High Octane Fuel

How to tame the octane octopus
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Every time gas prices creep higher, you might find yourself wondering if you really need to pay the extra change for the upgrade to premium or super duper grade gasoline. The answer to that question is no. Unless you're driving one of the very small percentages of performance vehicles with an engine designed specifically to utilize the properties of higher-octane premium gasoline, there is no performance gain in using premium. It might make you feel better, but your engine won't know the difference.

For the most part, vehicles made after 1985 or so should run just fine on regular 87-octane gasoline. The good majority of engines out on the road today have relatively low compression ratios, and are well suited to use with regular gasoline. There are of course exceptions to every rule: if you're out on the street running a high-compression engine in a racecar disguised as a grocery getter, then you know who you are and know what you need. Understanding what octane is and why there are different grades of gasoline begins with a simplified version of engine operation and straight chain hydrocarbons.

Knock. Ping. Detonate.

In a four-stroke engine, the pistons are doing one of four things: taking in a breath of fuel and air into the cylinder through the intake valve, compressing the air and fuel mixture for a spark induced burn, turning the energy created by that burn into a downward power stroke, or expelling that same burnt up mixture out an open exhaust valve on the upstroke. When a piston travels up in the cylinder it reaches the point called top dead center. At this point the piston can travel up no more and, by virtue of being connected to an ever-spinning crankshaft via a connecting rod, must travel back down the cylinder again. For peak efficiency the compressed fuel should start to burn only an instant after the piston reaches top dead center on the compression stroke and the spark lights the mixture.

Even more importantly the fuel-air mixture must burn in an even flame front originating from the spark created by the spark plug. If the fuel-air mixture ignites on its own before it is sparked, this out-of-time explosion produces an audible knock that makes your engine suddenly sound like a clothes dryer full of rocks. What's happening is that an explosion is occurring out of time and ahead of the burning mixture created by the spark. Instead of one even burn propelling the piston back down the cylinder to make power, two out-of-time-explosions are competing against each other. The sound this competition creates is known as knocking, pinging, or detonation and is unfortunately the sound of engine damage!

Detonation is extremely tough on pistons, valves, connecting rods, bearings, and cylinder walls. If your vehicle is not one of the high performance few, but knocks away anyway when you hit the gas after pouring the 87 octane in the tank, then there are more nefarious causes at work under your hood.

Hexane, Heptane, Octane, Flame

From the sludge that is crude oil, straight chain hydrocarbons are refined and lined up in a row to put the tiger in your tank. The greater number of molecules in the chain, the more resistant to compression-induced ignition the fuel is. Octane has eight molecules in the chain like an Octopus has eight legs. The real difference between 87-octane regular gasoline and 91-octane premium is the rate at which it burns when compressed and ignited while inside your engine.

The compression ratio of a given engine is what determines its octane requirements. Since most engines have a compression ratio designed for 87-octane unleaded fuel, all should be well in staying regular with regular. Even if you have a super-performance sportster with a big "91 ONLY" sticker on the gas cap, no harm will likely come if 87 is added by mistake. Modern engines come equipped with a knock sensing system, which listens for detonation and tells the engine computer to adjust spark timing until detonation is eliminated.

Conversely, the engine computer in a vehicle built to run on regular will not sense an increase in octane and adjust ignition timing to take advantage of the higher octane, so pouring in the premium in an engine not designed to take advantage of it is the equivalent of shooting money out of the tailpipe. All this is, of course, is a massive oversimplification, yet the fact of the matter is this: If your engine is designed to use 87 octane, yet rattles away like Carmen Miranda shaking her maracas every time you hit the gas pedal, then something is wrong-and it's only going to get worse.

Less Space, More Problems

High mileage engines often suffer from a buildup of carbon deposits in the combustion chamber. This buildup causes the total volume of the combustion chamber to become smaller. This decrease in chamber volume effectively raises the compression ratio of the engine, and causes low octane gasoline to detonate, and your engine to knock. If a high-mileage engine designed for regular gas knocks unless premium is used, then carbon buildup may be the culprit.

Other causes of detonation include overheating, a malfunctioning EGR valve, too far advanced ignition timing, or a defective knock sensor. Overheating caused by a defective thermostat or cooling system negates the engine's ability to channel away heat from the combustion chamber and causes the compressed fuel to detonate. If the timing is advanced too far, the spark ignites the compressed mixture too early on the power stoke, causing detonation. A defective knock sensor can send the ignition timing pell-mell, and direct sparks into the cylinders astray.

A defective EGR, or exhaust gas recirculation valve, can also wreck havoc on fuel mixture temperature. Always follow manufacturer's recommendations for gasoline but, if your regular old gas engine knocks while drinking its regular old gas, first try switching gas stations, then consider making repairs or adjustments. You could end up saving money in the long run.

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