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Fuel Octane rating experiences with the 2.4L Turbo engine (FA24F)

116K views 430 replies 133 participants last post by  Robert.Mauro 
#1 ·
So I bought a 2020 Subaru Onyx XT. Still have a half tank of gas, but going to a fill up probably this weekend. Just wondering about people thoughts on using premium versus regular unleaded gasoline. What about mid-grade? I know there are benefits to using premium gas.
 
#4 ·
As I understand it, higher octane ratings are intended for engines with higher compression. Most of my experience on this topic is related to motorcycles, but don't know why it wouldn't carry over to car engines too. Using gasoline with a higher than recommended octane rating is generally not going to provide any benefit, and obviously it will cost you more. I have no intention on putting anything other than 87 octane in my Touring XT.
 
#8 ·
Plenty of discussion on the topic all over these, and other, forums. I've never seen a shred of convincing evidence that there's much, if anything, to be gained from higher octane on a stock tune. But there has been a fair amount of data posted from tuners that show it's possible to see some benefits, even in NA engines.
 
#14 ·
Higher octane supports higher compression pressures in the block by not igniting until a higher temperature. Lower octane ignites more readily at a lower temperature. Thus high octane fuels are typically only required for blocks that develop high compression. Choosing between high and normal octane fuels isn't usually optional -- you can get pinging or fouling if you don't use the right fuel for the engine -- but the symptoms are pretty obvious. I lived four years with an Outback in Dallas and never even had to think about premium fuel. You shouldn't either. It takes a heck of a lot of ambient heat to do more than a hot engine block is going to do anyway. Get standard and be happy. If you think you need premium, you probably have a problem with your engine.
 
#17 ·
So I know Subaru says regular gas is fine for the 2.4 Turbo but I wonder if premium would deliver more Umph/ MPG/ lifespan. This is my first Turbo vehicle but I have had several other naturally aspirated vehicles in the past that ran better and/or gave me better mileage by regularly stepping up at the pump. Has anyone tried running some level of Premium in their late model XT or know of some test info out there? A net search has produced nothing for me.
I'm wondering if there is any value in trying it on this one.
 
#26 ·
So I know Subaru says regular gas is fine for the 2.4 Turbo but I wonder if premium would deliver more Umph/ MPG/ lifespan. This is my first Turbo vehicle but I have had several other naturally aspirated vehicles in the past that ran better and/or gave me better mileage by regularly stepping up at the pump. Has anyone tried running some level of Premium in their late model XT or know of some test info out there? A net search has produced nothing for me.
I'm wondering if there is any value in trying it on this one.
Since premium gas is about 24% more than regular, you would have to gain 7 mpg to make it worth the extra cost. I don't see that happening. If there were any MPG benefit from using premium, Subaru would have "required" it in order to get better MPG numbers. It's funny how most luxury brands "require" premium yet the added cost of it (at least .50/gal) is never really discussed by anyone.
 
#19 ·
Not sure if all are same, but Shell Canada boasts their 89 has less ethanol and 91 is E0. Is there any benefit to removing corn from the XT's diet?

If not, we'll have to wait for Accessport. :) My 05 Forester XT also wanted 91 and was very expensive to run. That drove me to 2.5 NA Outback, but I'm very tempted by new XT. :)

Sent from my SM-A530W using Tapatalk
 
#20 ·
As noted premium fuel is not needed but the “top tier” fuel additives help keep the injectors clean but DO NOT do anything for the values as it is a direct inject engine (meaning the fuel does not spray on top of the values to which normally keeps them clean. The fuel sprays directly into the combustion chamber. )

As an FYI, Direct Injection puts greater need to make sure you change oil per recommendations to help keep values clean.
 
#22 ·
I suspect this is going to be one of those persistent threads...

 
#28 ·
I have decided to offer a new service. For those of you who have decided that it is better to run premium fuel in your 2.4T engine, for only $9.99/month I will send you weekly email's telling you what an awesome choice it was. If you subscribe to a full year for $139.99 I will also send you a special email on your birthday. Act fast, because I'm sure I will be busy. We all know Subaru knows nothing about octane! PM me for details!
 
