Higher octane fuel in my 98LGT

Hi guys, I have read a few dozen threads and can't for the life of me get a straight one way answer to this.

I work a few towns away, most of the way there is all an incline, and I myself live on top of a mountain. So there's climbing both on the way to work and home. Now I notice that my 98 Legacy GT idles really rough at stop lights. (In drive with brakes held. She idles around 600 rpm.)

Now the owners manual says to put in 82 octane fuel. There's tons of people saying that putting in the high test stuff will make her run smoother going up hills, idling and over all just make it perform better. On the other side there are people saying that if you use high test and it was not suggested it'll destroy your engine.

Sooo.. My question, would it be worth the few cents more to try it? Will it actually hurt anything? (I can't imagine it would..)

Thanks.

The difference between grades of fuel is the temperature at which it ignites. If you're having problems with the air/fuel mix detonating before the spark plug ignites, you need to use a higher octane fuel. There are a lot of claims that it runs smoother or gets better gas mileage or is just "better" for your engine. Most of those claims are not scientific and usually just from people seeing what they want to see when it's probably not running any different (placebo effect).

Given your situation with the climbing and experiencing rough idle, higher octane might be worth a shot. It sounds like your engine probably gets very hot. If it fixes it, great; if not, you can start looking at other potential problems. You certainly won't hurt anything other than your wallet if high octane fuel wasn't the cure.

man, you suck, there's literally 3 ongoing threads about this very thing lol

I only saw one, which was asking about the 96 GT which was when they first swapped out to the 2.5, That one says in the manual to only use 92 octane, By the time 98 came around they had re-engineered the injectors so that it no longer needed the 92, thus suggestions in the book were for 82. Went back 9 pages and could only find the one still :/

Given your situation with the climbing and experiencing rough idle, higher octane might be worth a shot. It sounds like your engine probably gets very hot. If it fixes it, great; if not, you can start looking at other potential problems. You certainly won't hurt anything other than your wallet if high octane fuel wasn't the cure.

Thank you, that was very informative, I'll fill up next time with high stuff and see.

The engine never really gets overly hot, or at least never says it does. Which alone has concerned me a few times. Like on my way to work it's about a 10 mile drive, all up hill. The engine heats up to just under the half way mark.

{photo borrowed from http://sl-i.net/FORUM/viewtopic.php?f=22&t=23650}

Right to where the needle is here and never fluctuates. Not one line. No matter how fast I drive or how slow I go. I found that odd.

I only saw one, which was asking about the 96 GT which was when they first swapped out to the 2.5, That one says in the manual to only use 92 octane, By the time 98 came around they had re-engineered the injectors so that it no longer needed the 92, thus suggestions in the book were for 82. Went back 9 pages and could only find the one still :/

The octane required by a motor has absolutely nothing to do with the injectors... And I have no clue where you're getting the 82 octane requirement from.. 96 25D recommended 91 octane and 97 to 99 recommended 87 octane.

If you actually read all of the posts in those other thread you would understand exactly why. The combustion chamber is completely different between them... Thus, the requirement of 91 in the 96 25D models.

There is no harm in running 89/91/93 in the 97 to 99 25D motors. The compression rating is 10:1 which is plenty high enough to efficiently burn those fuels. Whether or not you will actually see any gain in performance is debatable. The ECU only adjust ignition timing when it detects certain ranges of knock. The motor itself efficiently burns 87 octane, so running anything higher to reduce knock and alter the timing is almost pointless because there isn't much knocking from pre-detonation in the first place. And if it does alter, it won't be by very much at all. And being the motor it is, I highly doubt any gains whatsoever would be worth it. I ran both regular and premium (93) in my 98 LGT and there was absolutely no running difference between the two on the butt dyno, but it seemed to run a bit healthier.

The octane required by a motor has absolutely nothing to do with the injectors... And I have no clue where you're getting the 82 octane requirement from.. 96 25D recommended 91 octane and 97 to 99 recommended 87 octane.

 

If you actually read all of the posts in those other thread you would understand exactly why. The combustion chamber is completely different between them... Thus, the requirement of 91 in the 96 25D models.

I confess that I really know and understand very little of how the modern engine works, beyond the whole piston/injection/firing and repeat. I was raised by mechanics but my time was spent wholly engorged in computers and electronics. I just typed what I could remember and what my bad memory skewered into thinking I remembered. Thus 82 and 97 octane vs 87 and 92. Thanks though.

Try and find fuel at a reputable station with brand name gas like BP Shell or Conoco. Then find one with no ethanol high octane gas. My car gets great mileage and pulls hills better on 91 octane. There is a point however where you need to try a tank of each and evaluate the cost increase versus the driving and performance benefit.

Worst case.... Your car will have no performance improvement and you can justify lower octane fuel.

How is your car mechanically? When was the last tune-up?

I ran 91 in my ej25d legacy I had as my first car and wish I hadn't. In the long run I would of had a couple hundred extra in my bank account. It might be worth it if premium was 100% gas but it is mixed with ethanol 10%+- at most stations anyways these days. So I use regular in anything non turbo

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I ran 91 in my ej25d legacy I had as my first car and wish I hadn't. In the long run I would of had a couple hundred extra in my bank account. It might be worth it if premium was 100% gas but it is mixed with ethanol 10%+- at most stations anyways these days. So I use regular in anything non turbo

Truth.

The car doesn't produce the same level of energy per stroke that it would on 87. It will only run smoother yet need to be run harder to produce the same work, which gives the "more eager to move" illusion.