If you’re paying too much at the pump, read on as our Mechanic Next Store explores the mysteries behind gasoline pricing and octane ratings.
When it comes to gas for your vehicle, is it all the same? Is there a difference between the “name brands” of Exxon, Mobil, Shell versus the “grocery store gas” at Kroger or WalMart or even the less common “off-brand” names sold at discount stations with odd-sounding names that include “Kangaroo,” “Pure,” or “Liberty”?
And what about the different grades of gas available at starkly different prices. Call it what you will – regular, mid-grade, premium, 87, 89, 93, or even “V-Power” if you happen to be filling up at a Shell station. By choosing one fuel over another, are you risking damaging your engine in the interest of saving money?
Let’s start with the easy answer. Unless the station attendant is bringing your gas out in a metal bucket or dispensing it from a pump with a glass globe on top so you can see the “quality” or lack thereof, like they did at the earliest gas stations, there’s little difference in quality no matter where you buy gas. Gas quality today is regulated and legally required to contain certain levels of detergents, octane, ethanol and other ingredients. And while “name brand” gas might contain more engine-cleaning detergents, there’s a good chance that the gas found at “off-brand” stations was actually produced by the same name-brand manufacturers you know. Save some money and buy gas where it’s convenient for you and easiest on your wallet and comfort level.
The bigger, and age-old question and debate on most motorists’ minds is, “do I need to spend more money on a higher grade fuel, and if so, which one, and why?” There are generally three grades of unleaded gasoline available at nearly all U.S. gas stations, regardless of name, with the price per gallon rising in tandem with the fuel grade. Depending on what you drive, these grades matter.
To make an informed decision, you need to first understand what those numbers mean. The results might surprise you. Spoiler alert – a higher number doesn’t necessarily mean the gasoline supercharges your engine.
The three numbers in question are simply octane ratings, which mean nothing to most drivers unless they’re a chemical engineer, or work in the petroleum industry. When crude oil is refined (cracked) into gasoline and other byproducts, the end results are products composed of hydrocarbon chains of varying lengths. For example, methane has one carbon atom, propane has three, hexane six, and octane eight. Thanks to Mother Nature, it turns out that octane – technically iso-octane – with its eight carbon atoms and 13 hydrogen atoms, resists detonation really well, as compared to, say, heptane, which ignites fairly easily. An 87-octane rating means the gas is composed of 87 percent octane and 13 percent hexane and/or other ingredients. Pushing the “91” button at the pump delivers gas that’s 91 percent octane, and so forth. You get the picture.
Fascinating – but what’s this got to do with your engine, and possibly saving some dollars at the pump? Simply put – as the octane rating goes up, so too does the gasoline’s ability, when mixed with air in the engine’s cylinders, to withstand compression without spontaneously detonating or igniting. In gasoline engines, the air/fuel mixture inside the cylinder is supposed to ignite only when a small flame is sparked by – you guessed it – the spark plug. As that small flame gradually grows and spreads out within the cylinder, the air/fuel mixture should ignite in one detonation. Problems arise, mainly in the form of an audible “knock”, when more than one detonation occurs within the cylinder. And that “knocking” or “pinging,” or “pinking” if you’re in the U.K., can be more than just an annoyance and rob your engine of power – it can also destroy it, quickly or over time.
As that initial flame grows, pressure and heat within the cylinder rise. Under the right circumstances, those increases will cause the air/fuel mixture that hasn’t yet been reached by the flame to detonate, resulting in two detonations – one from the flame and a spontaneous one from the increased pressure and heat. The knocking sound results.
Most modern vehicles have knock sensors on the engine that can tell when a knock is about to occur and can adjust the spark’s timing just enough to prevent the premature explosion. A higher octane fuel is better able to withstand the increased pressure or compression, thus preventing spontaneous detonation.
Does your vehicle need higher octane?
But that doesn’t answer the question of which engines need higher octane fuel. It’s a question with several answers. For starters, high-performance engines need higher octane fuel. That’s because the engine’s designers engineered it to generate higher compression within the cylinder and increased power. Higher pressure and lower octane, however, isn’t a good match.
To help determine what octane rating your vehicle needs, start by looking in the owner’s manual. Other good sources are two lists in this article that specify which vehicles require premium gas and those for which it’s just a recommendation. For example, Acura’s MDX, RDX and RLX are all on the “premium-required” list, as are Audi’s A4 through A7, several BMW models, Chevy’s Camaro and Corvette, the Dodge Viper, and numerous other vehicle manufacturers and models. On the “premium-recommended” list are again Acuras and Audis, Ford’s Escape, Subaru’s WRX and several Volvos. High-performance engines that require a higher-octane fuel and don’t get it will deliver decreased power and performance.
Still other drivers determine whether they need a higher octane fuel through experimentation. If the vehicle runs great on 87 with no knocking, pinging, or performance issues, and choosing the lower grade fuel doesn’t run afoul of any warranty requirements or specific manufacturer guidelines, why spend the extra money on a higher octane fuel?
Knocking or spontaneous detonation can be caused by other factors as well. For starters, the environment can be the culprit. Areas with high temperatures and low humidity can increase knocking and the need for higher octane. So too can vehicle age. Older vehicles can have a buildup of carbon within the cylinder, creating hot spots that lead to pre-ignition. These deposits can also decrease cylinder volume leading to higher pressures. Other culprits include a malfunctioning EGR system that increases cylinder temperature or an improper or malfunctioning spark plug. Increased load – like those that occur when towing or during steep uphill climbs – higher RPMs, or a malfunctioning cooling system that results in higher engine operating temperatures can also bring on the knocking.
Leaded gas and older vehicles
Many drivers will remember another choice available at the pump in addition to the three grades available today – leaded or unleaded fuel. Around the 1920s, a partnership between GM and ESSO, now Exxon, discovered that adding tetraethyl lead (TEL) to fuel helped raise the octane ratings above what they were listed at by increasing the compression ratio. Leaded fuel also came with the added benefit of helping protect soft valve seats, like those found in many 1970s-era vehicles and earlier.
During engine operation, heat from combustion gases causes valves to temporarily weld themselves to valve seats, if only for a tiny fraction of a second. Each time the weld between the two is broken, minute metal pieces from the soft valve seat are torn away, attaching to the valve. Over time, these deposits oxidize and further harden, inflicting damage on the valve seat as the valve continually hammers down. Lead in fuel helped prevent the two from welding, reducing valve seat recession or wear. Unfortunately, lead – which was spewing from the exhaust of millions of vehicles worldwide by that time – is bad for the environment and devastating to human health, which is why it was gradually phased out beginning in the 70s.
That begs the question of what’s a 1970’s muscle-car owner to do to prevent damage in the absence of leaded fuel, short of spending a lot of money to install hardened valve seats or replace a cast-iron head with an alloy one? For starters, don’t overwork your engine, turn consistently high RPMs, or let her get too hot. And, consider adding a lead substitute with anti-wear properties to your gas tank.
For the rest of you, consider using one of the countless octane boosters available, most of which are designated as being safe for turbos, oxygen sensors and catalytic converters, if your vehicle needs it.
And remember two things. If you hear some knocking and there’s no one at your door, it might be time to switch to a higher octane fuel. On the other hand, if the vehicle manufacturer doesn’t specify high octane and there aren’t any performance issues, save some money by sticking with a lower octane fuel, and purchasing it where you want.
Editor’s note: Whether you need a lead substitute, octane booster, fuel additives or even a new engine, stop by Advance Auto Parts is here to help. Buy online, pick up in-store, in 30 minutes.