I have a friend who owns an expensive car and he always insists on filling it up with the most expensive gas (the highest octane rating???) because he says that high-quality engines needs a high-quality gas. Is there anything to back this claim up or is this just a myth?
High-performance engines require high octane; “high performance” means “operating at high engine speeds.” ALL gasoline from a given refinery has the same QUALITY – octane has nothing to do with quality, it’s an anti-knock rating and nothing else. If he’s filling the tank of his M-class with premium gasoline, he’s throwing money away (in more ways than one). For that matter, what the hell is a “high-quality engine,” and what is there about it that requires “high-quality” gasoline? (Answer: a high-quality engine would be manufactured to very tight specifications, which has bollocks to do with “gasoline quality”)
If the car in question is less than 15 years old, he is throwing away his money. Virtually every automobile made lately is designed to function on regular unleaded.
The engine in a car needs whatever grade of gasoline the owner’s manual says it needs. Using a different grade, higher or lower, will probably result in poor performance.
“Premium” gasoline isn’t more expensive because it’s somehow “better.” It’s more expensive because it costs more to refine higher-octane gasoline. And “higher octane” isn’t somehow “better,” either - it’s just different.
The only exception I can think of is the situation I ran into with my old car. As it crossed the 10 years/100,000 miles barrier, it started knocking on very hot summer days, going uphill with the A/C on - in other words, with a very heavy load on the engine. I found that nudging up from 87 octane fuel to 89 octane made the knocking go away, so I’d stick to that for the worst of the summer, then drop back to 87 octane.
Read the owner’s manual for the car. It will recommend an octane rating for that vehicle. Buying more expensive gas is just a waste of money.
If he really thinks he’s doing his car a favor, he can always go to an airport and fillup with Avgas. It’s 100 octane, and with gas prices up, it must be around $3, $3.20 a gallon right now. It’s also got tons of lead in it, so it must be good, eh?
No, there are still many new vehicles whose engines require high-octane fuel, and their owner’s manuals say so, explicitly. Take a look at the specs on a lot of Mercedes, Acuras, BMWs, etc.
Out of my league. On a teacher’s salary, I’m more knowledgeable about Nissans. 
Shoot, silenus, on a teacher’s salary, you should probably be posting in that thread about the gear ratios on bicycles.
Here’s a little blurb I happened to read yesterday on wwwhowstuffworks.com. It’s from their article on how a car engine works, specifically the section on producing more power.
The 97 Honda Accord gets regular.
The BMW mototcycle and the John Deere lawnmower get higher octane.
Why?
I don’t know. Cause that’s what the owner’s manuals tell me.
Just because a car is expensive does not mean it needs high octane gas. Usually it’s high performance cars that need high octane gas. My cadillac for example is designed for 89 octane. Anything higher or lower will make it run worse.
High octane gas isn’t “high quality” even though the big evil gas companies like to adverise it that way. The higher the octane rating, the more the gas can be compressed more before it will explode just from the compression. If you have a high compression engine then you need high octane gas, or else the gas will explode before it’s supposed to, and the engine will ping, run bad, and cause damage to itself.
The engine computers in most modern cars that require premium gas can compensate if some dolt puts regular into the tank, but you still should follow the manufacturer’s guidelines unless it’s an absolute emergency.
Why? Two reasons:
You don’t gain anything. True, regular is cheaper than premium, but the engine is designed to run most efficiently on premium, and the adjustments that the engine computer makes to compensate for the regular gas decrease performance and increase fuel consumption. You may think you’re being clever for saving $2.00 when you fill up, but you won’t feel so smart when your gas mileage drops in half.
By putting regular gas into the tank, you’re trusting the engine computer and its array of sensors not to malfunction or somehow goof up. If it fails to detect that you’re too cheap to pay for premium, it’ll happily let the engine ping and knock itself to oblivion.
Really, I don’t see why people buy $30,000+ cars, and then try to skimp on premium gas. Weren’t they aware of what they were getting into? If you want value and economy, buy a Honda.
Switch to a higher octane gas if your engine is knocking. Otherwise, you’re just wasting your money.
The people saying read the owners manual are correct.
