BWalker:
I researched the technical specs for several race fuels and found that indeed, the highest grade racing fuels are highly parrafinic, low volatile content fuels. Before you crow victoriously, be aware that these fuels were all over 115 R+M/2 octane rated, and contained TEL levels "at or near the highest allowed presently by the US Surgeon General". These fuels are NOT recommended for use in any air cooled engine, but were for 'Pro Gas', F1 and the like.
All of the other race fuels for which I could find published distillation fractions(including the ones you mentioned) showed aromatic contents in the usual 30-50% range, with the balance of the blending stock being paraffinics. This is exactly as we would expect, since without TEL additive, the octane rating of the basic blending stocks will be greatly reduced. It is generally undesirable to add large amounts of oxygenates as a sole octane improver since the energy density of the fuel will suffer excessively. The only logical choice is to increase the aromatics.
All the unleaded racing fuels were of the high-aromatic type blend. It should be noted that a few years back, F1 racers all used a standard fuel blend of 86% toluene and the balance a filler stock with no appreciable octane rating. These engines would have run fine on 100% toluene, if the rules had allowed it.
If you read up on the refining process, you will discover that various aromatic species (primarily toluene) are desirable by-products released from "heavy" gasoline during catalytic cracking. These aromatics cause a dramatic increase in the octane reserve of the raw fuel stock. It takes further refining (and expense)to remove this aromatic content, and doing so reduces both the octane rating and energy density of the resulting fuel. Currently only the US seems to be in the business of removing aromatics from fuels, even though the majority of Americans breath cleaner air than they did even 20 years ago. Canada and most of Europe have actually increased the allowable aromatics in their gasolines. Canada and Europe allow about 50% aromatics.
To compensate for the resulting increase in HC and CO emissions that accompany this increase in aromatics (again, one of a only a few downsides), Canada has permitted the use of certain organo-metallics of manganese, long banned (since 1978) here because they contain heavy metals and are almost as toxic as TEL. Europe is still in the process of phasing out TEL and will probably follow Canada with alternate organo-metallics rather than the US EPA model.
The approach adopted here is to add so-called "oxygenates" which are nothing more than partially burned hydrocarbons. As I mentioned in an earlier post, these reduce energy density, but have the advantage of making the fuel burn faster and more completely. This facilitated burning results in a net increase in anti-knock properties. Note that available heat energy is inevitably reduced, though.
Toluene got a bad name as a fuel additive when some dishonest fuel wholesalers began adding a low grade (waste product of other industrial processes) toluene which is usually used in paint factories as a thinner. This was done to save money during the late 70's and early 80's when crude oil prices were at historic highs.This lower grade of Toluene contained a lot of 'junk' isomers that were hard on BuNa rubber. Engines at the time still had many seals made of Butadiene-Acrylonitrile (BuNa) rubber which worked well with the lower volatility fuels of the 70's, since aromatic content did not need to be as high. TEL was still allowed then and TEL is still the most efficient anti-knock additive available. But since the early 80's engines of all types have all come equipped with fluoroelastomer (Viton) fuel wetted seals making this a non-issue. I doubt you could even buy a kit for a Walbro or Tillotson that did not have Viton seals, where applicable. It's now a Viton world.
It should be noted that this is NOT the type of toluene sold to consumers as paint thinner. Since this low grade toluene blends poorly with resins, it blends best under mechanical power agitation, else it will may come out of solution. For this reason, consumers are sold standard industrial toluene which blends much more reliably, and is a close match for what was in gasoline originally.
The very modest decrease in volatility that I referred to in an earlier post is nowhere near severe enough to cause the wholesale precipitation/evaporation you described in your previous post; at worst it could mean harder cold starts, especially in cold weather. It will not be an issue in a warm engine whatsoever, especially given that the mixtures recommended will still be within the ASTM definition of gasoline. Anyway, isn't it true that So-called 'power enrichment' in a piston engine generally increases engine power output over stoichiometric mixtures by evaporative cooling? It does so by reducing peak combustion temp thus delaying the onset of detonation which in turn allows more ignition advance. It does not take a rocket scientist to understand that some fuel in a mixture which is over stoichiometric is not going to burn (oxidize) since there is not sufficient oxygen available to burn it. Therefore it must be increasing power some other way, like evaporative cooling.
My point that the octane of 100LL is over-rated by 2-3 point compared with auto gas still stands, irrespective of the fact that I was unaware of the exact method used to rate its octane. For that reason, it will still be cheaper to get 97-98 octane gas (equal of 100LL) by blending 93 with toluene than by buying avgas, which is approaching $4 a gallon. The difficulty for most folks of obtaining 100LL just puts an exclamation point on it.
That's All
Jimbo