Australia... settled by convicts...
Queensland... settled by convicts of the country settled by convicts....

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Again ......
This is part of what I think is so wrong with Australia at the moment.

We pay $1.40 for unleaded and more for 98RON. All sorts of gobblety-gook about the pricing structure, excises (its not a tax !!!!), transportation costing and of course, volume discounts (our case lack of ??)

Yet different states have different pricing structures.

God, let some sanity and commonsense happen here. We cant afford to be a nation divided over an economy matter.

Rediculous part of this whole fuel thing is, 80% of petroleum crude still comes from OUR reserves in Bass Strait. We pay `somebody' (WHO???) Singapore prices for it.
Wasn't always so... thank Fraser I think?

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Hold on there seems like theres a bit of confusion. Octane is not some sort of an additive or an active particle - so the way I understand it petrol doesnt "loose it relatively quickly" the higher it is nor does it make your petrol denser. Octane rating relates purely to the ability of petrol to resist detonating when it is compressed.

You dont want the fuel to detonate on the compression stroke before it is ignited by the spark and is at the optimal compression for the power stroke. So the higher the compression ratio of the engine the higher is the octane rating of the petrol needed to run in it effectively (that is without detonating prematurely).

Higher octane petrol is not neccesarily higher in calorific value per litre, so it is not neccesarily more energetic by volume as compared to the lower octane petrol. In fact my understanding is that that 95 Octane petrol we get here at Shell pumps marked as "formulated with 10% ethanol" is actualy less energetic than standard 95RON and even 91RON because the calorific value of ethanol is so low.

So in essence the only difference between different octane ratings is the potential for energy production not the actual energy. Compression is one of the main factors affecting engine performance - In essence by compressing the most petrol/air mix into the least ammount of space before ignition you achieve the most efficient energy production. Because higher octane rating allows achieving higher compression ratios w/o detonation the potential for the production of energy is higher in higher octane rated petrols.

But you need an engine unit capable of utilising that potential - ie an engine with a sufficiently high compression ratio. Performance mini engines, like many modern engines have higher compression ratios and are designed to run with high octane petrols which will not detonate at the engine's operating compression ratio. Lower octane petrols will detonate prematurely in such engines and cause them to operate inefficiently. At the same time standard engines with lower comp. ratios (all your 998-1100 standard engines have a comp ration of about 8.5/1) will derive no benefit from higher octane ratings as they just dont operate at such compression ratios.

So to conclude if your engine has a high compression ratio, high octane (98Ron +) petrol will be of benefit plus the engine will run as a dog with anything lower (though you can modify the ignition timing if you insisit on running low octane) But if your engine is stock standard and has a low comp ratio and runs fine with low octane petrol then anything else will be a waste of money. Off course the petrol companies say that the higher octane petrols are their premium products and so are designed with a higher calorific value - but who knows for sure....just try for your self ( I can't I have an 11/1 comp. value and have to use 98RON as a minimum).

sorry for the long post and this is just my layman's understanding of the whole octane issue.

I agree with most of what Mike says.
But, in the old days a high octane rating was achieved by adding tetra-ethyl lead. It stays in the fuel until burned.
Nowadays with no lead, it's achieved by adding higher fractions and volatile solvents of one sort or another, eg toluene. These tend to evaporate out first, potentially lowering the octane rating of the remainder in your tank.
Someone posted here a while back that the BP Ultimate 98 (for example) starts to lose it after 3 days...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Wouldnt be surprised, if it does, anyway I feel that my petrol starts to evaporate immediately. After being parked for a few days I could swear that the fuel level seen when shining a torch (of the electric type, not the fire type) into the filler neck is lower. Anyway just wanted to emphasise what octane (or octane rating) was.

And would evaporation of hidrocarbons be a problem in modern cars with pressurised tanks?

Great info Mike, thanks,

So in an opinion other than mine is my 998 engine Deluxe (at 8.3:1 according to the BMC shop book) considered high, medium, or low compression?

I use PULP in my 8.7:1 alloy head, 1960's engines in my other classics to great success.

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YDO005 1965 stock standard MORRIS MINI DELUXE

8.3 is pretty low.

