SCed VXs should be running a MINUMUM of 91 octane - run lower and step into it a lot and the computer cannot compensate for it and will soon destroy the engine through probable piston burnthrough. ?_The U.S. uses the following formula: Road Octane Number = (RON + MON)/2 It is a common practice in many European countries to advertise the Research Octane Number on their pumps, so you may see unexpectedly high octane values when travelling abroad.
Unless SCed, use the 87 Octane the owners manual recommends. One - and a very important - exception is that when a vehicle gets older, the normal build-up of fuel- and lubricant-related deposits in the engine can increase the fuel octane number a car requires to prevent engine knock. For this reason, if a car more than a couple of years old experiences engine knocking, the problem may be solved simply by moving to the gasoline with the next-higher anti-knock index.
Following is a great article on selecting the best gasoline for your needs
By: Phil Coconis/autoMedia.com
Many people pick a gas station based on price and/or convenience, probably because they feel that most brands of fuel are pretty much alike. Is that really the case, though? Since your choice of gasoline directly affects engine performance, economy and longevity, along with the environment, it pays to know the facts about different types of fuel.
RON, MON
Let's start with some basic information about octane rating. The term octane is a familiar one and it's important to follow the recommendation that your vehicle's manufacturer specifies in the owner's manual. That number is actually an average of two different octane numbers (Motor Octane, or MON, and Research Octane, RON). These refer to the fuel's ability to resist "knock" (fuel igniting before the ignition spark and resulting in a "flame front") under different driving conditions. MON affects knock at high engine speeds or loads, RON at low ones.
What do these two types of octane measurements have to do with fuel selection? Well, if your engine knocks or pings under only one of these conditions, changing to a different brand of fuel may solve the problem. But if knock occurs under both conditions, then there's likely a mechanical problem that needs attention. It should not be necessary to use a fuel of a higher octane rating, unless all other remedies have been tried.
[b[]Fuel Myths[/b]
We should address a common myth about octane. Know that using fuel of an octane rating higher than that of the manufacturer's recommendation will not increase performance, fuel economy, lower exhaust emissions or increase engine/fuel system life. Usually, only vehicles with high performance engines (fitted with multi-valve heads and/or a supercharger) require a higher-octane fuel.
Does this fact about octane mean that "gasoline is gasoline" and they're all made according to the same formula? Should you just go for the bargain du jour and pocket the difference? Actually, oil companies are competing with each other, and that means that they don't share formulas with each other, nor do they necessarily get their crude oil from the same place. That means the final product will vary, from brand to brand and grade to grade. (And there's even some slight variation within brands, as gas is reformulated both seasonally and geographically, but that's another story.) You do truly get what you pay for—but is it worth it to you?
Price & Grade
Let's examine some of the differences in price and grade. The first is in the quality of the crude oil used. Cheaper grades have more undesirable compounds, such as sulfur, olefins, and diolefins, which cost the refiner to remove. The more of them removed, the higher the price. Since these compounds contribute to higher exhaust emissions, reduced service life of catalytic converters and sludge/varnish buildup in fuel delivery/intake systems, those who plan to keep their vehicles for many years would wisely opt for the better quality gasoline. One easy tip-off that marginal-quality gasoline is being used is a foul sulfur smell emanating from your exhaust (assuming there's no engine malfunction).
Besides having more of the aforementioned compounds removed from them, higher quality fuel also includes more energy enhancing and fuel system cleaning (detergent) additives. These can yield better fuel economy and performance, plus result in reduced maintenance costs. Reformulated Gasoline (RFG), sold in many states and in most urban areas, has also added to the cost and complexity of modern gasoline refining. Although refined under tight government guidelines, there are a variety of formulas, with varying results in terms of cost and performance, with the better performing formulas generally costing more. RFG, deemed necessary to reduce exhaust emissions, differs from earlier gasoline formulas by having more aromatics removed, such as benzene, sulfur, and olefins, as well as the addition of compounds called oxygenates (which raise the oxygen content in gasoline, thereby reducing CO emissions). There are two popular oxygenates used presently—MTBE and Ethanol.
Oxygenates
MTBE (methyl tertiary butyl ether), produced as a result of reacting methanol with isobutylene, is the more stable (less volatile, or prone to evaporation) of the two, and yields a greater octane increase. It tends to produce fewer driveability complaints, and is friendlier to older-car fuel systems. It can be blended, under present rules, at up to 15 percent concentration levels. It is also the more controversial of the two, as it has a higher level of toxicity (not necessarily more toxic than the gasoline it's blended with, though).
Ethanol is produced from fermented grain or corn. Yep, distilled from mash! It's not any more toxic than hard liquor and can be blended at up to 10 percent concentration levels. In Brazil, many vehicles with some modifications to fuel delivery systems run exclusively on the stuff! For environmental reasons, this is the more politically correct of the two oxygenates.
On more modern vehicles with electronic fuel injection, RFG poses no real problem (except for a slight drop in fuel economy), but owners of vehicles with carburetor engines may notice driveability problems under some conditions, due to the effectively "leaner" working fuel mixture. Carburetor modifications, if done skillfully, can solve this problem.
So what's the bottom line? Don't pay extra for a higher octane, unless your owner's manual specifies it or you've modified your engine for higher horsepower. When it comes to choosing between brands of gasoline, however, you may find that paying a bit more at the pump has a definite impact on performance.
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