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Everything You Need To Know About Octane

All About Octane: What it is, how it’s done, and the best race fuels, concentrates, and alcohol injection kits on the market

Rick Jensen Sep 7, 2015
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With neck-snapping beasts like the 650-horse C7 Z06 roaming around, factory performance cars have never been faster. Cheers to the Corvette team for cranking out that pump-gas power number, because that thing is flat insane.

The rest of us spend thousands to equal or exceed the supercharged LT4's output. But normally, high-compression or boosted engine builds tuned for 91-octane street gas leave power on the table. If you want to add more timing or crank up the boost for more ponies, you'll need more octane to keep that engine-killing detonation at bay.

And adding octane can also benefit our lightly modded Vettes, too. Since the high-octane glory days of the 1960s, premium pump gas octane ratings have dropped, and then leveled off in the mid-1990s. They also vary widely around the United States. I travel between the East Coast and the Midwest a lot, and seeing an NYC-area Sunoco station with an Ultra 93 pump gets me all tingly. Does that make me weird? Yes, but let's see how you feel at rural Midwestern gas stations with 89-octane "premium." Throw in the fuel quality and octane variances between different stations and tankfills, and you can understand why our thriving performance aftermarket craves octane-boosting products.

But deciding to add octane is easy—the hard part is deciding how to add it. For example, let's say you decide to run race fuel: how much octane does your car need? Do you buy leaded or unleaded race fuel? Should you buy oxygenated race fuel or not? Is mixing race fuel with pump gas recommended, and if so, how much do you use?

And those questions are just about race fuel—there's also race fuel concentrate to consider. And while alcohol injection is a slightly different solution to the octane equation, it also can solve your octane-deficiency problem.

All offer benefits and drawbacks, so we're going to discuss each method in depth. And then you can go forth, head swimming in octane knowledge, to choose the best octane enhancer for your Corvette.

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Race Fuel

Race fuel is the old guard of high octane—it's been around for decades, it's produced by numerous companies, and it's available in many octane ratings and formulations.

It's a fascinating concoction, so to further educate ourselves we reached out to two gentlemen with a combined 80-plus years of fuel experience: Jack Day, president of Rockett Brand Racing Fuel, and Tim Wusz, Rockett's VP of engineering. These two made and sold race fuel at Union 76 during its NASCAR partnership, then started Rockett Brand in 2004. These days, Illinois-based Rockett Brand offers unleaded and leaded racing fuels with 100-, 110-, 112-, 114-, and 118-octane ratings, and E85 with a 112-octane rating.

Race fuel is comprised of fewer than 100 hydrocarbons: there's maybe six prominent ones, and the rest are present only in small quantities. A good race fuel manufacturer only includes fuel components that will give desired qualities—high octane, a fast burn, good vaporization, and for leaded fuels, the ability to play nice with lead.

Compare that to normal pump gas: it is a mixture of several hundred hydrocarbons, with many of them occurring during the refining process. As they can't be separated out, these components are just along for the ride—they burn just fine in non-race, non-high rpm engines anyway.

Race fuel is a fantastically complex product, but of course the first thing we all wonder about is its

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Octane

A fuel's octane number is its ability to resist detonation. And it's important to note that octane isn't power—the benefit to raising a fuel's octane quality is only valid if the engine it goes into is detonating. The higher a fuel's octane number, the more resistant it is to detonation.

An engine only needs enough octane to prevent detonation; if it's tuned for 91-octane pump gas and has a slight ping on a hot day, filling up with 118-octane race fuel won't add crazy horsepower.

Fuel has what's called an anti-knock index, or AKI, which is calculated based on the average of two different tests: the research octane number (RON) test, and the motor octane number (MON) test.

The research octane number test consists of a single-cylinder engine that runs at 600 rpm. Using 125-degree F intake temperature and standard barometric pressure, this part-throttle test determines a fuel's detonation resistance at light load.