Yeah, that was what I was thinking of. I seem to recall the advice was to buy your fuel from the busy petrol stations not the quiet ones.

Yeah, 8.3 is low - look on here - http://www.ausmini.com/forums/viewtopic.php?t=33719 - and look at what they're talking about with boost... 10+psi boost achieveable at 8.5:1 compression... Obviously that would be with the appropriate components (head gasket, bottom end, etc) and fuel. But it shows that 8.3:1 isn't high. Modern atmo cars often run over 10:1, although pretty well never over 12:1 (I can't think of one that high). WRX's are in the 8:1 range...
I could be wrong here, but I'm of the understanding that the reason for relatively low compression ratios in older cars was due to the inconsistency in fuel quality available back in the '50's and '60's? Correction appreciated if necessary. Modern engines have knock sensors and alternate fueling/timing maps to allow for such situations. As shown by a lot of Mini owners on here, you can easily run a higher CR on an A Series. It's only a matter of proper tuning and the appropriate fuel.
Methyl Benzene is another octane booster that can be used. Yes, octane rating is just a measure of how well the fuel resists detonation under compression. But like Doc said, octane rating is adjusted using additives - otherwise all fuels would have similar octane ratings. And, with current pollution laws, the additives permitted to boost octane are those which tend to evaporate out pretty quickly... Which is a pain when you only drive you car once a week... Put in some 98RON fuel, drive it a week later, and you can feel loss in octane. If you don't fill up for over a month, you can hear it, too, on the top of your pistons...
I'm like most Queenslanders, in that I moved here. Unlike the majority of them, I wasn't fleeing Victoria (most of the Gold Coast), South Australia (my neighbours), South Africa, or New Zealand! That was only 12 years ago - I'm not convict stock, not that convict stock is anything to be ashamed of - my wife can trace back to 1814 in Sydney. I don't care too much about Queensland's history, just its future... And cheaper petrol prices!!! And other things which are cheaper - I believe housing stamp duty is cheaper... No annual rego inspections (so you don't have to have working brakes unless you're selling the car)... It's pretty easy going up here! Just need some water... But we're not alone there.
DrMini, as the people of Australia don't own the oil in Bass Straight (it's not drilled by the government), what else would we expect..? Just like we don't own the phone system, the power grid, airports... Do we own anything any more? If you don't own something, and you need it, it's easy for the holders of said necessities to extort whatever price they want. Please note, I am not saying oil companies are doing anything illegal. They're just doing what any business should do - making the most they can with their product. If we owned (i.e. extracted, rather than selling off licenses) the oil, we wouldn't have to compete with Singapore to buy it.
Back on topic, 91 RON should work fine with a stock Mini engine's compression (8.3:1 or 8.7:1), but, as I asked earlier, is my memory about ethanol fuels possibly causing carburettor seals to perish correct?
The End!

The issue of whether ethanol will damage the seals within a carburettor is probably more prevalent for carburettors with a rubber diaphragm such as Mikunis etc. Not as big a problem with SUs. The main concern would be the O-ring on the float bowl, rubber fuel lines etc. I don't run ethanol mixed fuels myself but I don't think a 5-10% blend would do much harm.

I believe some petrol stations have 100 octane fuel with 5% ethanol, one of my mates runs it in his Mini and has had no problems, reckons it runs better than 98.

Thanks for the answer Anto.
I believe the 100RON fuel is Shell V-Power. Not available everywhere - I haven't seen it yet.

I think it is V-Power Plus or V-Power Slightly Better or something, the V-Power down at my local shell is 98 ethanol free as far as I know.

I only use Vortex or the highest octane unleaded in Sally with her Flash lube.

I filled her up 2 weeks ago with Vortex and drove here everyday in and out of town (5km each way) 3 times each day....good fuel economy I think

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1978 Leyland Mini 998LS All Original *Sally*
2002 Mini Cooper *Coopa*

I run Mobil 8000, it doesn't seem to evaporate as quick as some other 98s.
I've given up on Caltex 98, my local Woolworths/Caltex usually either has a queue of people out onto the road, or the Vortex 98 pumps empty.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.