Before you can understand detonation you must have a basic grasp of the concept of "timing" as it relates to spark ignition. Timing, in that regard (as opposed to cam timing, an entirely different thing,) is the subject of when your sparkplug fires to ignite the gas and air mix in your cylinder. It takes place at a specific (but variable) point in the piston's travel up the cylinder. For a more complete explanation, see this article: Timing in this website.

Let us assume that you now understand that the fuel/air mixture must ignite at a particular time to be most effective in producing power in the cylinder.

Detonation has two forms, and several names. First, let us deal with the forms.

The first, and simplest, type of detonation is often called "preignition." It is brought about when there is a hot spot in the combustion chamber (usually on the head or on the piston, but in some instances, it can even be the spark plug itself) that is hot enough to ignite the fuel/air mixture present in the cylinder on the compression stroke. Usually the culprit is a speck of carbon stuck to the head or piston, or a rough spot, or even a sharp corner that, in a good design, does not belong there.

Unlike the other form of detonation (more on that later,) preignition takes place largely independent of compression ratio or temperature of the intake charge. The only thing that will make it quit is getting rid of the source. That usually requires cleaning up the combustion chamber by pulling the heads and removing the offending object. Rarely, very rarely, using a bore scope, you can see inside the chamber what the problem is and deal with it, but that's so unusual as to not be worth talking about here.

There are also fuel additives (like Chevron's Techron) that you can run through your engine. Over time, they'll often do a good job of cleaning up the carbon.

This type of detonation can take place silently; you won't necessarily hear it as a rattle or ping, or knock. The next section will explain why not. You may notice it though, as a roughness or seeming miss.

What makes preignition bad is that it fires off the mixture before it ought to be ignited. That means that as the piston continues to rise it is fighting the increasing pressure of the burning mixture. That reduces or eliminates the power produced by that cylinder on that stroke. It also generates excess heat that the engine absorbs, and it can set off true Detonation.

The other type of Detonation is brought about by high compression, and affected by other factors that will be explained.

Compression of a gas creates heat, as anyone who has ever used a manual tire pump can tell you. Put your hand on the tube of the pump after inflating a tire, and you can feel that it got quite warm.

So let's put a mix of air and gasoline vapor in a closed cylinder and suddenly compress it. Compress it enough, and quickly enough, and it will generate enough heat to light off the fuel charge. And that, in a nutshell, is what detonation is. It's also how a diesel engine works, but that's neither here nor there for this explanation.

Detonation though, is not a burn. It is an explosion that is so near to instantaneous that its duration cannot be measured. Essentially, all the fuel involved goes off at once.

Detonation does not generally take place purely due to mechanical compression by the piston, though. Usually, it does not take place until after the spark has fired, and in fact, the spark is what usually contributes to the detonation. Here's how.

The burning fuel charge develops both heat and pressure. That heat is absorbed by the end gas, bringing it closer to the detonation point. The pressure is not only pushing down on the piston and out against the chamber walls, but it is compressing the end gas too, heating it further still.

At some point the end gas can explode on its own, detonating. That detonation will probably not involve the entire fuel charge (some of it has already burned; right?) or even all the end gas. It can be pretty localized, especially if it takes places in a "corner" of the combustion chamber, some little out of the way pocket.

High compression also helps induce turbulence, so it is a two-edged sword. You don't want the compression too high or it can induce deto, but you do want it high enough to induce turbulence to fight deto. You walk a thin line in a high-performance engine.

Timing that is too advanced will induce detonation. There are several factors that contribute to the problem, but one is primary. It starts the fuel burn before the turbulence has become really effective, so you have a more or less solid flame front compressing and heating the end gas. Those effects, coupled with the mechanical compression of the piston going up, will build toward detonation faster than the flame front can consume the mixture. Instant deto.

A factor that can contribute strongly to detonation is the temperature of the intake charge. That is why your scooter may usually be OK while running around town normally, but ping noticeably on a hot day. It's borderline ready to ping anyway, and the extra heat of the day pushes it over the edge.

A large cylinder bore will also result in the flame front taking longer to burn all the gas if there is just one plug, too, so pay attention to that when you build an engine. The pros, like S&S and Axtell have taken the time to make their combustion chambers work well with large bores, but if you just drop a set of 4" bore jugs on your motor, and slap on some normal heads that have not been designed for max turbulence, you're begging for detonation.

Dual plugs can actually contribute to the problem, too. I saw that in an engine that I built, and was puzzled as hell. After all, I thought, dual plugs oughta make the burn faster; right?

Well, a phone conversation with the guys at Axtell revealed to me that dual plugging by itself can cause such a rapid burn that the pressure spikes very, very quickly. If you have dead spots in your combustion chambers, places that the turbulence doesn't reach (and I had a couple,) the quickly developing pressure can cause the end gas in those pockets to compress and explode before the flame front from either plug can get there. In my case, the detonation was silent, but it eroded the hell out of the piston edges in that zone. Guys with better ears than mine had ridden that bike and never heard a thing.

Take all those factors together, and project them onto a day when you and the ole lady and a load of camping gear are headed up Raton Pass in July, you just itching to try out your new big-bore built motor. Now there's a recipe for detonation!

The pass is long enough that you'll spend lots of time with the problem. Long enough that you might even melt the tops off of your pistons if you don't pay attention. Make sure you've got the highest-octane gas you can get, and keep the RPMs up. That reduces the lugging load, and gives the burning fuel less time to compress and explode the end gas.

While occasional mild pinging is to be expected under some circumstances, pinging is never a good thing in and of itself (although it can be an indicator of an engine running at a mixture that is close to "best".) If you can work away from it, do so. If you hear it, roll off the throttle or drop a gear; make it go away.

You will not always hear detonation as a ping or knock or rattle though, so know the situations that can lead to it, and avoid them.

A mixture that is too lean can detonate from leaness. (But not necessarily. It's a very complicated equation as to when it will, and won't. In our engines it can be a factor, though.) "Too lean" depends on many circumstances, so it's impossible to define here a definite ratio of fuel to air that will blow up instead of burn.

If you regularly get pinging though, the first thing to do is make sure you've got good gasoline, then think timing, then think mixture.