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Exposing the Myths of High Octane Fuel and the Definition of Detonation


It is often stated that " The higher the octane, the slower the burn" AND there is PLENTY of discussion on the internet that would support this statement... BUT... It is my opinion that it is really not that "cut and dry"

First, a little tech info..

Octane is the fuel's ability to resist detonation. The higher the octane, the higher its resistance to detonation.. This is understood.. It is not this article's intent to dive into how these octane numbers are derived. Suffice to state that different companies/countries derive these ratings quite differently. We can leave it at that for the time being.

Detonation occurs AFTER IGNITION... This is very important point in the discussion. MANY confuse/equate detonation to pre-ignition. Pre-ignition is just that.. PRE IGNITION and occurs BEFORE IGNITION.. So, detonation and pre-ignition are very different. Now, pre-ignition will generally lead to full blown detonation.. but it is its own entity and should be treated as such.

During the compression process, the fuel/air mix is being "squeezed" into a small area. Shortly after this squeezing process begins, ignition occurs. Ignition will start a flame kernal / flame front that will propagate and expand from the ignition source (generally a spark plug in a gasoline engine) outward towards the oncoming piston.  Meanwhile, the fuel/air charge is still being compressed and forced into the combustion chamber area. This compression creates a pressure and this pressure creates heat. The flame kernal is also creating heat and aiding in the rapid rise is pressure.

Side note: The speed at which this flame kernal propagates is ,greatly, influenced by the head geometry, the amount of pressure surrounding it, the residual "mix" present and its chemical makeup.

OK.. back to the topic , at hand... So, we have a rapid expanding flame kernal, a fast approaching piston, and a fuel/air mix all occupying the same space and this space is getting smaller, VERY QUICKLY! This, my friends, is compression in an internal combusted engine.  When the temp gets to a point that this fuel/air mix FULLY ignites, then we have combustion.

The key to proper combustion is to have it occur at the best time to extract the most energy from the fuel/air mix , at the best rod angle to impose the max amount of leverage on the rod, and to perform work on the the crank train via force.

IF the combustion process before or after the above criteria are met, then you still have combustion but it is not optimum combustion. For this tech article, we will only concern ourselves with combustion that occurs too soon with respect to the rod angle. This is what we will term uncontrollable combustion or DETONATION.

Detonation from Wikipedia:

Detonation is a process of combustion in which a supersonic shock wave is propagated through a fluid due to an energy release in a reaction zone. It is the more powerful of the two general classes of combustion, the other one being deflagration. In a detonation, the shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration, which propagates at a subsonic rate (i.e., slower than the sound speed in the material itself). Because detonations generate high pressures, they are usually much more destructive than deflagrations.

Detonations can be produced by explosives, reactive gaseous mixtures, certain dusts and aerosols.

In more simplistic terms (via RK Tek): Detonation is when the charge is prematurely combusted due to very high heat/pressure. This is more of an explosion than a combustion and the forces produced from this are VERY large and are super-sonic in nature.

So, why is detonation so bad? After all, you have extreme pressure occurring at very high speeds. This should produce some serious force on the rod and spin that crank with much more authority. Yes, it produces forces that we would love to have acting on our crank, unfortunately, these great forces occur at the wrong time in the stroke.  They occur at a rod angle that causes great STRESS on the crank, piston, bearings, and rod. So, this force becomes destructive vs. productive and since the force is so powerful, it WILL cause engine damage if not addressed.

So, how do we address and prevent the onset of detonation?

1) Proper combustion chamber design (i.e. geometry and volume) See RK Tek for this variable.

2) Proper ignition timing (adjust timing until optimum)

3) Proper Fuel amount and Octane (resistance to detonation) fuel.

OK, FINALLY, we get to fuel octane...

The octane rating of the fuel is directly related to its resistance to detonation. The higher the octane the higher pressures/heat the fuel can withstand before ignition. The rate at which this fuel burns has VERY little to do with the octane rating. The chemical soup of the fuel will determine the fuel's burn rate under IDEAL conditions. Since we KNOW that under combustion (active radicals, residual un-burnt mixes , squish action etc.) is NOT IDEAL, then this burn rate can not be properly established nor calculated. We do not need to know this burn rate because we will adjust our ignition and combustion chamber design to accommodate ANY burn rate.  So, it is really irrelevant for us. 

Does the higher octane fuel burn at a slower rate than lower octane fuel? IMO.. MAYBE, SOMETIMES, YES, and NO..  See above.. the rate of burn is dependant on many variables but to state that just because the octane is higher, then the burn is slower, MAY NOT always be a correct assumption.

