What Grade Do You Give Your Engine?
By the way, this is no “knock-knock” joke!

You pull up to the pump and usually have three choices: Regular, Mid-grade and Premium. Are these grades the equivalent of “good, better and best?” Have you ever been tempted to give your car a “treat” and upgrade? Do you really have to feed your high performance car that outrageously priced Premium grade?

These questions are not “fuelish,” and the answers are based on something called the octane rating of gasoline. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. This is meaningful when you recall how a four-stroke, gasoline-fueled engine works. One of the strokes is the compression stroke where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a sparkplug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. High performance engines generally have a higher compression ratio. Higher octane fuel can better tolerate the greater pressures in high compression engines.

When gas ignites by compression rather than because of the spark from the sparkplug, it causes a condition known variously as knocking, pinging or pre-ignition. Knocking generally occurs when climbing steep grades, rapidly accelerating or driving at unusually high altitudes.

Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like Regular-grade 87-octane gasoline) can handle the least amount of compression before igniting. So, the compression ratio of your engine determines the octane rating of the gas you must use in the car.

The gasoline grades have corresponding octane ratings, typically 87 for Regular, 89 for Mid-grade and 91 for Premium with slight variations around the country. Most filling stations sell three grades, but some offer a few more choices between Regular and Premium. Even so, the typical gas station has just two underground storage tanks. Midgrade and other octane ratings are created by blending these two at the pump.

The basic question we want to answer is, “Should you consider spending the extra money for Premium gas?” The answer is a qualified, “No.” The fact is, for most car and truck engines, Regular grade 87 is fine.

That’s good news, but what if your owner’s manual recommends Premium grade fuel? The key word is ‘recommends,’ which really means it’s okay to use Regular under most driving conditions. Virtually every expert – even those from oil companies and auto manufacturers – say that using Regular will not harm engines, but you’ll probably not get the ‘advertised’ performance. In other words, you may add a second to the zero-to-60 mph time a reviewer said you should expect from your car. Again, using Regular gas will not damage the modern engines of most of today’s cars.

Modern engines with advanced computerized engine management systems rapidly adjust their ignition timing at the first indication of knocking. For this reason, using Regular will not void your manufacturer’s warranty unless, in the rare case, your car’s owner’s manual states that Premium grade gasoline is REQUIRED. The owners of high performance engines that require the highest octane are well aware of this fact because high performance was an important factor in choosing their particular cars.

However, if you own a “classic” car or a vehicle that is 10 years old or older and lacks such stuff as electronic fuel injection, computerized engine management and knock sensors and the owner’s manual specifies Premium grade fuel, DON’T USE ANYTHING OTHER THAN PREMIUM! If your vehicle has a supercharger and you drive aggressively, you might also want to use Premium. In this latter scenario, knock sensors cannot sense the condition fast enough because the supercharger boosts pressure too quickly. Of course, if your engine does experience knocking, no matter what vintage or sophistication, feed it the next higher grade of fuel or bring it in for a diagnosis and possibly a tune-up.
Using Premium grade fuel in a vehicle designed to operate on Regular is a complete waste of money. It won’t add to performance since the engine is not designed to make use of the higher octane. Don’t believe that bunk about Premium gas doing a better job of keeping an engine clean. All grades of gasoline contain detergents and additives intended to promote clean combustion.

I under stand that ,used to ran turbo corvair in early 70s,Gosh Im old!
even back then they pressure retard on dist.