Premium Gas?

May 20, 2004 7:41 AM

Should I be using premium gas for my ’96 Subaru Outback? [mi]

My wife and I got the car as a hand-me-down from my father-in-law. According to the car manual and the dealership the car is "designed" for premium gas, and that is what we should use. My father-in-law took the advice seriously and as far as I know, only used premium gas. I, for one, am a little skeptical if it makes that much difference. The collective wisdom of the web varies between definitely use premium if the manufacturer says so to it doesn’t seem to matter. Anyone have an informed opinion? Now that gasoline prices are sky high and increasing daily it would be nice to save that money if it wouldn’t harm the car.

posted by Tallguy to travel & transportation (10 comments total)

Personally, I find 93 octane to be a cleaner burning fuel that causes the engine (fuel injectors especially) to behavior normally for longer periods of time. Less likelihood for misfiring as the car ages. If the car has always used 93, I would suggest continual use.

On the other hand, if the car will only be around for another 2-3 years, save some money this summer.
posted by BlueTrain at 7:47 AM on May 20, 2004

According to the FTC, if your manual requires it then yes.
posted by drezdn at 8:06 AM on May 20, 2004

The requirement for premium gas generally comes from a higher compression ratio -- a more heavily-compressed fuel-air mixture means a higher temperature, which means higher octane is required to prevent early detonation. That said, modern (newer than 1981) engines have knock sensors that can change the timing in the case of early detonation, so there's not really much danger to using lower-octane fuel (although you will likely experience somewhat worse performance and fuel mileage). Literally, YMMV (severe knocking could be a problem; maybe try mid-grade fuel first).

As another note, a friend of mine drives a 2000 Subaru Impreza, which (if you've got the 2.5L 1996 Outback) has the same engine as yours (but with a slightly higher compression ratio), and I'm pretty sure he runs 87 octane fuel without any problems (although 87 octane is what's indicated in the owner's manual; curious, that).
posted by uncleozzy at 8:19 AM on May 20, 2004

You might want to check out the Gasoline FAQ. Everything you might want to know about gasoline, except a specific answer to your question.
posted by Mark Doner at 8:45 AM on May 20, 2004

Mark Doner's link is comprehensive. A must read if you have a few hours to spare.
posted by BlueTrain at 9:01 AM on May 20, 2004

My 97 Prelude says that it wants 91+, and the owner's manual agrees.

It also says that you can run it on regular if you really want to. You'll lose performance and mileage because the engine and software are optimized for high-test, but you can do it.

Experiment. Run it on two or three tanks-worth of whatever it asks for, noting performance and mileage. Then run it on a couple-few tanks of the cheap stuff and see if you're losing anything you care about.

The premium you pay for high-test seems to be pretty constant at $.10--.15/gallon, even as prices rise (at least in the parts of the country where I drive). It's at least possible that the car might run sufficiently more efficiently on high-test to pay for the difference, at least in the current high-price setting.
posted by ROU_Xenophobe at 9:18 AM on May 20, 2004

Using a lower grade of gasoline if the engine is made to user a high grade of gasoline can also decrease the engine life. On the flip side, using a high grade of gasoline when it's supposed to receive regular gasoline will also kill your your engine life (the higher energy output from higher grade gasoline put undue wear and tear on the engine).
posted by jmd82 at 9:34 AM on May 20, 2004

The answers so far have been precisely right. If your manual calls for high octane gas, you should run it. If you have a modern car, the engine will adjust should you run too low an octane and start pinging, but it will do so at the cost of performance.

(And, should your knock sensors ever die, bad things could happen)

The choice is yours.
posted by jammer at 10:46 AM on May 20, 2004

uncleozzy has presented you with the absolutely correct, textbook answer.
posted by trharlan at 10:47 AM on May 20, 2004

A lot of the advice and information above is correct and useful. From what I can see, none of it is wrong. However, the octane question is can be summed up pretty easily:

Just because Michael Jordan drinks Pepsi doesn't mean drinking Pepsi turns you into an all-star ball player.

The higher octane fuel "burns more slowly," in layman's terms, and has little to nothing to do with performance or mileage It just happens to be the fuel of choice for high performance, high compression engines. Generally, one only uses a higher octane fuel if there is some symptom, such as "knocking."

As it seems your manual recommends premium fuel -- probably because you have a high-compression engine -- I'd suggest going with exactly the octane level mentioned in the manual unless you know why you should do otherwise.
posted by majick at 12:06 PM on May 20, 2004