Don't believe me?? Why should you? there are countless articles contradicting what I just said.. Well.. see an interview from the Sunoco Engineer below (I will highlight the best parts LOL)


Popular Hot Rodding Magazine, January 1998
By Scott Parkhurst

Octane is a measurement of a fuel's resistance to ignition. Ideally, the air/fuel mixture will ignite at the proper time and burn smoothly through the power stroke. The idea is that one powerful combustion of the air/fuel mixture is better than several randomly-ignited small flame fronts. When you can precisely control the point at which the fuel will ignite, maximum performance of the engine can be achieved, and power-robbing knock and ping will be eliminated. Knock and ping are a result of abnormal ignition, or multiple flame fronts colliding within the combustion chamber during the compression stroke.

All reputable fuel manufacturers determine the octane rating of their gasoline in the research lab using a special, dedicated single cylinder engine. Comparing the gasoline to a series of standard reference fuels in the test engine results in either a research octane number (RON) or a motor octane number (MON) depending on a set of operating conditions. The RON is determined with the test engine operating at 600 rpm, at standard barometric pressure, and the intake air temperature set at 125 degrees Fahrenheit. RON is primarily used to address part-throttle knock and ping problems. The MON addresses wide open throttle operation and is determined with the test engine spinning at 900 rpm, also at standard barometric pressure, and the intake air temperature pumped up to 300 degrees.

The best predictor of a fuel's performance in a street/strip machine is the Anti-Knock index (AKI). This is simply the average of the RON and MON numbers, or (RON + MON) / 2. Most all octane ratings posted at the pumps are determined by this AKI formula, and are the minimum values you could expect to see. The minimum octane requirement
of your engine is determined by several variables besides the compression ratio. The engine and cylinder head configuration, air/fuel mixture, timing, coolant temperature, atmospheric pressure, relative humidity, and ambient air temperature will also affect the octane required to make your mill produce maximum power.

The burn rate of a fuel is a measurement of the time required for complete combustion of the air/fuel mixture. The notion that octane ratings affect the burn rate of fuel is about 180-degrees from reality; burn rate is a function of several variables, and the two are completely independent, although there is generally a correlation between octane ratings and burn rates.

To give you a good example of this, we contacted Jim Wurth from Sunoco Race Fuels. He explains, "A perfect example is Sunoco Maximal, which is our fastest burning fuel, and coincidentally one of Sunoco's highest octane fuels at 116 (R+M) / 2. A lot of Pro Stock teams rely on Maximal for those sub-seven second runs. When they are turning 9,000 rpm or more, the fuel has to burn pretty quickly to achieve complete combustion".

Octane boosters offer little help in the quest for higher octane. Most popular street-legal octane boosters claim increases in octane ratings up to five points, and those boosters intended for off-road use only claim up to seven points. That's a lot of octane to hope for simply by pouring an additive in a tank. Sunoco told us that before they launched their GT-100 Unleaded retail pilot program, they wanted to be sure that a 100 (R+M) / 2 octane street legal fuel would be of value, and that enthusiasts would not be able to get the same (or better) results using an octane booster. Nine of the most popular retail octane boosters were put through a series of tests to determine where the consumer could get the most bang for the buck. The test results were verified by an independent testing facility, using several brands of regular unleaded and premium gasolines, just to make sure everything was legit.

According to Mark Borosky, Vehicle Test Engineer for Sunoco,"Of the nine octane boosters tested, none showed a significant increase, and one actually lowered the octane number of the test gasolines."Testing repeatedly showed a maximum increase in octane of 3.5 points by only two of the six street legal octane boosters when the recommended treatment rate was blended with lower base 87 octane gasoline. The best the remaining four products could muster was less than a one point increase. "While clearly no one would actually use an octane booster in a low base octane fuel, we wanted to give the manufacturers the benefit of the doubt relative to their claims of five-to-seven point increases," explained Borosky.

When tests were performed using 93 and 94 octane fuel, even the two best products from the previous tests produced a disappointing 1.5 to 2 point maximum increase. The remaining four street-legal octane boosters showed less than a .5 point increase. Those products designated for offroad use only didn't fare any better than the street-legal products. Subsequent tests where the dosage of octane booster was doubled, tripled, and even quadrupled produced only minimal improvements in octane, regardless of the base octane number of the test gas. In fact, quadrupling the treatment rate of the most powerful additive produced only a 3.5 point increase in octane when added to 93 premium, resulting in a cost of $3.25 a gallon.

