The info below was copied from cartalk.com
It provides an excellent explanation of what different octane ratings are and what you should be buying, as well as exactly what “pinging” and knocking (a.k.a. predetonation) are.
Best of all, it’s written in laymans terms – no engineering degree required.
The gist of the article – almost nobody actually needs to pay a premium for premium.
Q. My owner’s manual says my car will run just fine on regular, unleaded gas. Will “treating” it to premium gas provide any benefit?
A. Let’s be perfectly clear about this: NO!
The only thing you’ll be benefiting are the portfolios of impoverished oil company executives.
Q. But, won’t premium increase the performance of my car’s engine?
A. No. Higher octane provides no additional performance for cars that are designed to run on regular gas.
Q. You’re telling me I’m wasting my money by using premium?
A. You’re starting to get the picture.
Q. Okay. My owner’s manual does call for premium gas. Do I need to use it?
A. In that case, the answer isn’t quite so clear. In a high-compression engine that’s designed to run on premium fuel, premium will provide some additional power. But, if you want to save some money, you can probably still fill up with regular, unleaded fuel much of the time.
Q. How much will I save?
A. About 20-30 cents per gallon. For an average driver, that can run as high as high as $200 a year.
Q. So, why would my owner’s manual calls for premium gas instead of regular?
A. Because your manufacturer decided to use what’s known as a “high compression” engine.
Q. My owner’s manual says that if I don’t use premium, I might get engine pinging or knocking. Is that right?
A. It’s possible, depending on the load you put on the engine.
Q. What is pinging or knocking?
A. They’re noises, which are signs that the gas and air mixture in your car’s cylinders isn’t burning exactly as intended. The sound you hear is the result of the fuel and air mixture combusting unevenly in the cylinder.
Pinging or knocking will reduce the efficiency of your engine and, over a very long period of time, it can cause damage.
Q. How does it occur?
A. Let us show you! Here’s what’s going on in the cylinders inside the engine during pinging or knocking:
(obviously, the original site’s illustrations can’t be provided here. nonetheless, the explanations that accompanied those illustrations are provided here since they may be useful. if you would like to see the illustrations, google the article. – icot)
Illustration #1 The piston begins to move upwards during what’s called the, “compression stroke,” compressing a mixture of fuel and air.
Illustration #3 However, it’s possible for the mixture of gas and air to burn somewhere else in the cylinder, simply from the pressure (or compression) within the cylinder, and not from the spark. That’s called “pre-ignition,” and it’s more likely in a high-compression engine.
Illustration #4 When the fire you want (that’s the one from the spark plug), collides with the fire you don’t want (that’s the one resulting from the pre-ignition), the result is a ping or, if the pre-ignition fire is big enough, a knock. The degree of severity, including the amount of noise generated, is determined by how big that unwanted flame is.
Q. How does high-octane gas help?
A. Compared to regular gas, high-octane gas requires higher temperature and pressure to ignite, so it can withstand higher compression without reigniting. As such, it’s less likely to result in premature ignition, and therefore prevents pinging and/or knocking.
In other words, it’s a form of protection for high compression engines.
Q. So, I should use high-octane gas to prevent pinging?
A. High-octane gas will prevent pinging.
Q. How does a knock-sensor work?
A. The knock-sensor uses auditory detection to actually “hear” the knocking or pinging. When necessary, it uses this information to delay the spark and to minimize knocking or pinging.
That’s good news. That means you can use gas with a lower octane rating. And, if your engine detects a knock or ping, it can usually adjust the timing of the spark until the knocking is gone.
Q. You’re telling me that modern cars automatically detect this problem, and therefore won’t ping?
Load is the amount of work you’re asking the engine to do, and it’s affected by a number of factors, including climbing steep hills, pulling heavy weight, hard acceleration or driving in hot weather. Any combination of these factors can affect load. So, if you live out in the desert Southwest, it’s 115 degrees out and you’re hauling your mother-in law up, you’re going to need premium.
Why? Because there’s only so much that your car’s knock sensor and computer can do to adjust the engine’s timing.
Q. Is it a big deal if my car pings for a few seconds here and there, under these circumstances?
A. No, it’s not.
Q. So, are there any circumstances under which premium gas would be a good idea?
A. If you plan to haul that aforementioned mother-in-law in hot weather, or are going to be driving up monstrous mountain passes with a heavily laden car, then you might consider filling up with a tank of premium gas.
Q. But, under most normal operating conditions, do I lose anything if I use regular unleaded instead of premium?