Subaru Outback Forums banner
  • Hey everyone! Enter your ride HERE to be a part of October's Outback of the Month Challenge!
1 - 12 of 12 Posts

·
Registered
'09 gen3 2.5l "sports" automatic OB wagon
Joined
·
4 Posts
Discussion Starter · #1 ·
Hey all. I am the proud owner of an '09 gen 3 outback with the 2.5 litre engine and 4 speed auto. Stock except for roof-racks and towbar
I've been using PULP, 95 octane fuel and getting just under 10 litres / 100 k's fuel consumption (so far, only on 2nd tank).
What octane do you use, and why? What mileage do you guys get from the regular (91 octane) stuff, the premium (95 octane) and the "pay through the nose super dooper premium" (98 octane) fuel?
 

·
Registered
2011 Outback 2.5i 6 spd manual
Joined
·
20 Posts
You may be interested in the results I got on a prolonged fuel comparison test I ran a couple of years back. I had a manual 2007 Forester X at the time which I am pretty sure has the same 2.5 L engine as your Outback. I did the test properly and tabulated the results. The table is attached below. I ran fuel in blocks of either 91 ULP or 95 PULP (all Caltex fuel).
At no time using either fuel could I pick any difference between them in terms of power, acceleration or, as the attached table shows, fuel consumption! If anything, the ULP gave marginally better results but not better in a statistical sense.
So the conclusion was that the PULP was costing 10 cents per litre more for zero benefit and consequently I ceased using it.
I have experienced a big improvement in fuel economy in the 2011 manual Outback I now drive with it typically giving around 8.8 to 9.0 L/100 km. I can only put this down to the 6 speed gearbox over the 5 speed in the Forester and possibly some technology improvement as the engine is essentially the same. I have not as yet tried PULP fuel in this vehicle to see if it is any better than the ULP I have used in it exclusively to date.
 

Attachments

·
Registered
'09 gen3 2.5l "sports" automatic OB wagon
Joined
·
4 Posts
Discussion Starter · #3 ·
Thanks mate, thats great info.
Looks like my next tank (and most likely subsequent ones) will cost me $5 less. :D
 

·
Registered
2009 2.5i, H4, Auto.
Joined
·
838 Posts
A quick guide to octane.

The Octane Rating is a measure of how well a fuel resists detonation due to engine compression. Basically how high can you compress the air/fuel mixture before the engine starts to knock.

An engine will have an octane rating based on how it operates and the compression ratio. Preference engines with higher CR's will either knock and damage the engine with lower grade fuel than what it was designed for or modern ones will change the timing , resulting in lower performance and mileage.

If you use a higher grade fuel than what the engine was designed for, it will have no effect on the car at all. The engine is not designed to take advantage of higher grade fuel.

Look in your owners manual and read what the recommended octane rating is for the car. Using anything higher than that is a waste of money in a properly functioning engine. There are a few cases where higher grade fuel will help with mechanical problems but they are not very common.

One other thing to keep in mind is that using higher grade fuel commonly can give you slightly less mileage. The reason for this is the chemicals that they use to increase the octane have a lower energy in them than regular gas. In a performance engine, the higher compression boost efficiency more than any losses in the chemistry. A normal engine can see a small loss of power and mileage. However the loss is usually very small and most don't notice it.
 

·
Registered
2011 Outback 2.5i 6 spd manual
Joined
·
20 Posts
A quick guide to octane.

If you use a higher grade fuel than what the engine was designed for, it will have no effect on the car at all. The engine is not designed to take advantage of higher grade fuel.
I must say I am a little skeptical of that statement as a generalization about all engines. If it is accurate then a lot of fuel companies are guilty of false advertising in that they claim your fuel consumption will decrease on premium fuels, assuming that the engine is not actually required to run on them.
The key thing is whether the gain is at least as good as the difference in price. I was willing to believe that it was and was in fact challenged to prove it by my boss who said it had made no difference in his vehicle. So I did the trial properly intending to prove it, which I failed to do.
I have also seen comments by others that the engine management system will adjust to make use of the higher energy content fuel. I can only assume that NA Subaru 2.5 L EJ engines do not but remain open to suggestion that other engines do.
 

