There are several unsung ingredients required in the recipe for any high-performance engine to perform safely to its potential, but one of the least understood is fuel. For a surprising number of hot rodders and racers, their understanding starts and ends with the typical 87-, 89-, and 91-octane numbers on the fuel pump. Even the R+M/2 formula just above each number is a mystery to most. We're here to rectify that, because what kind of go-juice you run through your engine can mean the difference between getting the most out of your engine or being forever limited below its potential. And in the worst-case scenario, it could be the difference between safely churning out power and catastrophe.

At our annual festival of horsepower, the AMSOIL Engine Masters Challenge, we pull together some of the top engine builders in the country, give them a set of parameters to follow, and let them create the best engines their years of experience can produce. It's always an amazing assortment of engines that run the spectrum from Y-block to LS. However, no matter how good the engine is on paper, it'll fall flat or eat itself on the dyno without proper fueling. That's because at this level of power extraction, all the variables need to be known, so we provide a spec fuel for each class: Street gets VP100, and Xtreme Street gets Q16 from VP Racing Fuels (VP).

Race Fuel vs. Pump Fuel

What is the main difference between street and race fuel? Well, first and foremost will be the octane rating. While daily driven commuter cars can easily putt around on 87- to 93-octane gas, the cylinder pressures found in performance and racing engines are much higher, which requires higher octane to resist engine destroying detonation. VP's fuel catalog starts at 100-octane with the unleaded 50-state legal VP100, and progresses up the ladder from there. On the unleaded side, VP has many options that climb all the way up to MS109. Of course lead isn't allowed in pump gas for a variety of health and environmental reasons, and it's also pretty hard on oxygen sensors and catalytic converters and will significantly shorten their lives. It's still the best-known blending component for suppressing detonation, so it's still allowed in race-only fuels where compression ratios can reach 18:1 or more, and where huge boost numbers or nitrous shots require such stability. VP's leaded fuel catalog is extensive, covering any application imaginable all the way up to C23 which has an octane rating that's off the testable scale at 120-plus.

Those big numbers are great, but only if you can rely upon them to be consistent. "Pump gas can vary region to region, season to season, even station to station on the same day across the street from each other. You basically never know what blend you're getting day to day," Steve Scheidker at VP explained to us. "By contrast, race fuel in general, and VP fuels in particular, are consistent from drum to drum, pail to pail, regardless of where or when you buy them. Consistency is a VP hallmark; you can always count on a particular fuel blend having the same characteristics as the last time you bought it." That may not be a big deal for factory-tuned or very mildly modified street cars, since the factory ECU can and does compensate for fuel fluctuations, but it's very different for highly modified cars that have been custom tuned.

Also, remember that pump gas is blended with the cheapest components possible to meet the minimum government-mandated standards to maximize profit. Race fuels are designed from scratch with only the highest-quality components to maximize power and performance for each specific application, with cost only a secondary consideration. That's why the stuff is so much more per gallon than pump fuel. But in the final analysis, race fuel often generates more performance gains per dollar invested than many hard parts, because it lets you maximize your combination to its potential.

That careful attention to components shows up most in the distillation curve. Vaporization and atomization is how you make more power; liquid doesn't burn, we need vapor. The distillation curve refers to the rate at which all the components in a fuel vaporize. Ideally, for maximum energy per combustion we want droplets of fuel to turn to vapor as soon as the intake valve closes. Street-driven engines spend much of their time at low rpm ranges and have a much longer window of time to get heat into the combustion chamber and burn components, so heavier components will be chosen to create a smoother curve. Some of those components may require 300-400 degrees F to vaporize and burn. In performance and racing engines where engine speed is much higher, the time available to burn the fuel drops dramatically so racing fuels will begin to vaporize around 100 degrees F and will completely vaporize by around 250 degrees F. Racing fuels are blended with that in mind: vaporize quickly and burn completely at lower temperatures under high cylinder pressure to extract the most power possible while leaving little unburnt material. Jason Rueckert, a Tech Specialist at VP, says their Q16 blend may be the best vaporizing race fuel in the world with nearly instantaneous vaporization in the combustion chamber.