Is higher octane petrol actually worth more?

[ATAl]

Something I was thinking about on my way home last night:

Does higher octane petrol cost the company more to make? i.e. does 98 actually cost more to make than 91?

Or are petrol companies just ripping us off because it's a higher number therefore worth more (like sunscreen)?

Anyone know?

[wde_bdy]

Quick answer -
Yes, as you refine petrol you get less and less high end product compared to low end product.

Callum

[MrOizo]

Quick answer -
Yes, as you refine petrol you get less and less high end product compared to low end product.

Callum

Arent there more addatives also?

[Flannelman]

I got told that all NZ refinery does to get 95 is bubble LPG through 91...

The best thing is also Gull, or any bio-fuel using ethanol, is that ethanol has an octane rating of 114. Adding this to 91 yealds an easy 98. A poorer feedstock is needed to make 91.

On that note - octane rating is only needed under full throttle. Some tests with and XR6 Turbo showed up some very interesting results regarding power and ethanol. On E85 (85% ethanol so an octane close to 110) they squeezed out over 350rwkW on stock internals (yet larger fuel delivery system) and found that by running too much ignition timing made the torque curve fall rather than detonate.

So as a fuel, if you have a shitbox (like i do now - 92 telstar) there is no point using anything more than 91.

[ATAl]

I don't like ethanol based fuels, I get about 50km less out of a tank of gas with them which makes them effectively more expensive.

Interesting about the bubbling lpg thing, do you have a source for this?

[Mr Revhead]

Yeah never heard the LPG thing....


Alcohol based fuel is EVIL and a con.
It kills your economy and they don't lower the price. So the more alcohol they put in out fuel, the more it costs us to run it.

[Malcolm]

I got told that all NZ refinery does to get 95 is bubble LPG through 91...

The best thing is also Gull, or any bio-fuel using ethanol, is that ethanol has an octane rating of 114. Adding this to 91 yealds an easy 98. A poorer feedstock is needed to make 91.

On that note - octane rating is only needed under full throttle. Some tests with and XR6 Turbo showed up some very interesting results regarding power and ethanol. On E85 (85% ethanol so an octane close to 110) they squeezed out over 350rwkW on stock internals (yet larger fuel delivery system) and found that by running too much ignition timing made the torque curve fall rather than detonate.

So as a fuel, if you have a shitbox (like i do now - 92 telstar) there is no point using anything more than 91.

You should be careful about randomly throwing around hearsay and conjecture as facts.

The LPG thing is BS, as is the "octane rating is only needed under full throttle", as is the last statement about how shitty your car is = level of benefit to running higher octane. Some shitboxes will get better fuel economy on higher octane petrol and some cars (depending on a number of factors) could experience detonation at less than full throttle on lower octane fuels.

Ethanol blends will always give worse fuel economy because ethanol's specific energy is much lower than petrol - with E85 you would expect to use roughly 50% more fuel than 98 petrol

[Lith]

Ethanol blends will always give worse fuel economy because ethanol's specific energy is much lower than petrol - with E85 you would expect to use roughly 50% more fuel than 98 petrol

Thats a bit exaggerated, and depends on the application. I tuned a high compression NA motor on 98 recently and then E85 back to back and it "only" needed about 20% more fuel for E85. We used way less E85 than anticipated and the left over E85 is still being slowly eaten through. Being its basically an occasionally street driven race car, he is not going to bother with having it tuned for regular pump gas as its just not worth the effort swapping - even though its pretty easy.

For what its worth the car made 7-8% more power in that instance to give an idea of the loss vs gain of the stuff.

[Malcolm]

FYI the specific energy of E85 is approximately 30-35% lower than petrol (depending the constitution of both), so if you fully combust 1L of petrol, to get an equivalent energy release from combusting E85 you'd need approximately 1.5L. If you're only adding 20% more fuel and getting the same/more energy output then it suggests the previous tune was doing a bad job of extracting energy from the petrol - I can see how this could be the case with a turbo charged engine as increased octane of the ethanol could allow you to operate the engine in a more thermally efficient manner (i.e. better ignition timing, higher cylinder pressure)

edit: figured you would be talking about a turbo but re-read and saw it was NA - I'm guessing knock limitation was a big factor on the petrol (and potentially still a bad/rich tune on petrol)

[Malcolm]

giving the NA thing a bit more thought - the increase in power makes sense, as an NA engine's limiter is typically the amount of air that can be drawn into the cylinder in an intake stroke, E85 comes with the benefit of bringing some of its own oxygen with it, increasing the amount of oxygen available to assist in combustion within the chamber.

Still, on the fuel use side of things...physics/chemistry doesn't lie, so it had to have been running too rich on the petrol so half the potential energy was pissing out the exhaust ports as unburnt hydrocarbons and CO.

