Carburetor Jetting - Should you go lower to run higher octane fuel?

beyond my knowledge. doubt if jet change needed.

 

from my understanding, higher octane generally gives the A/F mixture a better strike rate and yes, can be run a little leaner safely. the fuel is refined in the evaporation chamber, im told by a Shell rep that when it is harvested in the higher parts of this chamber, it is in fact the 98 and upwards ron yeilded. lower parts of the chamber give off 91ron fuels. so therefore in my logic..... the 98 would be a much 'finer' fuel.... so does this mean you could in fact lower jets sizes due to this finer fuel? this could also mean the fuel swirls much better with the air mixture as it gets drawn through the manifold? in turn increases the total % of fuel utilised when the plug fires.

interesting experiment would be to down size jets and re-do the idle mixture..... see if it makes a big difference? using heat range 8 irridium plugs too could top this off well.

 

The answer is yes you could jet down slightly, but not for the reasons you may think.
He's the easy way to think about it:
A higher octane rating doesn't mean the liquid itself holds any more potential energy, it's just that you can get more energy out of it (when mixed with air) because you can squash it up (compress it) more before you light it (Ignition).
When you do light it you get a multiplication of pressure rather than an specific addition, so for more power squash it up as much as you can.
The actual a/f ratio it's best to burn it at stays broadly the same however as maelstrom says. So, if the world was ideal, it would make no difference what fuel you put in, and your jetting would always be right, irrespective of what octane rating the fuel had - just so long as it was high enough not to cause detonation.
However, the world isn't ideal, there's death, worse than that the tax man, and worse still oil companies, who to reduce cost are sneaking ethanol into all fuels in small amounts - not just your E10. Ethanol of course makes you run lean, so you should jet up accordingly. So, if your jetting has been done recently on a dyno on 91, then it will have taken this into account, and the lower ethanol content of 98 will allow you to jet down slightly to compensate.
If your jet specs are from a time (80's) when things were a little more honest then they should stay as they are, even on the 98.
More likely your jetting is just rich anyway as it's specified for the whole world, you've got some warm weather, a slightly clogged air filter etc, so the ethanol in 91 got you back to the correct ratio - that's why it felt better.
The only way to tell is to take it to the dyno, or use it as an excuse for more extreme tuning.
Looking back I have a feeling this post doesn't really answer your question!

 

115 octane. do not know jetting but if you run methanol or nitro you need bigger jets.you can also run hydrazine but it explodes at room temp so needs to be kept chilled.has been banned for 50 years. was magic fuel.

the avgas allows you to run higher compression which means more explosion so more power.it is a leaded fuel but now hard to buy in 20 litre cans[shell a]

 

detrimental-NO, downside-COST OF FUEL.plus-RELIABILITY.if you rn methanol it will melt the plugs and pistons by running lean.pistons develop holes where the plug fires and the electrode gets funny deposits.nitro will grenade a motor if you get a backfire.never leave methanol in an alloy container as it corrodes it[carbies]

avgas is run in plane engines for reliability. i do not know their compression ratio but it fires well with magneto spark.you can put in in your car engine with no problems,

 

im liking all these facts!