#29 ·
Yeah, the "climates" thing sort of cracks me up, too. What temp is that intake manifold, exactly? ;-]

I used to work on a tank-farm, as a mechanic/hand (great job to pay for college, interesting, and mostly hazard-pay), and we transferred LOTS of fuel between various companies, the "trading" was CONSTANT, whenever someone got low, for distro.
This applied to fuel being sold to the airport, military, and gas-stations, it ALL got mixed.
Supposedly, some places re-mixed their additives, after receiving it, but if that was the case, then we often got their "post mixed" fuel sold back, to us (not naming any specific fuel companies here, but I can say it covers most of the "big players"), also pretty regularly. Perhaps they re-mixed the mix, when we sold it back, but at some point it gets a bit "weird", if you ask me.
I know I've seen various "test metrics", the standard ones used post-transfer, and they rarely varied, at ALL, even after we got a "big push", say from another company that was "known for their additives"...
The only fuel that was treated really carefully, IME, was the AV-Gas, intended for (partly turbo) prop-planes, which was often in the 100+ octane range (also used in really high-compression race cars and such).

I'm sure if you use an aftermarket tuning package, that advances the timing "enough", with an XT-turbo engine, you would see some benefits (to using >87). The downside is that if it can't de-tune enough, too, you'll get a fair amount of "knock", when using 87-ish fuel. If/when these packages exist, I'm sure those aftermarket tuners will publish the numbers, showing the improved torque/HP curves...
 
#39 ·
Something else to consider is that there isn't any reason to stop selling/making lower octane fuels as long as somebody is buying them. Years ago I worked at a Sunoco station and we had 5 grades of fuel ranging from 86 octane to 94 octane unleaded. We only had 2 in the ground though, 86 and 94. The other octane ratings were blends of the two at the appropriate ratios to deliver the desired octane. So it isn't like they are stocking 3-5 different fuels in underground tanks, just two, plus whatever ethanol free, diesel, kerosene etc. which is obviously different.
 
#56 ·
Cobb's own website says:

Subaru truly calibrated the ECU to use an 87 octane fuel and testing with higher octane fuel on the factory calibration did not impact the overall power of the engine, though it did clean up occasional minor knock events.

So using 87 octane produced minor knock events that using a higher octane did not. Given that fuel quality may vary, my choice is to use higher than 87 just to avoid those occasional minor knock events. Subaru says 87 or higher so I'm just following guidelines.
 
#59 ·
Additional notes:

The 2.4T has a high 10.6:1 compression ratio.

Torque (android app) saw 14.7 pounds of boost spike using MAP method on my car in very limited testing. That amount of boost in a 10.6:1 compression engine doesn't sound like 87 octane to my old school mindset.

An Ascent owner reported much higher boost:

peak boost for ascent is 14.4 after doing some research. On stock air filter I saw boost spikes up to 16.8psi, using AEM intake filter I saw boost spikes up to 15.7psi. at least its better than the wrx. I had boost spikes all the way up to 23.1 and i think peak was suppose to be 15.5psi

So for me, using higher than required octane, even if I don't get a performance boost, gives me peace of mind that to me is relatively cheap in the scheme of things. Some people spend thousands of dollars on paint protection, aftermarket sounds systems, wheels, whatever. I spend it on fuel, filters and oil.
 
#61 ·
So for me, using higher than required octane, even if I don't get a performance boost, gives me peace of mind that to me is relatively cheap in the scheme of things. Some people spend thousands of dollars on paint protection, aftermarket sounds systems, wheels, whatever. I spend it on fuel, filters and oil.
That’s my philosophy as well. I’ve tried to get my wife to consider at least running a tank of premium every now and theN in her Onyx but so far she’s refusing. That was the one thing she always complained about with her previous Outback , having to use premium. I think an occasional fill up with premium doesn’t cost that much and doesn’t hurt anything even if there’s no obvious benefit.
 
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