I could psycho-bable a bib full of verbiage, addo, but won't!
First rule, is find the optimum combination ratio. Basic, cheap process is to add the highest grade leaded to the lowest grade regular gasoline/petrol.
Our Kiwi 91, 96, or 98 (give or take a number or two) is similar to your Aussie 92, 98, or 100. The Yank grades are 89, 93, but they don't sell a 98 grade just yet. The rating difference is
RON+MON
...........2
in the US,
while we Pacific Prisioners of Mother England use RON ratings.
The difference varies, but its about 5 points lower in the land of the US 55 gallon drum. Who ever said Texans always have something bigger haven't tried to embarress them over there conservative octane ratings!
David Vizard noted that the US octane situation whent critical in 1973 when there unleaded 87 to 89 (RON+MON)/2 fuel was made mandatory on all new cars. Before hand, air craft style 105 octane leaded fuels were common.
Today, everywhere except the Middle East gets unleaded 91 RON, 96 RON, and Japan, Aussie and New Zealand get 98 to 100 RON (Mobil Synergy 8000, Shell Optimax, and others), while air craft fuel is still leaded 105 RON in most countiries due to safety issues. The risks of running low shelf life is too much to allow plans to run on 96 unleaded unless some severe restrictions are made on the conditionon plastic pieces and shelf life. With leaded avgas shlf life is very long.
The best option is to rate the octane of the cheapest base fuel (91 or 92 RON) with the octane of the heighest octane avgas additive (105 RON avgas).
On the top use an Imperial 22 gallon drum, and create two columns.
Ist, the total average fuel costs per combination ratio
100% base fuel, 0% additive
90% base fuel, 10% additive
80% base fuel, 20% additive
70% base fuel, 30% additive
60% base fuel, 40% additive
50% base fuel, 50% additive
Then work out the charted calculated average octane rating for the combination.
Your aim is to get the recomended RON the weed wacker maker recomends. This is often 95 RON, (and these days, they set it up for unleaded fuel, and warn that leaded fuel pollutes the sparking plugs or hurts tune).
You make no allowance for the poor shelf life of the base penaut oil grade 91 or 92 RON fuel, becasue the higher the octane the unleaded the worse the shelf life. Regular grades are okay for months. Super and Ultra High are very short life fuels because the are just stock regular with a huge spike of armonatic hydrocrarbons like benzine and toulone. They try to keep the total aromatics below 40%, but benzine goes up to 5% in 95, rather than 3% for the 91 Any avgas added will increase shelf life because Tetra Ethyl Lead is a long life boost to octane, benzene and toulone is a easily evaporated aromatic.
Second thing is to keep a 22 gallon drum upside down, and turn and shake it each week.
Add 0.5 to 1% Ethyl Methyl Ketone or acetone to the mix to keep from precipitate forming.
Do these calculatuions for each combination of fuel, and you'll see something which will suprise you. Petrochemical companies are very good at making certain fuels like 96 and 98 octane, and especially additives like 105 Plus and VP Plus. They do so at the real cost, plus a margin. The retail cost ischarged in additon to its real cost.
This is because they have to refine the oil so its total aromatics% is below 40, and this has to occur even if they add another few % of benzene to rasie the octane rating. You get nothing for nothing with petrol.
All the extra effort must be paid for, or else they will loose a margin and end up having a loss leader. As the farther of the Ford Falcon and four place Thunderbird, Robert Mac Namara would tell you from his grave, loss leaders are evil, and there is no place for it in modern society. Unless your a lover of giving gifts to the poor.
If you want to not get paid for brewing your own long life high octane AddoBrew, Adam, then Shell, Mobil, Caltex, Ampol or Challenge won't stop you.
Whew! Hope that made some sense. I can do some calculations, but then you wouldn't learn to do it yourself!
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