Petrol for 2008 Subaru Impreza S-GT variant. 91 OK?

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Mehrts

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#3269825 9-Aug-2024 11:11

Running a lower octane fuel can end up being false economy too for a lot of vehicles.

Sure, it might cost less to fill up each time with 91 vs 95/98, however your fuel efficency can often take a hit and you end up losing range on that same amount of fuel so you end up refuelling sooner.

That, and some engines really do not respond well to lower octane fuels, to the point where engine damage by detonation can occur. So for the few bucks saved, you'll potentially be up for an engine replace/rebuild in a worst-case scenario.

The best thing to do for fuel savings is to get a car which is capable of running a lower octane fuel. Of course these won't be any performance vehicles, but it's a tradeoff. Power vs outright efficiency.

richms

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#3269858 9-Aug-2024 12:57

If you care about saving money, buy a NPC car like an aqua and drive slowly.

In almost all cars I have had, putting 91 in makes less power at low revs, so you have to use more accelerator, then it will downshift and rev more so it ends up less efficient. That's before you even worry about grenading the motor. New stuff with variable boost will usually just give you no boost and therefore no power, but hey, "saving money" right?

Older or manually tuned cars will just detonate and kill themselves. Some have so much compression with the wastegate open fully that 91 cant be adjusted for and will detonate regardless of how much the timing is shifted and extra fuel is put in.

The constant extra fuel to prevent detonation is hard on catalytic converters, and will often make check engine come on in order to save the cats from being cooked up.

Richard rich.ms

JPNZ

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#3269930 9-Aug-2024 15:35

Mehrts:

Running a lower octane fuel can end up being false economy too for a lot of vehicles.

Sure, it might cost less to fill up each time with 91 vs 95/98, however your fuel efficency can often take a hit and you end up losing range on that same amount of fuel so you end up refuelling sooner.

That, and some engines really do not respond well to lower octane fuels, to the point where engine damage by detonation can occur. So for the few bucks saved, you'll potentially be up for an engine replace/rebuild in a worst-case scenario.

The best thing to do for fuel savings is to get a car which is capable of running a lower octane fuel. Of course these won't be any performance vehicles, but it's a tradeoff. Power vs outright efficiency.

Finally someone said it, it may cost more at the pump but higher octane in cars that need 95/98/100 will get your more range case for case.

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mudguard

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#3269933 9-Aug-2024 15:44

JPNZ:

Finally someone said it, it may cost more at the pump but higher octane will get your more range case for case.

I'm not sure about that. Take car one, my Corolla, runs on 91. It will not make anymore power with a higher octane fuel. So it's a waste for me to put in anything higher than 91. Take car two, my Civic which has an Integra Type R engine. I have to run it on 98, and was told by the engine builder if I get stuck I can put 96 in it and drive straight to the nearest station with 98 in it. It has more aggressive timing, and higher compression I assume than my Corolla. But if I put 100 in it, I'm unlikely to make any more power without getting the laptop out. And my fuel economy won't improve.

I think it goes without saying that there won't be any performance or economy difference if you run an octane higher than recommended.

In the OPs case though, I worry about a JDM version, which if it was the same as the Honda versions, tended to required the higher octane fuel standard over the NZ new models.

To be honest, I could probably test the octane thing in the Corolla. It went through 2545L of 91 last year to do 43,451kms.

Mehrts

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#3269943 9-Aug-2024 16:08

mudguard:

JPNZ:

Finally someone said it, it may cost more at the pump but higher octane will get your more range case for case.

I'm not sure about that. Take car one, my Corolla, runs on 91. It will not make anymore power with a higher octane fuel. So it's a waste for me to put in anything higher than 91.

Try going the opposite way though, this is what we're talking about. Using a lower octane fuel in an engine that expects a higher one.

Efficiency won't be as good, or as you said yourself with regard to your Civic "I have to run it on 98, and was told by the engine builder if I get stuck I can put 96 in it and drive straight to the nearest station with 98 in it." This is because you run the risk of detonation, which trumps any potential savings in fuel cost by trying to use a lower octane.

Yes, you're probably right about using a higher octane fuel than what the engine is expecting with regard to power & efficiency. This basically comes down to how well the engine management or tune can deal with this. A lot of OEM ECUs can handle some change in octane and adjust timing etc accordingly on the fly, but a lot can't. In these cases, you're spending more money on fuel which is giving you no real benefit.

Basically, use the octane your car was designed to run on, and nothing less.

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