July 7, 2005
Who’s got gas?
Quick question: What octane gas do you use in your car, and why? With gas prices the way they currently are, can you afford to make the wrong choice?
To start, let’s discuss some physics of an automobile engine. A car engine has four phases, or four-strokes. It’s intake-compression-ignition-exhaust. Or, to the enthusiasts, suck-squeeze-bang-blow. Here’s how it works. During the intake, the engine draws in fresh air and fuel. The piston then compresses that air. The spark fires, igniting the fuel and propelling the piston downward. And as the piston returns to the top of the stroke, it forces the spent fuel and air out of the exhaust valves of the engine. Then, the cycle begins again with the next intake stroke. (For a great diagram and illustration of how this works, please see this link.)
So how does octane affect this cycle? Simple. Gas is flammable (duh!). As gas is exposed to the extreme heat and pressure of the compression stroke, it is always possible that it can spontaneously ignite before the spark comes. In fact, diesel engines operate on just this principle, where the gas is not ignited by a spark, but by the pressure and heat of the engine. In a normal (non-diesel) engine, however, this is a bad thing. This is the definition of “knocking” in an engine. Knocking is very bad, and can destroy your engine if you’re experiencing it.
So what is octane? Octane is the rating of how much compression the gasoline can take before it spontaneously ignites. The octane rating of the gas your car needs is largely determined by the compression ratio of your engine. Power is partly a function of compression ratios, so higher performance engines are typically high-compression engines (not counting forced-induction, which is a whole new ballgame). Higher octane gasoline actually is less likely to detonate prematurely, and thus it is required for high-compression engines. And since higher octane is needed for higher compression, you need an 89 or 93-octane gas for most sports cars. You are actively trying to stop the gasoline from igniting before the spark arrives. On the opposite side of the coin, most standard sedans are not high-compression, and require only 87-octane gas.
Simply put, read your car’s manual. If it says to use 87 octane gas, you don’t need to use anything higher for your car to run properly. However, we’ve all seen the marketing, and high-octane gas is usually given names like “Premium”, or as I saw at Shell today, “VPower”. The question that inevitably follows, is whether it is beneficial to use higher octane gas, even though it’s not required? Is it better for my engine? Will in increase performance? The simple answer is no. There is absolutely no benefit to using a gasoline higher than required by your manual:
If you are already using the proper octane fuel, you will not obtain more power from higher octane fuels. The engine will be already operating at optimum settings, and a higher octane should have no effect on the management system. Your driveability and fuel economy will remain the same. The higher octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy. You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of driveability.
Once you have identified the fuel that keeps the engine at optimum settings, there is no advantage in moving to an even higher octane fuel. The manufacturer’s recommendation is conservative, so you may be able to carefully reduce the fuel octane. The penalty for getting it badly wrong, and not realising that you have, could be expensive engine damage.
Always remember, your car’s manufacturer does not recommend a fuel that will damage your engine. And physics and chemistry easily explains that there is no advantage to using a fuel beyond the specifications of your manual. When you see words like “Premium”, understand that you only need that if you have a “Premium” car that requires it. For most of us, it’s just a big fat waste of money. Gas is expensive enough as it is, there’s no reason to make it worse.
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Hi Brad, Good post. Maybe I will do a blog on this. My Toyota Solara uses regular gas. The Z3 requires premium gas. When Bonnie was living in Utah, she drove from Provo to Salt Lake City five days a week. She was on the highway all the way. At the time, she drove a Saturn and when she put in regular gas it got about 30 miles to the gallon. One time she tried premium and she got 40 miles to the gallon. After that she used premium while she was freeway driving. When she moved back to Houston she went back to using regular gas. She was back to the usual bumper to bumper driving again. Now she drives a VW Jetta and uses regular all the time.