RexNuts Subaru Forums

Hey all, Just have a question about fuel types in different types of subaru's.. I currenty own an 04 RX now from that do i use the normal 91 octane fuel or the "Cleaner" 98 octane fuel. Here is a discussion i've red from another forum.

BP have a clever marketing team, they've successfully conned people into thinking that 98 is a"cleaner" fuel, and it'll be better for your car and will make more power or get better range etc... It's BS. In some cases 98 is often worse of a quality fuel because they can 'cheat' with additives to achieve the octane rating. I've heard and seen proof that United and Gull use a small amount of ethanol in some batches of their 98 octane fuels. This causes problems with some cars with fuel lines made of a certain material and certain fuel pumps I hear. I think plastic reacts badly to ethanol but I'm happy to be proven wrong...

Unless the car is tuned for 98 octane, you are wasting your money if you're buying 98 thinking it'll make any difference to performance. The exception is Japanese imports, because they have better quality fuel over there, so their normal '91' fuel is closer to our 98.

TL;DR version: Unless your car is tuned for 98 or it's pinging, don't bother with higher octane fuels, it's a waste of money.

Incorrect.

Late model cars all have what is called an Ignition Advance Multiplier, which basically advances timing to an optimal point for 98RON fuel. Using cheaper fuels causes this IAM to retard ignition timing and increase fuel to offset which ALSO means that all of the fuel is not being burnt. With the right software, you can actually look at the IAM of various cars and you can definitely tell the 91RON fuel cars from the 98RON cars, even in non-turbo applications

Also, BP do not synthesise their fuels like cough *Shell* cough which means that there is more calories of energy per litre(Calorific value), which means with more ignition timing along with 'leaner' fuel mixtures thanks to the IAM, it is possible to burn more calories of fuel and thus turning that energy into a force which pushes the crankshaft to generate more torque than shittier 91RON etc. More timing advance means the fireball ignites more calories, rather than richer mixtures with less timing which burns minimal calories.

Yes, I would never use Gulf or United 98RON fuels as they use ethanol to increase the RON rating.

Your also wrong on some of the Ethanol problems with fuel systems, ethanol only strips fuel varnish off old fuel lines which clogs fuel pumps, fuel filters and injectors etc on cars which have run petrol for most of their life which is where the sudden flush of exposure in the fuel lines can cause rusting. However, converting to ethanol early in the cars life has no side effects
Many high-performance engines are designed to operate with a high maximum compression, and therefore demands fuels of higher octane. A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.

Higher octane ratings correlate to higher activation energies: This being the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition or detonation.

However, burning fuel with a lower octane rating than that for which the engine is designed often results in a reduction of power output and efficiency. Many modern engines are equipped with a knock sensor (a small piezoelectric microphone), which sends a signal to the engine control unit, which in turn slows down the ignition timing when detonation is detected. Slowing down the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency. Because of this, under conditions of high load and high temperature, a given engine may have a more consistent power output with a higher octane fuel, as such fuels that are less prone to detonation.

After reading that and confusing myself even more i still find myself with the same question. Should i run purely on 91 or 98? Or should i run on 91 and every month run a 98 tank through? Hopefully some1 can help Thanks for reading guys. Palmer

I used to think 98 was better on any car and would cost more but give more kms as well....then i read about people with non turbo engines like yours who tried 98 and ended up with starting issues like it would crank longer before starting...
Its safe enough to try so you can test a tank or 2 on 98 then switch back and see the difference..

Bottom line is, You need to tune the car to the fuel you plan to run. Otherwise, stick to the factory recommended fuel stated in the manual.

Yeh I'll give that a try thanks WRXSTI02, Fair Call also MY00; Thanks for the feedback fella's appreciate it Palmer.

On my old RS, i found 95 ran better than 98.

okay interesting, I tanked up the 95octane this week and found it did run ALOT smoother! This was from BP not shell this time aswell?

My car is tuned to 98 and even then some fuels just dont sit well with her. I refuse to use BP Ultimate anymore cos i never get the same kms out of a tank.... a few times i've had it ping and had to get computer reset... i only use Shell V-Power... never had any problems with that.

I think best think to do is listen to ur car.... if u using the 91 or 95 and getting no problems or issues then continue with it. I used to take notes for a few weeks on what fuel i used, wat day filled up and how many kms i got out of it... gives u a good indication of what fuel is best for ur car.

Cheers for the advice Saha; Starting the log this week - So far it's running 100x better on 91 octane - I'll keep updating my results

I find my car gets 'used to' certain fuels.. thats the only way i can explain it anyways..
I've only ever put 98 into it.. but i vary between V-Power and Ultimate.

If i put one fuel in more than 3x, car seems to gradually get less KM's out of a tank. If I then switch, its as if it shocks the car.. and i get more KM's per tank until I've used it 3x, then i switch again :p
I live in a very flat area, I haven't boosted it since trying this theory, and the drives each week are generally the same time/distance/leg.

Also, I've found that the Caltex 98 gives me the least KM's per tank, and feels more sluggish.
V-Power and Ultimate are close, but I'd choose V-Power over Ultimate.

Very very interesting read above tho!

I'm the same as Mikey, generally stick to BP Ultimate 98 or Shell VPower unless I get really stuck. Now that you mention it, it seems I get more kms to a tank whenever I switch between the two fuels. Normally I struggle to hit 300kms to a tank but as soon as I change I seem to just clear the 300 mark...