The motor octane number test is more severe: the single-cylinder engine runs at 900 rpm, and breathes 300-degree F intake air. This is a heavy-load test that determines how a fuel will perform at wide-open throttle.

When buying gas at the pump, you're choosing a fuel based on its AKI—the pump wears EPA-required decals showing (R+M)/2 and fuel grades like 93, 89, and 87. So as an example, clicking the "93" button means you're selecting a fuel that might have a 97 research octane and an 89 motor octane. (97 + 89 = 186) / 2 = 93 octane.

But with race fuels, companies sometimes only list the MON number for fuels they consider "track only," and can list the MON, RON, and R+M/2 numbers for street-friendly formulations. Both the MON and RON have value; however, unless you need to choose a race fuel based on its RON or R+M/2 number for a certain racing class, experts recommend that you base your fuel choice on its MON.

But Day adds an important last word on octane. "All racing fuels are not the same, or made the same way. Between two different brands of race fuel, the one with a higher octane number doesn't necessarily mean that it's a better fuel. You only need an octane number to satisfy octane requirement, but a quality racing fuel is about the entire package, not just its MON."

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Race Fuel Properties

Speaking of the entire package, let's get into how race fuel works—we've already discussed motor octane, research octane, and R+M/2 values, so let's cover a few more important properties of race fuel:

A fuel's burning speed describes how quickly it releases energy. Different race fuels contain different amounts of additives, which affect the speed at which that fuel burns. This gets ultra-technical real fast, but basically, the goal is to create peak cylinder pressure (and power) for your specific engine type and motorsport.

Its energy value describes how much potential energy a fuel can make, and is measured in BTUs.

The cooling effect describes how well a fuel helps cool an engine's intake mixture as the fuel goes from a liquid form to a vapor. Related to the heat of vaporization, cooling effect benefits all engines as it increases the density of the inlet air and therefore, improves volumetric efficiency.

Its specific gravity is the ratio of a liquid's density compared to water, at a specific temperature. The specific gravity of race fuel can range from the .690s to the low .800s at 60 degrees Fahrenheit.

A fuel's Reid Vapor Pressure (RVP) is a liquid fuel's evaporation characteristics, measured in pounds per square inch. You've experienced RVP by buying summer and winter gas: summer fuels are blended to have 7 to 10 psi RVPs, preventing the fuel from evaporating in the summer heat (and creating high gas tank pressure and airborne pollution). Conversely, winter fuels can have RVPs of 10 to 14 psi, as there's less chance of evaporation in cooler temps. A quality race gas has around 6 to 7 psi of vapor pressure; carbureted engines can't have too light of a vapor pressure, or it can cause vapor lock.

Its distillation is a measurement of a fuel's evaporation tendencies when heated. As fuel is a mixture of many different hydrocarbons, it will evaporate at different temperatures until the end point (E.P.) is reached. An example could be, "10% evap @ 141.0 F, 50% evap @ 174.0 F, 90% evap @ 214 F, E.P. @ 260 F."

The oxidation stability measures a fuel's resistance to breakdown by using oxygen, pressure, and heat, and recording the amount of time it takes to break down while in storage.

A fuel's oxygen content shows if it is oxygenated or not. Some manufacturers simply say "Yes" or "No," and some show a percentage: "0" for none, or "3.5."

Its lead content shows if a fuel has tetra ethyl lead or not, and if so, how much.

And finally, the color is simply the color of that specific fuel.

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Oxygenated Race Fuel

Highly oxygenated fuels are the relative newcomer to the race fuel scene. They have oxygen molecules in them, thanks to oxygenating components like methanol and ethanol. Compared to straight hydrocarbon race fuel, they require richening the mixture, but they also make more power. However, some oxy fuels aren't as low-maintenance as hydrocarbon fuels, and are best for dedicated racing vehicles. See the "How To Choose The Right Race Fuel" sidebar in the extended web version for more information.

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