An alternative path to octane euphoria is to blend gasolines of different octane levels yourself. It's easier than you may think, safe, and the results are predictable. The formula for mixing gasolines of the same type is pretty straightforward. When you mix a 50/50 blend of two unleaded fuels, simply average the two octane ratings to determine what's in the tank If you mix 94 and 100, you get 97. The same generally holds true for leaded gasolines, assuming the lead content is
nearly equal.

Blending a leaded fuel with unleaded, however, pushes the octane up a bit more than the math would suggest, due to the effect of the lead. Just a gram or two of lead blended into the unleaded fuel will raise the octane number significantly. Commercial leaded racing fuels contain anywhere from a trace to six grams of lead per gallon. If you were to mix 50 percent 110 octane leaded fuel with 100 octane unleaded, you would actually end up with an octane number around 106 to 107. Keep in mind that even the smallest amount of lead or leaded gas line with unleaded, could spell the end of your catalytic converter or oxygen sensor. The same holds true for using octane boosters intended for off-road use only. A word to the wise, check for any lead content in all the additives you might mix with your unleaded gasoline. And check with your state emissions regulations for street use.

We asked Sunoco's Wurth about using aviation fuel in an automobile engine. He was emphatic when he said, "Don't do it. Even though Sunoco is a major producer of aviation fuel, this fuel is specifically blended for aircraft engines. Aircraft operate under very different conditions than automobiles, and the fuel requirements are quite different as well. Aircraft engines generally.... run within a very narrow rpm range. There's no need for transient throttle response in an airplane because after the pilot does the initial engine run-up, the throttle is set in one position and the rpm doesn't normally change until landing. Also, airplanes fly where the air is cold and thin, and the atmospheric pressure is low. These are not even close to the conditions your street machine will see on the ground. Also, since most piston-driven aircraft cruise at 3,000 rpm or so, the burn rate of aviation gas is much too slow for any high performance automotive applications."

What is it that makes race gas so different? What's it made of? Sunoco tells us their GT PLUS 104 octane unleaded race gas is only 15-20 percent traditional gasoline, and about 85 percent additives! Actually there are about 120 different chemicals in GT PLUS. One reason it isn't street legal is the high oxygen content. The EPA requires that the oxygen content of a street legal fuel cannot exceed 2.9 percent. GT PLUS is about 3.5 percent oxygen. This fuel is light in weight at only 6.14 lbs-per-gallon. The high oxygen content improves the octane, and when the induction system is properly calibrated, this fuel will help make additional horsepower. The high oxygen content has a supercharging effect, since 3.5 percent oxygen is the equivalent to about 17 percent more air. Different fuels can actually alter horsepower 5-to-10 percent or more.

We wanted to to know more about the different types of race gas Sunoco had, and didn't realize there were five different types of racing fuel alone.

GT-100 Unleaded, is a clear fuel with a pump octane of 100, and will handle compression ratios of up to 12:1, and is street legal in all 50 states.

GT PLUS, is also unleaded, and is rated at 104 octane. It is suitable for compression ratios up to 14:1 and is colored light blue. It will not harm oxygen sensors or knock sensors in computer controlled engines. It is not street legal.

STANDARD, is a leaded fuel rated at 110 octane, is colored purple, and is intended for drag racing, road racing, and race boats.

SUPREME, is also a leaded fuel and is dark blue. It was developed to help resist vapor lock and meet the demands of sportsman, modifieds, offshore powerboats, and endurance racing where engines regularly run in excess of 7000 rpm.

MAXIMAL, we mentioned earlier, is colored red, has 116 octane, and is leaded. It is intended for exceptionally high performance applications like Pro Stock where extremely high cylinder pressures are common. Its extremely fast burn rate is satisfactory where rpm exceeds 10,000.

Now that you're an expert on gasolines, you probably would like to know where to buy and store the stuff. If you are fortunate enough to live in the mid-Atlantic states, you can take advantage of Sunoco's GT-100 Unleaded retail pilot program and get 100 octane race fuel at pumps located at select Sunoco stations. The rest of us have to purchase from local speed shops, at race tracks, or directly from Sunoco distributors.

When you plan on buying fuel in quantity, say a 55-gallon drum, you'll be happy to know that racing fuel has a shelf life of about a year, if you store it properly. The container must conform to all safety standards, and should be made from metal or polymer. Make sure the container is opaque and solid in color. The white plastic jugs we see at the track should be used for short-term storage only. They let in sunlight, which will affect the fuel. The lead in leaded fuel and other chemicals in unleaded fuel are photosensitive, and will dissipate if they am exposed to the sun. Keep any container tightly sealed to prevent evaporation.


So, the next time somebody tells you that higher octane fuel burns slower, just refer him to this article.

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