What you feed your engine determines it’s ultimate performance. The nutritional performance needs of an engine are influenced by a number of factors. Some of the most important factors include compression ratio, rpm potential and boost pressure (on forced induction applications). Engines that run higher compression ratios (above 10:1), operate at higher speeds (generally over 6,500 RPM) or use some form of forced induction are usually the applications where improving the quality of the fuel results in a significant increase in performance. However, there are even some specialty fuels that allow additional power to be realized over pump gas with stock or mildly modified engines.

Text by Michael Ferrara

Octane: The Magic Number?

Blame that yellow and black sticker at the gas pumps for much of the misunderstanding about gasoline. Octane ratings are important, but they are not the only aspect of a fuel to consider. Each blend of gasoline has several important properties that can affect performance. Octane rating is just one of the performance specifications. Octane is directly related to knock.

Octane & Knock

The octane rating indicates the likelihood of the fuel to experience “knock.” Knock, the onomatopoeia named after the condition, is also known as detonation or autoignition. Whatever you call it, the important thing to keep in mind is that knock is detrimental to performance and reliability. Therefore, it must be avoided.

Fuel Octane ratingsKnock occurs when the fuel-air mixture in the cylinder does not experience an ideal burn. An ideal burn allows the mixture to combust evenly, initiating from the spark plug until all of the air-fuel mixture occurs. In a laboratory environment, the ideal burn will occur at about 100 ft/second in a vacuum. In the turbulence of an engine’s combustion chamber, good flame speeds may be up to 250 ft/second. During detonation, or knock, the burn rate will see a violent 2,000 ft/sec explosion instead of a burn. Burn rates are crucial to how the pressure builds in the cylinder.

The burning of the air-fuel mixture results in a pressure increase. Ideally, pressure builds in the cylinder after top dead center and peaks somewhere between 17-to-20 degrees. This allows the cylinder pressure to produce the most horsepower at the crank. When knock occurs, the pressure cycle within the cylinder doesn’t occur as desired. In fact when knock occurs, the original flame front and pressure wave from the desired spark-ignited front meet a pressure wave from an undesired auto-ignited flame front. When these two pressure waves meet, the pressure oscillations produce a “knocking” sound. When knock occurs, power is reduced. Rod bearings, connecting rods, head gaskets and pistons may suffer slight damage or catastrophic failure depending on the severity of knock. Elevated temperatures generally result from knock and this can lead to preignition problems that cause the air-fuel mixture to ignite even before the spark fires.

Knock or detonation is not the same as preignition. Pre-ignition occurs when the air-fuel mixture becomes ignited before the spark plug fires. Sometimes temperatures that are higher than normal, or a hot spot in the cylinder, can lead to preignition. While both knock and preignition cause undesired burns of the air-fuel mixture, the difference between the two is simple. Knock or detonation occurs after the air-fuel mixture has started its burn, preignition occurs before. Both produce undesirable pressure waves that affect performance and can result in engine damage.

Avoid the Knock

Fuel in combustion chamberThe sticker at the pump (or the octane rating provided by the race fuel manufacturer) expresses the “antiknock index” or (R+M/2) octane rating of the specific fuel. Fuel is generally tested by both a “Research Octane (ASTM D2699-92 [105]” and a ‘Motor Octane (ASTM D2700-92 [104]” method. The “R+M/2” number provided at the gas pump is the average of the values obtained by both methods. Both testing methods use a Cooperative Fuels Research engine that allows the operator to vary the compression ratio of the engine (from 4:1 to 18:1) and monitor for the presence of knock. The Research Method tests for knock at 600 RPM with the ignition timing fixed at 13 degrees Before Top Dead Center (BTDC). The Motor Octane method tests at 900 RPM but varies the ignition timing based on the compression ratio being tested. In general, Motor octane ratings will be lower than research numbers since the testing iS indicative of a more severe, high speed, high load condition.

This number is important but it is just the tip of the iceberg. Bigger turbos, ported cylinder heads, performance camshafts, free-flowing exhaust systems and air intake systems offer effective methods of getting more air into the engine. However, it is always the engine’s ability to turn the fuel into energy that allows horsepower to be produced at the flywheel.

Need for Higher Octane

If your engine is experiencing knock, you need to run a higher-octane fuel. The need for fuels with a higher octane rating generally occurs as peak cylinder pressures rise. Peak cylinder pressures tend to rise as compression ratio, volumetric efficiency, ignition advance and boost pressure rise.

The general rules are simple. Naturally- aspirated engines will need a higher-octane fuel if either compression ratio is increased or ignition timing is advanced. Forced-induction engines respond the same as naturally- aspirated engines, but octane requirements will increase as boost pressures rise.

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.

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