I recently got interested in gasoline and octane because the Ford Cobra engine in my Panoz Roadster has been down on power ever since CA introduced 91 octane ethanol gas. So I did a little research and here is what I found. I'm not a chemist, so I welcome any corrections and additions.

Octane measures how much the gas will resist self-ignition. Octane does not measure energy content or burn rate. You need just enough octane to prevent the fuel from self-igniting under compression, so it doesn't start to burn until the spark plug fires. How much octane an engine needs depends on several factors: compression, timing, A/F ratio, etc. More octane than the minimum needed to prevent detonation is a waste.

Below, "octane" means (R+M)/2 octane, and "octane spread" means the difference between Ron & Mon octane.

For an example of octane spread, take 2 theoretical gasolines, both 92 octane.
Gas A has a Ron of 94 and a Mon of 90 -> (R+M)/2 = 92.
Gas B has a Ron of 100 and a Mon of 84 -> (R+M)/2 = 92.

Both gasolines are 92 octane because pump octane is (R+M)/2 which is the average of Ron and Mon. But Gas B is more likely to ping than Gas A, because it's the Mon that determines the effective octane under heavy load.

So octane spread is as important as the octane rating itself. Just because the fuel meets the minimum (R+M)/2 spec doesn't mean it won't ping under heavy load. Ideally, for racing you want a narrow octane spread.

Natural gasoline has a low octane [what is it?] so even 87 octane gas has some level of octane boosting additives. However, in general, gasoline has higher energy content than most octane boosting additives. So the higher the octane, the more additives, the lower the energy content. However, while this is generally true it's not necessarily true in every case. It's possible for a 92 octane gas using higher energy additives, to have more energy than an 87 octane gas using lower energy additives.

MTBE is an oxygenate with a natural octane of 118. It helps boost the natural octane of gasoline, and also helps clean up emissions.

In CA, MTBE is being phased out and replaced with ethanol. Reasons for this are some combination of environmental / toxicity concerns or political lobbying for corn farmers, depending on who you ask.

The problem is, ethanol has a natural octane of 101 -- significantly lower than MTBE. Furthermore, it has a wider Ron/Mon spread, meaning its octane under heavy load is even lower. Also, ethanol has a lower energy content than MTBE.

This means when replacing MTBE with ethanol, one must use more ethanol than MTBE to maintain the same overall octane. But there is a limit to how much ethanol you can put into gas without causing problems, and this limit means we can't use enough. Thus when MTBE was abolished, the octane of high test dropped from 92 to 91 octane.

However, even more important than this 1 point octane drop, was that the Ron/Mon spread got wider. This means the effective octane under heavy load (Mon) dropped a lot more than just 1 point.

So what we end up with is that engines designed for 92 octane gas start to ping. Even engines designed for 91 may ping, since they were designed for 91 octane gas with a higher Mon (e.g. with a narrower octane spread).

This is why CA's new ethanol based gasoline provides lower HP, lower fuel economy, and pings in high performance engines. But supposedly, it will keep the air and water cleaner and the corn farmers sure aren't complaining.

Here is where Xylene and Toluene enter the picture. Most gasoline already contains Xylene, Toluene or both. I'm going to ignore Toluene because most places seem to have stopped selling it because it was being used illegally in drug labs. Xylene has an octane of 116 (or 117, my sources varied) with a narrower spread than ethanol. It's sold at hardware stores, paint stores, Home Depot, etc. Cost seems to be around $5-$10 per gallon. It's a non metallic aromatic, so it shouldn't hurt O2 sensors. However, high enough concentrations may dry up and crack rubber / plastic parts in the fuel delivery system. Most sources I've read say concentrations around 1:10 are safe and it won't hurt anything up to around 1:3. I have absolutely no idea how true that is.

1 gallon Xylene + 9 gallons 91 octane = 93.5 octane.
That's because the octane of a mixture is more or less linear:
(1 * 116 + 9 * 91) / (1 + 9) = 93.5

Even though the Cobra engine only really needs 92 octane, this gives you a little safety margin to compensate for the too-wide octane spread of the 91 octane ethanol gas (because it's not really 91 octane under load). I tried it and repeatedly romped the heck out of my car at WOT high RPM and not only is the pinging gone but some of the top end power and crisp throttle response that I lost seems to have returned.

I've done this myself with good results. But no guarantees. From what I read, I believe it's safe for me and for my engine when handled carefully and mixed properly but I'm not a chemist and I have NO IDEA whether that's true. I was willing to take the risk of damaging my car on this science project and feel that I got a positive result. If you try it, it's at YOUR OWN RISK of course. Everything I'm saying here could be wrong. Check up on it yourself and please post any corrections. Handle Xylene with all the care and caution you would use for gasoline.