·
Registered
2005 Outback XT Ltd
Joined
·
162 Posts
it was explained to me that cars from the factory are tuned to run an avg octane.
Our subaru's are set up run on 91 octane as it is more widely avail in the states.

to say ur car will run poorly on something else is a long shot, but if you want to use a higher octane fuel regularly, you should get it tuned for it.
 

·
Registered
2009 2.5i, H4, Auto.
Joined
·
838 Posts
I must say I am a little skeptical of that statement as a generalization about all engines. If it is accurate then a lot of fuel companies are guilty of false advertising in that they claim your fuel consumption will decrease on premium fuels, assuming that the engine is not actually required to run on them.

I have no idea what advertising in your area is like your area, so I can't say if it is false advertising or not.

But yes, using higher grade fuel than the engine is designed for can result in SLIGHTLY lower mileage. As I stated in the last paragraph of my post. The reason for this is the difference between regular and premium (PULP) formulations. To get the higher octane rating, they commonly add other chemicals to bump up the rating. In the US it is common to add extra Ethanol to the gas. Ethanol has about 70% the energy per volume as Gasoline. So when they mix gas and ethanol, the energy that you can get from burning a given volume will be lower the more ethanol. Other chemicals used have different energy densities.

In the case of a car designed for regular that is filled with Premium, depending on how the premium gas is formulated, you could see a drop in mileage because of the lower energy density, however, it is going to be small and in most cases you won't be able to see it in normal driving conditions and error rates. It takes a pretty sensitive dyno setup to be able to tell the difference.

The gas commercials in your area could be correct saying that you will loose mileage in a lab setting, but in the practical world of everyday driving you most likely won't be able to tell.

As to the other car makers who say that their engines can adjust to be able to use higher octane fuel, really the case is the opposite. In reality, the engine is a one that is designed for high octane fuel, but is engineered in such a way that if it detect that fuel is starting to pre-detonate, then it will adjust itself to be able to use the lower octane fuels without damaging the engine.

You can safely use lower octane fuel in these engine, but you also loose performance and mileage. Then it becomes a case of if the performance gain on the engine using premium is enough to make up for the additional fuel cost. The economics will vary based on the performance difference and the relative gas prices.

It's all a matter of marketing. You can market it to more people if your sales pitch is an economy engine that can give you better performance with high test gas. The car companies love it. They can sell it to two different groups of people but only have to design and build one engine. What is funny is I have seen a couple cases where they market it both ways. Same engine, they either have the economy spin or the performance spin based on the car it is in.
 

·
Registered
05 OBXT 5eat stg1.2
Joined
·
2,104 Posts
There will be no difference in power etc from using higher octane if your recommendation is for low octane. To put it simple, the reason car makers require higher octane is that they tune the engine with timing and fueling that would knock with lower octane fuel. If your engine is safe with regular, then you are just throwing away money by using higher octane.

If you had the ability to tune your engine, you would likely be able to safely run a couple more degrees of timing in high rpm/load areas which would give you a little more power. But without changing timing, you will see no gain from going to high octane fuel.
 

·
Registered
2013 Legacy Lim CVT Car: 2011 OB Prem 6MT Car: 2006 Miata GT 6MT mc: 2003 Honda GL1800A * Reunite Gondwanaland *
Joined
·
3,565 Posts
Here's a typical example cut-n-pasted from the owners manual of
a car "designed for" premium fuel (2006 Mazda Miata):

Your Mazda will perform best with Premium unleaded fuel with
octane rating (anti-knock index) 91 or above [(R+M)/2 method],
or (96 RON or above).

You may use a regular unleaded fuel with an Octane Rating from
87 to 90 (91 to 95 RON) but this will slightly reduce performance.

