I have not known where alse to turn for this answer. Which is better? The thing is I am intersted in a more in depth answer than ,“premium does not give you better gas milage so it is not worth the cost.” I have a suspiscion that if you take in the environmental cost, premium may be better in some way. Does it use more pure ingredients and therfore burn cleaner? Does it simply keep your engine cleaner and therefore bruning cleaner longer? What else?
Mixim
The difference in gasoline grades is octane. Most pumps these days have 87, 89 and 92 octane gase. Octane is a rating for the resistance to pinging, the clattering sound a gasoline engine can sometimes make when it sounds like a diesel. The pinging sound is detonation where the fuel and air explode all at once in the cylinder instead of burning progressively. Too much detonation will damage your engine over time.
As long as you don’t experience pinging there is no benefit from buying higher octane gas. Check your owner’s manual for the reccomended grade but listen to the sound your engine makes. Different driving conditions such as heavy load and high temperatures can cause ping.
My Jesus fish can beat up your Darwin fish but forgives it instead.
And before I forget my manners welcome to SDMB. That is unless you and an old timer here and just changed your handle then welcome back.
A higher octane rating means that the fuel burns at a higher temperature. High compression engines get a lot hotter than low compression engines. If the interior of the cylinder gets hot enough to ignite the fuel mixture before the piston is fully compressed (at the top of it’s cycle) and the spark plug sparks, you get “knocking” and “pinging”, and over time that can cause some serious damage to the valves, piston rings, etc. “Premium” doesn’t mean “better” by any means. Premium simply means that the fuel burns at a slighty higher temperature than regular gas. If you have a normal, everyday car, 87 or 89 octane gas is what you want. No need to pay for something that you don’t need.
To determine which octane to use, run your car on all available gasolines in your area. Keep track of gas milage, and you can see which is actually costing you more money per mile driven. The least expensive gas that doesn’t make your engine knock/ping will be the one to choose. 
The only other difference is that the premium usually has more/better additives or detergents that keep the fuel line and more importantly the fuel injectors cleaner than the regular.
I believe that all grades of gasoline have the same additives. At least that is the case in California regarding additives that clean fuel injectors.
I would also think that all types of gasoline would have to meet Federal air pollution standards also.
While that’s all true, it also needs to be said that most modern engines have knock sensors that will back off the ignition timing at the first hint of such a problem, since knocking is really bad for the engine.
There are a few engines such as the LS1 that use high compression ratios and thus like high octane gas. If you feed them a lower octane, even if this would cause knocking you probably won’t hear it because the engine will deal with it before it becomes audible to you. The most significant effect will be a reduction in the maximum available power due to retarted ignition timing.
So basically, if your engine is designed for high octane gas, then use it; otherwise, it’s a waste of money. But with certain engines, there might still be an advantage to using premium gas even if you don’t hear any knocking or pinging with lower octane gas. Following the manufacturers recommendation is probably the best course.
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peas on earth
You make a good point bantmof. My last car, BMW 325i, had that kind of engine management system and would not knock with any gas though it was capable of more power with higher octane. I wasn’t aware that was commonplace. My brother has a new Saturn LS1 (I think, whatever the big Saturn is) but he isn’t exactly a gear head so we didn’t discuss octane and ignition timing.
My Jesus fish can beat up your Darwin fish but forgives it instead.
And just to avoid confusion here, by LS1 I meant the GM engine (the next generation of the LT1), not the Saturn car also called LS1 :-).
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peas on earth
I recall in another thread a dutch poster mentioned that octane ratings lower than 91 (IIRC) are unheard of in Europe. Now I’ve driven in several European countries but never noticed the octane rating.
Do they use a different rating system over there?
Hell is Other People.
Some owners manuals for newer cars even state different horsepower ratings at different octane levels. This is due to the above mentioned engine management systems.
I just use the grade recommended in my manual. A fill-up lasts me 2 weeks, so the price difference isn’t really that great a deal.
Peace,
mangeorge
I only know two things;
I know what I need to know
And
I know what I want to know
Mangeorge, 2000
This question seems to come up every couple of weeks.
bantmof nailed it.
Also, a gold star to DSYoungEsq.
No. In a high compression engine, the compressed air/fuel charge will be at a higher temperature simply because it is compressed more, but this is a local phenomenon. It does not extend significantly beyond the cylinder. The coolant temperature will be the same for both engines.
And to the OP:
If your car does not need premium, don’t use it. Not only is it more expensive(duh), but harder to light off. In low compression engines, a significant portion of the fuel will exit the cylinder unburned or incompletely burned. Both states are bad for the environment and for your motor.
An earlier thread on this topic is at http://www.straightdope.com/ubb/Forum3/HTML/004555.html
Some literature I’ve seen says that the actual octane rating of any particular batch of fuel may vary wildly from what the sticker says. Also, each engine may have an octane “preference” several points away from identical engines in identical cars. Even worse, winter gas is more volatile than summer gas, in order to make cold starts easier (so don’t wait till summer to use up your y2k fuel cache…).
Since you’ll probably never hear a knock from a modern engine, the idea of tracking fuel economy sounds like the best bet (if your favorite flavor gasoline stays consistent over time).
Consumer Reports mentioned also that some companies (like Chevron) use the same additives in all their grades, just different amounts. Some others may not have decent injector cleaners in anything but premium.
Knock-proof engines have been around for a while. My 1992 VW has one. for pete’s sake. The owner’s manual says (more or less), “Use premium if you can get it, but use anything in a pinch.”
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
I have a cheapo Suzuki sidekick and used high octane for a while, but the dealer said don’t, it will damage the engine. I guess I’m just repeating what everyone else said, “depends on the engine”. Trashing your engine is about the least economical and environmentally friendly outcome you can have.
I know sod-all about octane ratings, but the owner’s manual for my 1998 Honda Civic Coupe (which, geek that I am, I sat down and read), specifies the octane of fuel to be used and states that you should NOT use a higher-octane fuel than recommended.
Jodi
Fiat Justitia