The Power Difference of E85 Vs. Gasoline

There are all sorts of units of measure to quantify energy content; you can measure it in British Thermal Units, joules or even calories, just like you do food. A standard gallon of 93-octane gas contains about 114,500 BTU of energy, which is around 450 calories, or the energy equivalent of a double cheeseburger. A gallon of diesel runs about 129,500 BTU, and pure ethanol measures out to about 76,100 BTU. If you're wondering why these fuels -- which are all liquid hydrocarbons -- have different energy contents, you need only look at their relative densities. Diesel is the heaviest, followed by gasoline and then ethanol.

You may be thinking that there's a direct relationship between the energy content of a fuel and the amount of power it can produce. And you'd be right, in a sense. Ethanol's 50-percent energy deficit compared to gasoline means that you'd have to inject 50 percent more to yield the same amount of power. That's not a particularly difficult task, especially if you install larger fuel injectors with an additional 50 percent capacity to handle ethanol. As long as the fuel system can keep up with demand, ethanol will only end up costing you money at the pump, not horsepower at the wheels.

Where pure ethanol and E85 ethanol -- 85 percent ethanol, 81,800 BTU -- triumph is in octane rating. Ethanol has an octane rating of about 108.6, which is only 0.10 octane lower than the methanol once commonly used in racing circles. But that's pure ethanol, not E85. While mixing 85 percent 108-octane ethanol with 15 percent 93 octane would yield something in the 102 octane range, the fact is that most manufacturers mix it with 84 octane or lower. That yields a final fuel rating of between 96 and 98 octane.

Most engines aren't designed to take advantage of E85's higher octane. Really capitalizing on a 98 octane fuel means more than simply advancing the timing -- it means using higher compression or some method of forced induction like a turbo or supercharger. Provided that the fuel injectors can maintain adequate flow at maximum duty cycle, using E85 in a turbocharged engine can allow the driver or computer to turn up the boost for serious gains in power. At that rate, you might as well run pure ethanol for a huge increase in boost and power potential.

So, the verdict is: E85 is bad for fuel economy, worth a few horsepower on a naturally aspirated engine and worth a lot more on a turbo engine. But if you have the injector capacity, computer programming and boost potential to run E85, then you're already getting terrible gas mileage and you might as well use pure ethanol. Granted, the timing advances permitted by using E85 will help to offset some of the fuel economy loss -- maybe by as much as 10 percent -- but you could say the same for pure ethanol. At the end of the day, it matters less what fuel you use than how you tune your engine to use it.