Fuel with a rating lower than 87 octane (91 RON) could cause the
emission control system to lose effectiveness. It could also cause
engine knocking and serious engine damage.


So, premium is "recommended," but 87 octane is fully approved
by the manufacturer -- with no warnings/cautions except "slightly
reduced performance." Maybe.

That's pretty much the same owners manual recommendation as
four other "premium fuel" cars we've owned in the past 15 years.
I usually compromise with mid-grade (89 octane, in the lowlands),
but I really can't say that I've noticed ANY consistent differences
between fuel grades -- in either engine performance or mpg.

...don't sweat the small stuff,

Looby
 

·
Registered
Joined
·
22,806 Posts
Here's a typical example cut-n-pasted from the owners manual of
a car "designed for" premium fuel (2006 Mazda Miata):

Your Mazda will perform best with Premium unleaded fuel with
octane rating (anti-knock index) 91 or above [(R+M)/2 method],
or (96 RON or above).

You may use a regular unleaded fuel with an Octane Rating from
87 to 90 (91 to 95 RON) but this will slightly reduce performance.

Fuel with a rating lower than 87 octane (91 RON) could cause the
emission control system to lose effectiveness. It could also cause
engine knocking and serious engine damage.


So, premium is "recommended," but 87 octane is fully approved
by the manufacturer -- with no warnings/cautions except "slightly
reduced performance." Maybe.

That's pretty much the same owners manual recommendation as
four other "premium fuel" cars we've owned in the past 15 years.
I usually compromise with mid-grade (89 octane, in the lowlands),
but I really can't say that I've noticed ANY consistent differences
between fuel grades -- in either engine performance or mpg.

...don't sweat the small stuff,

Looby
Looby the old 2.5 was tuned and designed to run on the cheap stuff and its not put in anything close to a Miata LOL

As Seabass says - you can run higher octane fuel but when the engine was tuned and designed to run on the lower octane ie less stable fuel - paying more for more stable fuel does nothing but cost you more at the pump.

Most people associate more $ means better something - that does not apply to Octane ratings and fuel pump prices. Octane ratings or fuel stability is designed to address engine design and tuning. An engine which is designed for more stable fuel due to heat - compression etc is not going to like lower octane fuel which ignites early ie at the wrong time see Looby's cut and paste - the engine damage happens when the early ignition of fuel is beyond the range of the cars systems to dial back the timing and throttle ie power which is causing the less stable fuel to ignite at the wrong time.

An engine designed and tuned to run on less stable fuel ie lower octane fuel is not going to give a RATS a S S that you just put the high octane fuel in it given it never had issues with the lower octane fuel igniting early at the wrong time etc.
 

·
Registered
Joined
·
22,806 Posts
Also folks usually think that the more costly higher octane fuel contains more stored energy and results in more engine power. Not true - the higher octane fuel contains the same stored energy the lower octane fuel has - the difference is that the higher octane fuel has additives added that make it more stable so higher performance engines dont have the fuel burning ie igniting at the wrong time.
 

·
Registered
2013 Legacy Lim CVT Car: 2011 OB Prem 6MT Car: 2006 Miata GT 6MT mc: 2003 Honda GL1800A * Reunite Gondwanaland *
Joined
·
3,565 Posts
An engine designed and tuned to run on ... lower octane fuel is not going to
give a RATS a S S that you just put the high octane fuel in it given it never
had issues with the lower octane fuel igniting early at the wrong time etc.
I agree 100%. The point of my post was that I've observed little/no difference in
power or mpg when running 87-89 octane in several engines "designed for" 91.
AND most modern engines "designed for premium" also run just fine on regular.

IMO, any differences related to octane ratings are swamped by other factors --
such as temperature, humidity, seasonal changes in gasoline blends, etc., etc.

...it's silly to go all OCD about gasoline,

Looby
 
1 - 12 of 12 Posts
Top