Does your car need premium gas or is the hype just a way to get you to fork over more money? What octane do you need? Do you know what you are getting when you pay an extra 10 to 20 cents for mid-grade or premium?
Numerous articles have been published on this topic, most of them saying you are pouring your money down the drain when you pour premium gasoline in your tank. That’s not necessarily true. Some cars require premium fuel, and it usually says so in the owner’s manual if not on the instrument panel near the fuel gauge. Some cars require a higher octane diet over time. Your driving style also affects what you should choose.
What should you put in your tank? First, follow your owner’s manual recommendations. Your car was designed, and the computer’s performance map written, for the fuel recommended. But, if your car knocks and pings when you put a load on the engine, and your technician cannot eliminate the problem, choose the next higher grade of gas. To save a little money, try blending half a tank of regular and half a tank of mid-grade at the pumps. Ultimately, do not buy a higher grade than your car requires.
On average, American motorists are using the right grade of gas for their cars. In an August 1996 research study, the American Petroleum Institute found that “Americans buy no more octane than the vehicle fleet needs. In fact, the market share of regular grade gasoline is higher than warranted by the technical requirements of the vehicle fleet.”
So some people are buying regular gas when they should be buying mid-grade or premium. Others, meanwhile, are wasting their money on mid-grade and premium when regular will do.
While the API’s report found that Americans are buying the right amounts of each grade, the report also showed that too many are buying regular and premium and too few are buying mid-grade. Are you buying the right grade?
It helps to understand just what you are buying when you pull up to the pump.
Octane rates a gasoline’s resistance to knock–that metallic sound that is sometimes called ping. The higher the octane, the more resistant it is to knock.
An occasional light knocking may be OK. But, if it is chronic, it may damage your engine.
Octane is measured in a lab using a special, single-cylinder engine. By comparing a fuel’s resistance to knock against a known reference fuel, the researchers find a Research Octane Number (RON) and a Motor Octane Number (MON).
RON is determined in the test engine at slow speeds and mild temperatures to see how the gas responds to light throttle, moderate driving. MON measures how well the fuel prevents knock at higher engine r.p.m., at wide-open throttle, with the intake air raised to 300 degrees.
To predict how a normal car will perform, the fuel is given an anti-knock index–the average of RON and MON or (R+M)/2. This is the number posted on the pumps at your local station.
Regular unleaded gasoline is rated at 87 octane, midgrade at 89 octane and premium at 92 octane. In the Rockies, gas is often two octane points lower because of the altitude and lower barometric pressure. Regular is 85, midgrade 87 and premium 90 octane.
Many factors determine what grade of gas is right for your car: compression ratio, the cylinder-head configuration, air/fuel mixture, ignition timing, engine temperature, air temperature, atmospheric pressure, relative humidity and altitude. A turbo or supercharger enters the equation as well. Use the lowest octane that prevents knock.
Like most rules, however, this one has exceptions. Today’s computer-controlled engines have knock sensors so you won’t hear the noise, except perhaps briefly. So, you may think regular gas is fine. What you may not notice is the drop in performance. When your engine knocks, the knock sensor tells the computer, which backs off the ignition timing until the knocking stops. Retarding the timing reduces performance when you want it most: in acceleration. You could be a candidate for the next higher octane.
Sometimes the same year, make and model cars behave differently. Your neighbor’s identical car may not knock, but because of a stackup of manufacturing tolerances, yours might. If the knock is persistent, you may need an updated programmable read-only memory (PROM) chip in your car’s computer or need to have it reprogrammed.
Because combustion chamber deposits can build up, your car may require two to three higher octane numbers as it ages than it did when new. The same goes for an engine that is out of tune and whose ignition timing has wandered, a condition more common on pre-computerized engines.
A restricted exhaust system or an engine running too hot (overheating) causes knock. As do malfunctioning PCV (positive crankcase ventilation) valves (an emission system device), excessive oil in the combustion chamber or a leaking head gasket.
Problems in your car’s emission control system can lead to engine knock. Faulty EGR (exhaust-gas recirculation) systems upset the air/fuel ratio. Though most emission system problems trigger a check-engine light, some EGR malfunctions don’t. In this case, higher octane probably won’t help. Nevertheless, don’t mask problems with higher octane; get your car fixed.
If you have a lead foot and like blasting away from a stop, you may benefit from higher octane. Ditto if you pull a trailer, carry heavy loads or climb steep hills.
What about octane boosters? Before launching its new, 100-octane street-legal fuel, Sunoco asked the same question. According to Mark Borosky, vehicle test engineer for Sunoco: “Of the nine octane boosters tested, none showed a significant increase, and one actually lowered the octane number of the test gasolines.”
Testing repeatedly showed a maximum increase of only 3.5 points by only two of the boosters when blended with 87 octane unleaded regular while the rest only increased the octane one point. When added to premium fuels, the increases were even more paltry.
Many premium gasolines also have more concentrated additive packages than regular grades and, like some ads say, can slightly improve your engine’s internal condition. But, lower grades of name-brand gasolines have adequate additives for most motorists. What should you use? Try a variety of brands and grades, but don’t waste your money on something you don’t need.
Don’t be lured by the siren songs at the gas pumps. Choose the gasoline with the lowest octane that gives you the best performance.
KNOCK, KNOCK?
NO JOKE
You may call it knock or ping, but the proper term is detonation. It comes from two flame fronts colliding in the combustion chamber.
In normal combustion, the air/fuel mixture is ignited by the spark plug and the flame moves smoothly across the combustion chamber. A controlled increase in pressure pushes the piston down.
Spark knock occurs when some air/fuel mixture ignites in another part of the combustion chamber.
