Fuel Octane Ratings & Recommendations
How much octane does your engine really need?
The least amount that’s necessary to prevent detonation (spark knock). On most vehicles, that’s 87 octane regular grade unleaded gasoline. But on higher compression engines, or turbocharged or supercharged engines, the engine may require premium grade 91 to 93 octane fuel.
Detonation (spark knock) occurs when the octane rating of the fuel isn’t high enough to handle the heat and pressure. Detonation is most noticeable when lugging the engine under load or accelerating. It may should like a pinging, clattering or rattling noise. Instead of a single flame front forming when the fuel is ignited, multiple flame front form spontaneously throughout the combustion chamber. These collide and produce shock waves that cause the noise. The hammer-like blows produced by detonation are very hard on the pistons, head gasket and bearings and may damage the engine if the problem is not corrected.
Other common causes of detonation (besides low octane gasoline) include: a faulty Exhaust Gas Recirculation (EGR) system, too much compression due to a build up of carbon deposits in the combustion chambers (use a top cleaner to clean the combustion chambers), a lean fuel condition (dirty fuel injectors), or engine overheating.
Most late model engines have a “knock” sensor to detect engine vibrations caused by detonation. When the sensor detects detonation, it signals the PCM to temporarily retard spark timing. This helps protect the engine from possible detonation damage, but it also reduces engine performance and fuel economy.
One way to prevent detonation is to use a higher octane fuel. The octane rating of a motor fuel is a measure of its detonation resistance. The octane that’s posted on the filling station pump is “pump octane,” which is an average of something called “research” and “motor” octane ratings (which are two different laboratory methods of measuring octane). The higher the pump octane number, the better able the fuel is to resist detonation.
A gasoline’s octane rating depends on the blend of hydrocarbons in the fuel and other ingredients that are added to it. Tetraethyl lead was long used as an anti-knock additive to improve gasoline octane. In fact, it was the most effective and least expensive octane-boosting additive that could be used for this purpose. But leaded fuel cannot be used in a vehicle with a catalytic converter because the lead fouls the catalyst. So
unleaded fuels use other octane-boosting additives such as MBTE or ethanol alcohol.
Most regular grade gasolines today are rated at 87 octane, which is sufficient for engines with compression ratios of up to about 9 to 1. Higher compression engines, engines with turbochargers or superchargers, or ones used frequently for towing usually require a higher octane rating or a premium grade of gasoline.
CAUTION: Follow the fuel recommendations in your vehicle owner’s manual. If your vehicle requires premium 91 or higher octane fuel, use a premium grade of gasoline.
If you can’t find pump gas with a high enough octane to prevent detonation, you can always add an aftermarket octane boosting fuel additive to your fuel tank. Such products can boost the octane rating of ordinary pump gas several points depending on the concentration used (always follow directions).
But even this might not be enough to eliminate a persistent spark knock if your engine has an underlying problem.