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What's the verdict on running Premium in a 2.5?


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Hello everyone! I've been thinking about running 91/93 octane in my 2013 2.5i, to see if I notice much of a difference. Gas prices have essentially been tanking here in Mass, especially the gas station near me! Is there anything I should worry about? Obviously I know the car should compensate for the new type of gas once I run the tank low and refill it with the higher octane. I'll make sure I've got below 1/4 tank as well! Thanks for all the advice!
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what are you running now? Generally speaking, once you get past the mid-grade, there isn't much difference for a NA engine, since they are lower compression than the effective compression of a turbo or supercharged engine. Higher octane gives more resistance to detonation/knock.
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what are you running now? Generally speaking, once you get past the mid-grade, there isn't much difference for a NA engine, since they are lower compression than the effective compression of a turbo or supercharged engine. Higher octane gives more resistance to detonation/knock.

 

I've got the naturally aspirated 2.5i, and it's at 85k miles now. Always been run on 87 octane. I would assume running 91/93 wouldn't hurt anything right?

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I datalogged, my 2012 EJ253 2.5i and it pulled timing with 87 octane. (My DAM was not anywhere near one) You have a FB25 which is a different engine, so your results may vary.

 

One thing that eventually figured out in looking at the timing tables is that during normal part throttle driving, you probably wouldn't notice a difference between 83 and 93. The timing that the DAM adjust really only affects the high load area and WOT part of the tune. You may notice a difference at WOT. I really don't notice a difference between 87 and 93 octane in the 2009 2.5i Outback I have either in daily driving, but I don't do a lot of WOT. At this point I usually only use 93 in my Outback when I am planning to tow my boat.

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I datalogged, my 2012 EJ253 2.5i and it pulled timing with 87 octane. (My DAM was not anywhere near one) You have a FB25 which is a different engine, so your results may vary.

 

One thing that eventually figured out in looking at the timing tables is that during normal part throttle driving, you probably wouldn't notice a difference between 83 and 93. The timing that the DAM adjust really only affects the high load area and WOT part of the tune. You may notice a difference at WOT. I really don't notice a difference between 87 and 93 octane in the 2009 2.5i Outback I have either in daily driving, but I don't do a lot of WOT. At this point I usually only use 93 in my Outback when I am planning to tow my boat.

 

So the higher ictane gas will mainly affect the Wide Open Throttle? That wouldn't be a bad trade off for paying a bit more money :p

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So the higher ictane gas will mainly affect the Wide Open Throttle? That wouldn't be a bad trade off for paying a bit more money :p

 

You can always run your tank down below a 1/4 of tank and fill it with premium gas. It is only like $4-5 or so more per tank. Timing should readjust the dam within 20 miles and you can be the judge to see if you notice difference or you feel its worthwhile.

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the car uses a knock sensor so it is possible that it can advance/retard timing based upon fuel quality

 

 

and 89 in a car that requires 91 may or may not cause knock. depends upon engine programming. a Honda J series in 2003 or newer accords can take advantage of high octane gas. Ditto for Acura 2001 or newer J series Type S variants they can de-tune for 89 octane based upon knock.

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It de-tunes based on knock feedback. If your car manual recommends premium it's a safe bet you should use premium. I doubt a 10-14 Legacy manual will tell you to use premium unless it's a GT.

 

Yah it says in the manual to use 87, or better. This is why I'm curious as far as if I could harm my 2.5 with premium gas. I wouldn't think it would do anything bad...

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the car uses a knock sensor so it is possible that it can advance/retard timing based upon fuel quality

 

 

and 89 in a car that requires 91 may or may not cause knock. depends upon engine programming. a Honda J series in 2003 or newer accords can take advantage of high octane gas. Ditto for Acura 2001 or newer J series Type S variants they can de-tune for 89 octane based upon knock.

