from Vp Racing fuels:

Too Much Octane

You may have heard the following: “don’t use too high of an octane fuel or you will lose power.” This is a half-truth. Having a fuel with too high of an octane will not cause your engine to lose power. The problem is that the popular components used to make the octane of a fuel higher slows the burn rate and a fuel with a burn rate that is too slow can result in an engine power loss. Of course, that is just typically what happens and it does not hold true for all fuels. As an example, VP Racing Fuels worked with the Scranton Brothers and their turbocharged Pro Class Celica throughout the 2002 season to develop a new fuel blend. This blend was designed to provide 120+ motor octane with the quickest burn rate possible. The result was the VP Import blend which produced six percent more power in the Celica than VP Racing C16.

Energy, Burn Rate and Vaporization

Let’s say we have a fuel that keeps our tune-up from experiencing knock. From this point, we would want to find a fuel that packs the most energy, has a compatible burn rate and has a vaporization characteristic that allows for peak performance.

Racing fuels are essentially blends of anywhere from three to 50 components. According to Steve Burns of VP Racing Fuels, “Putting together a racing fuel is like putting together a salad. There are different types of lettuce, olives, tomatoes and dressings. A great salad is one that complements the engine combination being used.”

Some fuel components add energy value to the fuel, some components are good burn accelerators and some components help vaporization. In general, energy values of different racing fuels are usually within three-to-five percent of each other. The burn rate and vaporization characteristics of different fuels are more likely to vary from fuel to fuel. Burn rates need to be fast enough so that all of the air-fuel mixture can combust in the cylinder. If the burn rate of the fuel is too slow, energy will be lost through the exhaust valve. As engine rpm increases, the amount of time that is available for the burn to take place is reduced. In that sense, a fuel that works well in an engine that revs to 6,000 RPM, may not be the best fuel in an engine that revs to 10,000 RPM.

As for vaporization, here is the ideal scenario: The fuel would enter the combustion chamber as a liquid. The reason you would want it in liquid form is that this would not displace any of the air that could potentially make its way into the cylinder. However, when the intake valve closes and the compression stroke gets under way, all of the fuel should change from a liquid to vapor state. With the fuel in a vapor state mixed with the air in the cylinder, the air-fuel mixture can be ignited and a high percentage of the charge will go from reactants to products.