-Pre-Ignition
Pre-ignition almost always leads to detonation, but... pre-ignition also has a terrible habit of destroying motors. Pre-ignition is when the air/fuel mixture is ignited BEFORE the normal ignition point (dictated by ignition timing) by means other than the spark plug firing. Pre-ignition causes the piston to compress an expanding mixture, which causes a huge strain over a long period of time compared to detonation (which happens quickly and is over). The mixture usually detonates when pre-ignited. Potential ignition sources could be an overly hot combustion chamber, glowing carbon embers, a glowing spark plug, and any other residual heat in the combustion chamber that shouldn't be there. The problem with pre-ignition is you simply don't know if the detonation you are hearing is from pre-ignition until your motor blows. Rest assured, most detonation encountered is not from pre-ignition -- however, if your motor detonates enough, it could raise the combustion chambers so much that your engine pre-ignites, which tends to put the piston on the ground, and we all know that isn't where it goes, now is it! That is why you address detonation, in addition to the fact that severe detonation can damage the motor as well.

-Octane
Octane is a rating used to describe a fuels resistance to detonation. It is also a compound, but we don't care about that.

-Ignition Timing
The point at which the spark plug fires before top dead center on the compression stroke to ignite the air/fuel mixture. More ignition means you ignite the mixture sooner. The idea is to expose the power stroke (piston moving down) to as much of the force of combustion as possible while not detonating and not compressing an expanding mixture by starting the burn too soon. Ideally, you start the burn while still compressing the mixture because it takes some time to burn. If you have a high enough octane fuel, you can actually run too much ignition timing and compress an expanding mixture -- this puts enormous stress on your rods and rod bearings and is akin to mini pre-ignition. You CAN over-advance the motor with water/methanol. Don't advance ignition timing without being on the dyno to see if it adds more power. If it doesn't add power, take it out, it's just adding stress. Ideally, you should be just below peak power ignition for a long lasting boosted motor tune.

-Latent Heat
Latent heat, for our purposes, describes the amount of heat absorbed by a liquid as it changes matter states to a gas when vaporized.


-Why do we run rich?
This is something many folks take for granted -- you run rich, or fuel enriched, under load. More so for boost, less for n/a, but not at all for cruise. Why? It's simple. We don't want our motors to blow up. Why would it blow up? Because it gets hot in there damn it! Running rich means there is extra fuel left over that didn't get burned. This fuel absorbs heat. This keeps the engine from being damaged. At cruise (in vacuum), the power produced by the engine is tiny -- you don't need to run richer than stoich to protect the motor, and catalytic converters need stoich combustion exhaust to work correctly.


-How exactly does fuel cool the combustion event?
The unburned fuel is vaporized (evaporated) -- it changes from a liquid to a gas. This change in matter state, from a liquid to a gas, takes heat with it. Think of when you sweat -- liquid exits your pores, wind blows across your skin, the sweat evaporates, and your arm gets cooler. The problem with blindly adding fuel to cool down the combustion event is that it makes the air/fuel mixture richer... and after a while, it doesn't want to burn, and it stops making power (and other problems as well). The amount of heat absorbed by a liquid as it changes to a gas is described via its latent heat.


-Enter methanol...
Methanol, an alcohol, is a fuel that your engine can burn. It takes more than twice as much methanol to make the same power as gasoline, which is directly related to it's stoich point being about half that of gasoline. It literally takes half as much oxygen to burn an equal mass of methanol as it does gasoline, and that directly represents it's energy potential. Ethanol is an alcohol also (om nom nom), and has a similarly low stoich point, and similarly lower power output, which is why e85 folks have to run huge fuel systems compared to gasoline folks. The good thing about methanol is that it has an octane rating of around 120, and cools about THREE TIMES as well as gasoline when vaporized in a rich mixture. I know that sounds wonderful, but just wait until we get to water. By adding a little bit of methanol, you can raise the octane of your mixture a decent amount, and any remaining fuel is partially methanol, and thus will remove more heat than if it was purely gasoline.


-Enter water...
Water is a.... wait, we drink this stuff, it shoots out of our pores when we're hot... perhaps we could use this for cooling?! Indeed! Water removes TWICE as much heat as methanol, and OVER SIX TIMES the heat of gasoline. Want to know the really cool part? (Oh man, I sincerely apologize for that pun, it wasn't intentional, but now that I see it, I'm not removing it because it hurts sooo good.) What's really cool is that any water injected into the mixture does not burn. ALL of its heat removing goodness is left there to absorb heat, regardless of the air/fuel mixture you used.


-The proper mixture of water/methanol to inject:
This is debated a lot, but it generally ends up being 50/50 by mass, not volume. Sorry guys who are mixing your own and filling two jugs equally, you failed -- you should have used a scale. Methanol is about 74% the weight of water, so if you thought you were running 50/50 and you measured by volume, you're actually running around 37% methanol to water. DOH! Don't worry though -- if you're not detonating, then you're in the money, because all that extra water is there to cool the combustion event and perhaps the boost charge as well, especially if it's hot enough and has some distance to travel (see turbo piping and high boost supercharger.) Ideally, in my opinion, you'd run the perfect mixture that had just enough methanol to up your octane rating enough to ward off detonation with the cooled combustion event -- this is because the water cools better than methanol. If you run 35/65 (-30F washer fluid, what I used to use), and it prevents all detonation, moving to 50/50 would just add octane while removing cooling ability. This is why I never messed with mixing before I went to 11 pounds. Now that I'm here, one 100F+ day and I detonate a little -- time to add more octane. By running 35/65 instead of 50/50 when that was all I needed, I cooled the combustion event (and to some degree, the charge) quite a bit more. Remember that the methanol that cools is what is left over, so more water means more cooling, not only because it cools better than methanol, but because it isn't burned. The methanol is still burned, and the remaining methanol from running rich is all that is left to cool! This is why I'm so against people running more methanol than they need -- not only is running pure methanol dangerous, it's not required most of the time. Any methanol you can replace with water means more cooling and less flash danger. Seems obvious now, but good luck convincing some folks.