[Mr Ree]

Good thread...

Quite surprised no one has mentioned the fact that the numbers we read at the pump do NOT relate to actual octane numbers though.

We use RON as the accepted measurement in NZ and as such, the actual octane rating it alot lower than most people "presume" it is

Rough rule of thumb is that with all other things being equal, that BP 98RON is the equivalent of 93 octane, 95RON equates to 90 octane, and 91RON=87 octane.

Some food for thought for those with snails on their engines.

[ATAl]

Do you mean the RON vs MON ratings?

[Mr Ree]

Do you mean the RON vs MON ratings?

No specifically that mate, just the fact that a huge portion of performance car owners refer to our pump fuel as 95 octane, or 98 octance when in fact, it is far lower.

Why oh why cant we have 110 octane pump fuel *sniff*

[Mr Revhead]

Or PON, they are all octane ratings, just different ways of measuring. Some better than others

[Malcolm]

Do you mean the RON vs MON ratings?

No specifically that mate, just the fact that a huge portion of performance car owners refer to our pump fuel as 95 octane, or 98 octance when in fact, it is far lower.

Why oh why cant we have 110 octane pump fuel *sniff*

that's not entirely true, as the meaning of "octane rating" is slightly ambiguous. There's Research Octane Number (RON) and Motor Octane Number (MON). Our gas is advertised as its RON value, where a lot of other countries use the average (RON+MON)/2

The difference is in the way the octane rating is assessed, the MON value is typically (always?) lower than the RON and based on a slightly more extensive test (both ratings are tested for anti-knock properties in special engines)

[Mr Ree]

Do you mean the RON vs MON ratings?

No specifically that mate, just the fact that a huge portion of performance car owners refer to our pump fuel as 95 octane, or 98 octance when in fact, it is far lower.

Why oh why cant we have 110 octane pump fuel *sniff*

that's not entirely true, as the meaning of "octane rating" is slightly ambiguous. There's Research Octane Number (RON) and Motor Octane Number (MON). Our gas is advertised as its RON value, where a lot of other countries use the average (RON+MON)/2

The difference is in the way the octane rating is assessed, the MON value is typically (always?) lower than the RON and based on a slightly more extensive test (both ratings are tested for anti-knock properties in special engines)

For the record, I would prefer RON+MON/2 in NZ

[Malcolm]

Why? At the end of the day it's just a number, and it's more the relative values of the numbers available that is important when you're at the petrol station, deciding which handle to grab.

[Lith]

Still, on the fuel use side of things...physics/chemistry doesn't lie, so it had to have been running too rich on the petrol so half the potential energy was pissing out the exhaust ports as unburnt hydrocarbons and CO.

I'd have to check the datalogs, I could be wrong there. I have in my head we went from 60% to 80% IDC for the same load/rpm zones. From memory the thing was running in the mid/high 12s AFR on pump gas vs ~7.9-8:1 on E85 at full load which is definitely on the rich side (in reference to mid/high 12s for pump fuel), but its ~13:1 compression and quite easy to knock. The car cruises near stoich on both pump fuel and E85.

Bare in mind I am just spouting those numbers off the top of my head, but they will be fairly close to those areas. Short of saying the O2 sensor was lying or that I was supplied with a lower ethanol %age than the supplier advertised I'm not sure what else could explain it.

Why? At the end of the day it's just a number, and it's more the relative values of the numbers available that is important when you're at the petrol station, deciding which handle to grab.

+1, doesn't change the fuel we get any more - so just research the make up and quality of the fuel if you intend on getting crazy with it. At the end of the day if you tune it to suit the fuel, it should be fine.

[Malcolm]

I'd have to check the datalogs, I could be wrong there. I have in my head we went from 60% to 80% IDC for the same load/rpm zones.

Well that's a 33% change for starters (even more if you consider the reduced flow periods on injector open and close)

On the reporting of octane ratings - as it happens I'm currently working on some fleet emissions modeling and have been looking at fuel properties and legislation - Australia has dropped the minimum MON requirements from their fuel specifications, stating that it is an out-dated parameter (as modern engines are less susceptible to high speed knock) and AFAIK will only be requiring fuel companies to meet a minimum RON for each fuel grade.

[Lith]

I'd have to check the datalogs, I could be wrong there. I have in my head we went from 60% to 80% IDC for the same load/rpm zones.

Well that's a 33% change for starters (even more if you consider the reduced flow periods on injector open and close)

<face palm> I don't usually have maths fails like that, oops You can probably see how I stuffed that one up. Duh.

On the other side, you are quite right in terms of how much more it needs - it all makes a lot more sense now....