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BD from Boston ($2.98 regular — Mass Pike; $3.30 - $3.80 premium) reminds us to discuss cars' need (or not) for high-octane fuel, particularly now that premium is being delivered by armed guards in Brinks trucks rather than truckers in tankers. The quick answer is that an engine's compression ratio (check the manual, or Google your car's specs) determines the need for a particular octane level. The long answer follows after the jump.

A fuel's Octane rating indicates the degree to which it can be compressed before it ignites; among gasoline grades available to consumers, 93 (94 in some places) can withstand the highest degree of compression, 87 (or 85 in some places) the lowest. Using low-Octane fuel in a high-compression engine can result in the gas igniting during compression, not by the spark plug, which is what engine "knocking" is, which can damage an engine.

Still, outside factors can also affect cars' octane requirement, for example, climate — hotter, drier air increases octane requirement — and altitude — the higher above sea level, the lower the requirement — along with driving style — heavy acceleration, stop-and-go traffic, trailer pulling increases octane need.

Carmakers have tweaked cars' tolerance to low-Octane fuel upward, using mechnical technologies and electronic engine management systems to adjust spark timing to acceleration and climate. But in many high-performance and luxury cars — and even some econoboxes — carmakers still recommend 91/93 Octane. The point is, while it's tempting to punch the low number at the pump as a matter of course in the short term, making sure a car's Octane needs are met will mean extended life and keep performance high. Hiho!

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