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Gas Octane

When is higher better?

Just as proper building implosion requires big bangs at the right times at the right places, internal-combustion engines rely on controlled explosions. The correct amount of compressed air and fuel in the combustion chamber, ignited by a properly timed spark, is what forces the piston and rod downward, in turn turning the crankshaft.

To function at maximum efficiency, an engine needs every component of the combustion process to be in sync. The easiest parameter for the driver to control is the grade of gas that's pumped into the tank. Because America is the land of "more better," many motorists automatically assume that only "high-test" premium gas is good enough for their automotive baby. This isn't necessarily the case.

Weather Changes

In reality, an engine's fuel requirements can literally fluctuate with the weather. Ambient air temperature, relative humidity and altitude all affect the burn rate—as do driving habits and the engine's age and condition. As a result, the proper grade of gas for your vehicle isn't always a choose-one-and-use it proposition. For example, high altitude doesn't require as high of an octane rating, which is why many Rocky Mountain states offer 85-grade gas. Factors that raise octane requirements include high air temperatures, very low humidity and high-load driving conditions such as towing uphill.

To oversimplify this gas-grading business, the higher octane rating, the more resistant the fuel is to pre-ignition ("pinging"). If the spark from the plug arrives in the cylinder before the air and fuel aren't at the proper ratio or compressed to the optimal pressure, the fuel mixture won't burn evenly and efficiently. This pre-ignition syndrome can lead to excessive heat and pressure in the cylinder, and it's usually audible from inside the car as pinging, knocking or rattling. In extreme circumstances, pre-ignition can actually burn a hole through the piston.

So, high-octane gas simply permits engines to maximize performance by compressing the fuel mixture more tightly, which then yields a more-powerful explosion when ignited by a properly timed spark. (As tragically demonstrated in Oklahoma City, more compression makes a bigger explosion; nitrogen fertilizer is inert when it isn't under pressure.)

Luckily, most modern computer-controlled engines have knock sensors that "read" gasoline octane and retard the spark timing if necessary to avoid pre-ignition. In other words, safeguarding the engine is prioritized at the expense of performance and fuel economy. The old-school means to the same end was to drive the car under a load (such as up a steep hill) and turn the distributor to advance the ignition timing until the engine pinged. At that point, the "tuner" pulled over, nudged the distributor in the opposite direction to retard the timing, and did another test run. Ear-tuning was complete when the timing was advanced as far as possible without making the engine knock.

Waste Not

AAA estimates that only about five percent of the vehicles on the road actually need premium gasoline. The best way to tell if your car is one of them is to begin with the owner's manual. Some newer cars' manuals might say to use mid-grade at minimum but that high-octane is recommended. When this is the case, the pricier gas can actually be cheaper in the long run: better engine performance means less right-foot pressure on the accelerator.

One final gas-grade litmus test: Mash that go-pedal occasionally. If the engine knocks or pings, bump up a level at the gas pump.