 

That's something I'm always fond of Acura cars that are the Type S. It's great that you can use them without Premium if needed. I was reading up on my 2.5 that I could use premium if I wanted to, but since my car has been on 87 octane since it rolled off of the dealership I'm a bit worried to do the switch to 91/93.

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Yah it says in the manual to use 87, or better. This is why I'm curious as far as if I could harm my 2.5 with premium gas. I wouldn't think it would do anything bad...

 

You won't harm anything. You also won't notice a difference unless your car has been running kind of like crap due to oil from the PCV into your intake.

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You won't harm anything. You also won't notice a difference unless your car has been running kind of like crap due to oil from the PCV into your intake.

 

My engine sounds great as far as I can tell, very nice and smooth. Also quiet. I'll probably go with either Sunoco's 91/93 blend, or Shell's 93 blend as it seems like the best choice near me. I'd like to keep my engine smooth, obviously :rolleyes:

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running higher octane certainly isn't going to hurt anything, and the cost difference isn't that great (for 18 gallons, if the difference is say 25 cents a gallon, it's only $4.50). On our van we run mid-grade just to get the better additive package.

 

 

for the RSX-S, running lower octane just makes the car pull timing (reducing power) - pretty much all modern performance engines will do that (lower octane results in detonation so the car pulls timing and limits boost to stop it for a turbocharged subaru)

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This question has been asked regularly since time immemorial. The answer is, definitively: “You can run higher octane gas in your car than it calls for without harm as long as it’s not higher octane because it’s leaded. Your (stock) engine *may* run better or be more efficient with higher octane gas, but it’s almost never worth the extra cost. If your engine runs significantly better with higher octane gas than with what it’s designed for, there’s probably something wrong with it.”

 

The only exception I can think of is certain recent turbocharged engines, specifically the recent VAG TFSI engines like the 1.8T they were using, would run fine on 87, but were performance rated on 93. They specifically said something like “Maximum performance is achieved using Premium gasoline. Using 87 octane gasoline will not harm your engine, but will cause it to run at a slightly lower output.” It’s heavily qualified cheating if you ask me.

 

Anecdotally, some N/A cars I’ve had have run a bit better on 93 than on 87. Mileage is usually a touch better, power not noticeably better. Day to day, it was not worth the cost. All the rest were exactly the same on 87 as 93. One, a Subaru with an EJ25D, ran significantly better with 93 than 87 despite its rating for 87. I decarbonized it and it ran correctly on 87. Putting in 93 made it ping under heavy load a touch less, but no longer made any other improvements.

 

 

Sent from my iPhone using Tapatalk

Edited by SchwarzeEwigkt
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  • 2 weeks later...
The more you keep switching grade back and fourth you may confuse the O2 sensor and ECU. I know on other vehicles the O2 sensor and ECU is already program for certain grade fuel. Edited by amusa
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O2 sensor readings are going to be independent of the fuel grade - they report what they see in the exhaust stream. The ECU is certainly programmed expecting a particular grade if fuel, but you aren't going to confuse the system or change the basic programming simply by running different octane level fuels from tank to tank (adjustable parameters might change, but they'll change back as the fuel grade changes back).
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The ECM is able compensate for octane of gas by using the knock sensor. The total timing seems to be aggressive in certain areas and it used the knock sensor for feedback on the octane. The ECM calculation ultimately creates a DAM variable.

 

Here is my 2012 2.5i knock correction map. The ECM multiples DAM and this knock correction MAP than that adds number to the timing map. Around 1200-2000rpm, between 0.7 to 1.0 gram/sec is a partial throttle high load driving that one would encounter. For the typical driver, this is where the ECM is most likely is dialing in the octane and ultimate calculating the DAM.

 

attachment.php?attachmentid=277487&d=1563411472

 

For instance 1.0 engine load and 1800rpm timing has adder of 9.84 degrees (lets use 10 for simplicity) If the ECM has calculate DAM of 0.5, the ECM would be reducing the timing about ~5 degrees at that load & rpm. WOT on my 2.5i peaked just before 1.3 gram/rev for reference.

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