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vlad01 wrote:

TdracerTd wrote:Just something that I don't think has been mentioned. Higher octane fuels burn cooler and require more energy to ignite. So, by running a higher octane fuel your combustion temp for a given set of conditions will be lower, reducing the heat in the heads/combustion chamber, added to that, the fuel requires more heat/energy to actually combust. That's where the knock resistance comes in.

Yes it does burn cooler for a given CR, but the idea is its more energy dense and requires higher thermal input to react it with oxygen completely. So its required to have a higher dynamic CR to be able to release its higher energy content.

That is the reason it yields better economy in engines that have a high enough CR, if the CR is sufficient enough better power is yielded too. At those scenarios 91 fuel is virtually out of the questions due to its low knock threshold.


I have noticed 98 fuel always burns blacker in the combustion chamber and exhaust in stock V6 with 8.5:1 compared to 91 with the same AFRs, that means the combustion can't be fully utilized on 8.5, not even 9:1.


On ecotec with 9.4, that would be coming into usable range.

But with the quality of 91, I'd be using 98 regardless.

I don't think the octane rating of a fuel has anything to do with energy density. Ethanol fuels have much higher octane rating than that of petroleum based fuels, yet are much less "energy dense", I.e. for a given mass of both fuels, ethanol yield less energy output.

The reason that ethanol fuels have a higher octane rating is that it is harder to ignite and burns cooler.

Maybe the 95 in Aussie is different to the stuff we get here in NZ?

I've seen the pics of fuel filters with the red stuff running out of them but I've not encountered that here after changing fuel filters. I'll have to do the one on the missus L67 again shortly so I'll see what that one is like. It lives on a diet of Caltex 95 fuel.

its probably different, as I only use BP fuel, 95 back then and 98 now.

99% of the servos are BP in all the areas I go.

immortality wrote:Maybe the 95 in Aussie is different to the stuff we get here in NZ?

I've seen the pics of fuel filters with the red stuff running out of them but I've not encountered that here after changing fuel filters. I'll have to do the one on the missus L67 again shortly so I'll see what that one is like. It lives on a diet of Caltex 95 fuel.

red stuff is normally additive dye (injector cleaner ect) a lot of ppl just throw the whole bottle in without rtfing

TdracerTd wrote: I don't think the octane rating of a fuel has anything to do with energy density. Ethanol fuels have much higher octane rating than that of petroleum based fuels, yet are much less "energy dense", I.e. for a given mass of both fuels, ethanol yield less energy output.

The reason that ethanol fuels have a higher octane rating is that it is harder to ignite and burns cooler.

Fuel BTU's ? yeah like lpg over 100 octane but lower btu than petrol....

Meh fuel octane rating's ect has been discussed to death there are so many POV on the subject, then factor in the different variables in a combustion engine, I spent 6 years drag racing in the 80's not to mention 20+ years R/C racing in the air on the ground and water,always chancing those extra nano seconds and HP. These days Im happy that the bloody car starts and lasts as long as it can on a tight budget

TdracerTd wrote:

vlad01 wrote:

TdracerTd wrote:Just something that I don't think has been mentioned. Higher octane fuels burn cooler and require more energy to ignite. So, by running a higher octane fuel your combustion temp for a given set of conditions will be lower, reducing the heat in the heads/combustion chamber, added to that, the fuel requires more heat/energy to actually combust. That's where the knock resistance comes in.

Yes it does burn cooler for a given CR, but the idea is its more energy dense and requires higher thermal input to react it with oxygen completely. So its required to have a higher dynamic CR to be able to release its higher energy content.

That is the reason it yields better economy in engines that have a high enough CR, if the CR is sufficient enough better power is yielded too. At those scenarios 91 fuel is virtually out of the questions due to its low knock threshold.


I have noticed 98 fuel always burns blacker in the combustion chamber and exhaust in stock V6 with 8.5:1 compared to 91 with the same AFRs, that means the combustion can't be fully utilized on 8.5, not even 9:1.


On ecotec with 9.4, that would be coming into usable range.

But with the quality of 91, I'd be using 98 regardless.

I don't think the octane rating of a fuel has anything to do with energy density. Ethanol fuels have much higher octane rating than that of petroleum based fuels, yet are much less "energy dense", I.e. for a given mass of both fuels, ethanol yield less energy output.

The reason that ethanol fuels have a higher octane rating is that it is harder to ignite and burns cooler.

Im referring to petroleum based fuels, totally different chemistry. For petrol the higher octane rated fuels do have a higher density. The higher octane fuel contains heavier hydrocarbon molecules, such as octane and less of the lighter fraction of heptane etc...

Thanks for all your recent replies guys. You have manage to make me understand the answers to the questions I was asking & some I hadn't thought of asking. Finally I now understand why the Top Swap L67 need the Spark Timing retarded from stock settings.
With regards to BP fuels, a close friend who worked at the Lab in the refinery here in Perth WA told me all the fuels were strictly produced to a standard. So in saying that a lot of different brand servos over here get their fuels from BP. In the Eastern states BP servos get their fuels from the Shell refinery. She said it didn't matter if you got Shell 98, Caltex 98 or BP 98 they were all the same level of quality. She couldn't say the same for the lower premium 95's & 96's though.

Cheers

vlad01 wrote:
Im referring to petroleum based fuels, totally different chemistry. For petrol the higher octane rated fuels do have a higher density. The higher octane fuel contains heavier hydrocarbon molecules, such as octane and less of the lighter fraction of heptane etc...

The point I am trying to make though, is that the octane rating of a fuel has zero to do with it's energy density. Octane ratings refer to a fuel's resistance to ignition.

TdracerTd wrote:

vlad01 wrote:
Im referring to petroleum based fuels, totally different chemistry. For petrol the higher octane rated fuels do have a higher density. The higher octane fuel contains heavier hydrocarbon molecules, such as octane and less of the lighter fraction of heptane etc...

The point I am trying to make though, is that the octane rating of a fuel has zero to do with it's energy density. Octane ratings refer to a fuel's resistance to ignition.

Thats also how I understand it...

Not saying octane it linked to energy density, just that density being higher is a byproduct of the chemistry used in making petrol higher octane rated. That doesn't apply to other types of fuel. Eg ethanol is low density and high octane and diesel is very high density yet it's octane is rated around 20 something.

I remember there was an article on the sidi flex fuel holdens when they came out and 3 or 4 cars were driven together from Melbourne to Sydney and 3 fuels were tested. 98 for the best mileage out of 91 and e85. And the fuel densities were mentioned and 98 was few % denser than 91 and was the argument used behind the better economy. Another note was the e85 was considerably more powerful despite way less density but economy was a lot less obviously.

One way to test this is see how much BLM change from 91 to 98.

I would think that the difference between 91 and 98 in energy density would be very small and wouldn't account for the increased mileage. More likely would be that the ECU was able to optimize ignition timing on the higher octane fuel, thus improving the efficiency of the engine. The drop in economy from petrol - Ethanol would certainly be due to the difference in energy density though, as ethanol is roughly only 2/3s as energy dense as petrol by volume.