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Does Premium Recommended = Premium Required?

Hi, I recently purchased a used 2004 Subaru Outback 3.0H6, at the time of purchase I didnt know that premium gas was recommended for it, but i like the car so i have decided to keep it. I have a couple of questions

1)the manual reads recommended 91 gas, but i get only 93 gas in most of the station, can i use the 89 gas?

2)the manual also reads that 87 can be used if necessary, so if i put in regular 87 gas am i damaging the engine in long term?

I accidentally put 87 gas instead of 93 once and didn’t notice much difference in gas mileage or engine noise, what the deal with the octane rating

Your manual is clear, 91’s recommended, not required, and it allows 87. So you’re fine, if you notice no difference save the money! That’s what I do with my ES300, it has very similar wording in the manual, and I notice no difference in economy or power.

I am also the proud owner of a Subaru (2002 model) with the 3.0 H6 engine, so I can share my experiences with you.

I have used regular gas in my car a few times, and I noticed that it definitely degraded the acceleration to a noticeable extent. The car was still fast, but clearly, it was not as fast in acceleration with regular gas, as compared with premium gas. Also, my gas mileage did drop a bit with the use of regular. Normally, my gas mileage is incredibly steady, at 23-24 mpg in mixed driving. With regular gas, my mileage on those tankfuls was in the range of 21.5-22 mpg. Not a great difference, but the drop-off in gas mileage was almost as noticeable as was the drop-off in acceleration.

That being said, our engines may not be exactly comparable. While my 3.0 H6 puts out “only” 212 HP, your newer engine was tweaked sufficiently for it to put out (if I recall correctly) 240 HP. My instinct tells me that, if anything, your engine is probably even more demanding of higher octane than mine is, but if the tweaks were sufficiently high-tech, it is even possible that yours is more forgiving of reduced octane than mine is.

The bottom line–if you don’t perceive that your acceleration has suffered, and if you don’t calculate a drop in gas mileage, then feel free to go ahead with the lower octane gas. (But, then again, there must have been a reason for that recommendation from the manufacturer, so you have to make your own decision on this issue.)

I bought a Ford F350 online several states away. I drove it over 100 miles without the Check Enbgine light coming on. I bought 87 octane because that is what I was used to buying with my older truck. Shortly, the Check Engine light came on. I drove a couple hundred more miles and filled up again with midrange, and the check engine light went off. I continued using midrange after I got home, and when summer came and I was using the Air Conditioning the Check Engine light would occasionally come on. I would thing that if the Check Engine light isn’t coming on that everything is ok. It is affecting the emissions and the sensors with the different octanes.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

If it was my car, I would use 93 octane, and here’s why.

Most “experts” will tell you that the engine has a knock sensor which will retard the timing to prevent engine damage. The problem is that this is a reactive device- it will retard the timing AFTER it hears knock.

Over time, the computer will try to add back timing, until it hears knock again and then will retard the timing again.

Now the engine can tolerate an occasional knock, but if you add up the damage of occasional knocks over 150,000 miles, you will probably be upset when your engine needs a new head gasket.

In the old days, a timing curve was set up by driving the car and listening for knock. But nowadays nobody knows what knock sounds like anymore, so those of us with “tuned” ears will still cringe when we hear it. The most knock occurs during upshifts and when you “stab” the gas pedal, and these short bursts of knock (which the knock sensor has a hard time eliminating) will have a harmful effect over time.

In addition, if the ECM has to retard the timing due to lower octane gas, your MPG will decrease. This decreased MPG will cost you money anyhow, so you might as well get the 93 octane fuel.

One more note about buying premium fuel- many of the “cheap” gas stations have a very low turnover of high-octane fuel, which means that it sits in the ground for a long time (and gasoline has a limited shelf-life). Buy your premium fuel from a station that has a fair turnover of premium fuel.

FALSE! The knock sensor and computer will always set the timing on the safe side at the edge of detonation regardless of octane. Without hearing detonation, the system can’t optimize timing. Premium gasoline is not perfection guaranteed, can preignite too.

A little detonation is harmless. My mid 80s Chevrolet often had continuous light detonation on the freeway in warmer weather. Mine was not the only one as Roger Smith, president of GM at that time addressed the complaints; called the light detonation the “Sound of Economy”. My engine was still good at 160,000 miles when the car was scrapped; rusty body.

why dont you just use the best fuel possible??its so cheap in the usa!!you should come to england where we still pay ?4.50($6.80)a gallon for regular unleaded!!ive always used shell v-power in my 2002 ford puma,and never had any problems,even though it costs ?5.00($7.50) a gallon!!

Use 2/3 93 octane and 1/3 87 octane to yield 91 octane (or, close enough).