that was E85 compatible. Would definitely use more E85 but when I calculated it out it was still cheaper per mile consistently.