E85 is a blend of 85 percent Ethanol, a grain-alcohol made from corn, and 15 percent gasoline. This mixture gives the fuel a much higher octane rating than gasoline, and is cheaper than pump gas, race gas, or methanol.
For max power, the air/fuel mixture for E85 is ideally about 30 percent richer than for gasoline alone. (For the sake of reference, an idea stoichiometric air/fuel ratio for gasoline is 14.7:1, but only 9.765:1 for E85.) Fuel economy suffers about the same percent. The fuel-heavy mix has a cooling effect, allowing for more timing and compression. The addition of alcohol to gasoline in such a high percent gives us a significantly higher octane rating, which is posted as 105 at the pump. At about $2 a gallon on average, which is about the cheapest octane in the country, even after you account for the lower fuel economy. This octane level can easily support compression ratios up to about 14:1. Experimentation is bringing that number even higher. With this increase of compression tolerance, many engine improvements can be made.
When you're building an engine with the intention of running on corn, your higher compression ratio limit gives E85 other advantages, but you have to take advantage of these in order to gain the full benefit of the conversion. High-compression engines by nature produce a lot of low-rpm power. This gives room for about 15 degrees longer seat-to-seat camshaft duration without hurting the low-rpm power, while increasing the high side. Lower-compression engines, 10.5:1 for example, produce most of their power at the end of the power stroke, while high-compression engines get their power from the beginning. It lets us open the exhaust valve sooner, in turn allowing us to have a smaller exhaust valve. The smaller exhaust valve can then make room for a larger intake valve. The heavy air/fuel ratio of the alcohol mix can really benefit from this change. Of course, if you've already purchased your heads and aren't looking to spend a ton of money re-machining them, you can still get some of the benefit with a camshaft swap.
A downside to E85 is that it's very dry. You may have noticed how rubbing alcohol leaves your skin white and dry. This fuel craves moisture the same way. It pulls water from the atmosphere and carries it into the fuel system. Water isn't necessarily a detriment to power, but it can cause premature failure of fuel system components along the way. Water accelerates oxidation, and can destroy fuel lines, pumps, and filters from the inside. We will talk about what makes a product alcohol-compliant later on in the story.
Another one of E85's downfalls is that the government doesn't control its content very closely. The assumed 85 percent ethanol is no guarantee of its actual content. Winter blends of the fuel can be as low as 70 percent to accommodate cold-starting conditions. Big Brother says the mixture has to be between 70 and 90 percent. With a late-model, fuel-injected car built to accept E85, the difference isn't noticeable, however, when a carburetor is subjected to this variance, it can't compensate. In order for the gas companies to avoid paying a liquor tax, they denature the alcohol. This 2 percent of poison added to achieve the denaturing element is only hoped to burn, not formulated to complement the alcohol. People in the racing fuel industry refer to this poison as "refinery drippings."
Aside from the changes in the mixture ratio, the gasoline side of the equation is hardly monitored at all. As with the blend percent, gasoline changes seasonally, and there are no minimum octane level requirements. Unlike gasoline, E85 is not required to have any additives to protect your engine or improve its burn.