Petrol prices! Collins to get to the bottom of it

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#1714708 2-Feb-2017 21:33

Waitomo opened one in mt eden somewhere. I asked them on facebook if they sold 95 or just 91 and diesal and they wanted me to install their app to see what was sold there. Ummm, no thanks, I try to have as few apps as possible and they really should put that on their website.

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#1714838 3-Feb-2017 10:01

@richms yes they have 95, currently at 189. 9 :)

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shk292

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#1715205 3-Feb-2017 23:09

I was a bit shocked a couple of days ago when filling at BP on Lake Road - advertised price for 91 was 186.9, but the only higher octane fuel there is 98, and this was 225.9. I've never seen such a high margin.

My car needs at least 95; 98 seems to offer no advantage. Has anyone drawn any conclusions over which filling stations have the lowest difference between 91 and premium? It's annoying that only the 91 price is visible before you get to the pump. 10-20c extra per litre seems OK, but 39c??

alliao

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#1715216 4-Feb-2017 00:29

You may object but as long as your cars relatively new like post 2005 or so adding 91 won't hurt the engine. If it's got more than one oxygen sensor it should be able to know if fuels too rich or lean, and will adjust the timing accordingly. All that's most likely going to happen is you will get less torque at lower rpm.

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#1715230 4-Feb-2017 02:39

alliao: You may object but as long as your cars relatively new like post 2005 or so adding 91 won't hurt the engine. If it's got more than one oxygen sensor it should be able to know if fuels too rich or lean, and will adjust the timing accordingly. All that's most likely going to happen is you will get less torque at lower rpm.

Be very careful saying that. The octane rating is a measure to the fuels resistance to preignition or predetonation. (how much heat and pressure you can subject it to before it ignites by itself). High compression engines need high octane fuels due to the higher cylinder pressures. Sure often the engine computer will detect the extra engine knock. It will then retard the ignition advance. But it will often do so throughout the entire operating range. So you will have to press the accelerator more just to cruise at the same speed. So more fuel usage. And on cars fitted with exhaust gas recirculation (EGR) the computer will often increase the amount of EGR used, as a means of lowering combustion temperatures. So again less power but also increased risk of EGR system failure. (Extreme cases the inlet manifold almost completely fills up with soot, google Mitsubishi GDI for examples of this). And definitely very dangerous to run a turbo car on 91 unless the manufacturer specificity states you can.

There are lots of cars out there which are fitted with ultra lean burn technology. Under cruise conditions some could goto as much as 20:1 air fuel ratio. Instead of the usual 14.7:1 They would get excellent fuel economy, but had to have high octane petrol. They got killed off as those cars has very high NOx emissions, but low carbon emissions. Now to comply with the new rules, We have to have low NOx emissions and higher carbon emissions as a result. And you get cases of a 10 year old car using less fuel than the brand new version of that car.

Short answer - only use what the manufacturer says is the correct fuel for your car. If you bought a $10K+ car and cant afford the correct grade of petrol for it. Then you obviously should have bought a cheaper car. As extra repair costs will quickly wipe out any savings on fuel costs.

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#1715237 4-Feb-2017 06:54

shk292:

I was a bit shocked a couple of days ago when filling at BP on Lake Road - advertised price for 91 was 186.9, but the only higher octane fuel there is 98, and this was 225.9. I've never seen such a high margin.

My car needs at least 95; 98 seems to offer no advantage. Has anyone drawn any conclusions over which filling stations have the lowest difference between 91 and premium? It's annoying that only the 91 price is visible before you get to the pump. 10-20c extra per litre seems OK, but 39c??

Extreme discounting is only occurring with 91 - so there isn't really a solution to your dilemma. If you need 95 you basically need to pay extra for it.

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#1715238 4-Feb-2017 06:56

I'm definitely not going to use 91, it's explicitly against the manufacturer's instructions and no doubt would void the warranty. It would also probably spoil the magic by which they get 200hp out of 2litres without using a turbo 😊

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#1715239 4-Feb-2017 06:59

sbiddle:
Extreme discounting is only occurring with 91 - so there isn't really a solution to your dilemma. If you need 95 you basically need to pay extra for it.

I figured that might be the case, the 91 price is a special discount that hasn't been applied to other grades. Makes marketing sense I guess

GGJohnstone

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#1715241 4-Feb-2017 08:15

I have cars close to 20 years old. After careful consideration and measurement it can be seen that 91 petrol has more energy per volume at a lower cost per volume. I refute the claim that higher octane equals less fuel is used to generate motor power regardless of the vehicle. I agree that wide open throttle is able to give a detectable increased output with greater octane figures. There is an argument that a penalty of harm to an older design engine exists at W O T through actively burning gases persisting in the exhaust with higher than required octane fuel.

The notion that higher is cleaner, ethanol is more powerful and benzene is the same stuff, has a place in advertising along with celebrity endorsements and encouragements to save the planet.

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#1715265 4-Feb-2017 09:16

shk292: I'm definitely not going to use 91, it's explicitly against the manufacturer's instructions and no doubt would void the warranty. It would also probably spoil the magic by which they get 200hp out of 2litres without using a turbo 😊

There's no reason why you can't mix some 91 and some 98 to get 95 octane in your tank. The relationship is linear, so a 50/50 mix of 91 and 98 would give 94.5 octane.

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#1715267 4-Feb-2017 09:20

frankv:

shk292: I'm definitely not going to use 91, it's explicitly against the manufacturer's instructions and no doubt would void the warranty. It would also probably spoil the magic by which they get 200hp out of 2litres without using a turbo 😊

There's no reason why you can't mix some 91 and some 98 to get 95 octane in your tank. The relationship is linear, so a 50/50 mix of 91 and 98 would give 94.5 octane.

Yes, I understand that, thanks. But there are enough places selling 95, easier to go there

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#1715292 4-Feb-2017 10:35

shk292:
sbiddle:

Extreme discounting is only occurring with 91 - so there isn't really a solution to your dilemma. If you need 95 you basically need to pay extra for it.

I figured that might be the case, the 91 price is a special discount that hasn't been applied to other grades. Makes marketing sense I guess

And the fact that 91 does make up the vast majority of petrol sales and Gull don't do 95.

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#1715298 4-Feb-2017 10:41

Still waiting for Collins to crush a petrol station.

tdgeek

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#1715303 4-Feb-2017 10:47

So these discounts, up to 40c if you buy a good amount of groceries. Who wears the discount? I assume most by the supermarket?

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#1715311 4-Feb-2017 10:51

GGJohnstone:

I have cars close to 20 years old. After careful consideration and measurement it can be seen that 91 petrol has more energy per volume at a lower cost per volume. I refute the claim that higher octane equals less fuel is used to generate motor power regardless of the vehicle. I agree that wide open throttle is able to give a detectable increased output with greater octane figures. There is an argument that a penalty of harm to an older design engine exists at W O T through actively burning gases persisting in the exhaust with higher than required octane fuel.

The notion that higher is cleaner, ethanol is more powerful and benzene is the same stuff, has a place in advertising along with celebrity endorsements and encouragements to save the planet.

I always felt, after anecdotal tests in my 84 Prelude, way back in the day, that I went further on higher octane, car felt better, and that going a lesser km on lower octane, at a reduced cost, wasn't worth it. But we are overdue for an Octane thread!

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