Regarding the cite from How Stuff Works, that agrees with what my engine builder has told me about my race car’s engine. The way he put it in simple words that I can understand is that the octane rating is the burn rate of the fuel. In other words, it is how fast the fuel burns in the combustion camber. A higher octane rating causes a slower burn.
Predetonation (pinging and knocking) is the problem. That’s when the fuel/air explodes prematurely due to compression (and possibly hot spots in the camber). This actually damages the engine. As other have said, a higher octane fuel helps control this problem.
I can buy 110 octane (and leaded) racing gas for my race car. It was $3.59/gallon the last time I bought some. I use this, but I mix it with normal pump gas at the recommendation of my engine builder to reduce the octane. According to him, as long as my engine isn’t predetonating I can keep dropping the octane rating of my fuel. There are two reasons to do this. Obviously the cost is one thing. The second though (and probably more important for the race car) is that with the faster burning of the lower octane rated fuel, you actually gain a small amount of power. Naturally we keep a decent safety margin. It’s not cheap to burn a piston in a racing engine.
I’m not 100% positive on this information. It’s possible that others will dispute it, but it did all come from the mouth of someone who has been building racing engines for at least 25 years. Still, if your friend is using high octane fuel in a car that is recommended to use 87 octane, it could even be hurting his performance.
The owner’s manual for my high-end vehicle states that I should run it on high-octane gasoline.
Bummer, since the price of premium coupled with really bad gas mileage is killing me.
But I hear and obey.
Here is an approximate transcript of a real conversation I had with a friend:
Friend: You should always use high-octane gas in your car.
Me: What for?
F: You get better performance.
M: What do you mean “better performance”? Does the car go faster?
F: No.
M: Does it get better gas mileage?
F: Not that I’ve noticed.
M: Less wear on the engine?
F: Not that I know of.
M: A smoother ride?
F: No.
M: Cleaner emmisions?
F: I don’t know.
M: Better acceleration.
F: I’m not sure.
M: So basically you’re just spending more money every time you fill up your car because you have the vague sense it’s better?
F: Well, I, um . . . don’t you care about performance?!
I guess if you’re going to buy the expensive stuff, you should either make sure you know what the difference is or else don’t make friends with people who have the annoying habit of asking questions.
In my experience, higher octane than necessary can actually degrade performance.
I have an old 91 Escort that I ran on regular 87 octane. (Anyone wanna buy it??) Once, prior to driving from LA to Denver, I thought it might be a good idea to fill up on premium 91 octane…
Bad idea. Heading up Interstate 15, over the Tejon pass leaving LA, the car nearly died on the incline. If I tried to increase the throttle too much, it would nearly stall. All I could do was creep up the hill in the truck lane at 35.
In New Mexico, the regular unleaded being sold was something unfamiliarly low, like 80 octane. My car ran just fine on that, up and down I-25 and over Raton Pass no problem.
My Nissan 350Z requires 93 octane.
The price of a car does not necessarily mean that it has to burn a higher octane fuel. My Porsche 911SC ran quite well on regular gas, except in the summer when it was hot. The rest of the time it did not need the extra octane. Same goes for my Yamaha XJ600. My Yamaha YZF-R1, on the other hand, advises to use 91 octane or higher.
In other words:
[ul][li]Use the recommended octane stated in the owner’s manual;[/li][li]If you don’t use the recommended octane stated in the owner’s manual, monitor your engine while you are driving. (It’s shocking how people don’t listen to their engines!)[/li]The price of a car has nothing to do with the recommended octane; it’s the design of the engine.[/ul]
Obviously YMMV, literally…
For me, I fill up my tank about every 2 weeks. That’s 15 gallons every 2 weeks. 'Round here premium pretty much stays a contant $.20 higher than regular.
So by my calculations, I spend an extra $3.00 every 2 weeks buying the gas my manual (and incidentally, Car Talk) instructs me to. Over the course of a year that’s an extra $78! Filling up every week adds only $158/year.
Just something to keep in mind. Even if regular goes up to $5.00/gallon, as long as Premium is $5.20, it’s the same thing.
USCDiver