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DETONATION explosion- A Bad Thing.
Entire books have been written on the subject of detonation, and the subject is not yet fully understood by engineers working with internal combustion engines. Some things, though, are clear. This is just a quick overview of the subject, to help you understand what you may be hearing from your engine, or from someone else's mouth when they speak about the subject.

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.

Other sometimes you can get rid of some carbon buildup that may be making the hot spot by adding some water to the fuel mixture by using a spray bottle to squirt a heavy spray of water into the carb or throttle body throat with the air cleaner off. That does not mean add water to the gas tank, or dump a cup of water down the carb throat.

The water sprayed into the carb throat or throttle body flashes to superheated steam when it hits the combustion chamber, blowing off pieces of carbon, which then (you hope) go out the exhaust valve. It may take repeated applications to get results, and you may never get the result you need, but I have done it occasionally, and it has sometimes worked. I'd rather try that than go straight to a head job.

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.

NOTE: I AM NOT TELLING YOU TO DO THIS! If you decide to, do so at your own risk! I do it, but then, it's my bike. Usually, a little spritz of water will do the trick, or several squirts. Sure as hell though, some idiot out there will dump in a cupful all at once, fluid lock the piston, bend a rod, and break a crank, then blame me. So I'm not telling you to do this.

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.

See, when a spark ignites the fuel charge, it makes the fuel burn up quickly, but it does not explode it. There is a definite burn period, measured in thousandths of a second, to be sure, but it is progressive. That's as it should be; a nice, controlled burn. Ignition timing is set so that the burn reaches its peak pressure roughly when the crank and the connecting rod are at right angles to each other after the piston has passed Top Dead Center and is on the power downstroke.

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.

Note that a sentence to the left mentions ". . . all the fuel involved." Detonation does not necessarily, or even usually, inolve the entire fuel charge in the cylinder. Typically, only a portion of the charge detonates. The amount that does so determines the severity of the detonation, and the loudness and character of the noise you hear.

That explosion creates supersonic shock waves within the combustion chamber that ring back and forth across and around the chamber. They are so severe that in some cases they can erode the aluminum surfaces of heads and pistons. They also generate heat that softens the aluminum so that erosion is still easier. It is those shock waves hitting the chamber walls that we hear as ping, or rattle, or knock, and that is why we may not hear the preignition type of detonation. It is a burn, but an early one, not generating shock waves. Well, it may do so, by a mechanism explained farther down, where preignition induces the other type of detonation.

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.

First, a term: end gas. End gas is that portion of the fuel air mix in the chamber that has not yet burned after the spark has begun the burn process. Remember, the burn is not instantaneous; it takes time, so a portion of the fuel/air mix is unburned for a good while.

Then another: Octane. Some fuels resist detonation more than others. A fuel's resistance to detonation is expressed in Octane Points. The higher the octane number, the more resistant that fuel is to detonation. A higher octane fuel may not be as "powerful" as a lower octane fuel, but in use it may be more effective because it can burn without exploding, which the more powerful fuel might do, thus losing its power.

"Powerful" is a loaded word that is shorthand for a bunch of other concepts, mostly having to do with how much pressure the fuel generates during its burn. Octane has nothing, absolutely nothing, to do with the raw power of the fuel.
Here's a fundamental, simple thought to keep in mind while we discuss more complex issues about detonation. What we really, really want is for all the fuel in the charge to burn smoothly before it has a chance to explode.
Now, onward.

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.

Or not so out-of-the-way. A hemi head with a domed piston and a spark plug on just one side of the chamber can be bad news that way if the compression ratio is too high, or octane is too low - think "shovelhead." The spark starts the fuel burn on one side of the dome, creating heat and pressure on the end gas on the other side of the dome. The dome, though, is in the way of the flame front, and the delay of the flame front gives the end gas on the other side of the dome a chance to overheat and explode before the flame front can get there to burn it.

Strong turbulence in the combustion chamber will help control detonation by swirling the flame front throughout the chamber, starting little fires all over, burning up end gas in lots of places, rather than leaving pockets to explode. Turbulence can be induced by shaping the port, by shrouding the intake valve, and by putting a squish band (also called a "quench band") in the chamber. That's a ledge or section of the chamber that the piston comes really, really close to (40 thousandths or less. 30 thou is good; less than 20 is too close) making the gas mix in that zone squirt out into the chamber. Ideally, it's aimed to squirt at the spark plug.

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.

"Quench band"
Heat is a primary factor in making fuel detonate, as opposed to burning. A hot mixture will detonate sooner than a cool one (which is why supercharged engines often have intercoolers: it's to cool off the fuel/air mixture that has been heated up by being compressed in the blower.)
Hot end gas will also detonate sooner than relatively cooler end gas. And the cylinder walls and head surfaces, although quite hot, are still cooler than the heated end gas. Soooo - the theory is that if you can create a squish band that is quite tight, the end gases will be cooled somewhat by contact with the relatively cooler metal surfaces. When that takes place, the area is called a "quench band" because it quenches the heat of the mixture being compressed in the zone.
In typical Harley motors, the rule of thumb is that if the clearance on the squish band is more than 40 thousandths of an inch, it will probably have no useful cooling effect, and will generate only minimal turbulence. That pocket will, however, be an excellent place for detonation to take place, and you may (as I did) find signs of detonation on the top of the piston or on the head in that zone.
Lugging the engine can induce detonation. The relatively slow piston speed, low turbulence in the chamber, and high load give the burning mixture a chance to set off the end gas before it can burn up.

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.

Go up one range in octane, and quit buying it at "Fred's Cheapo Shop 'n Rob;" all gas is not equal. Then check your timing. If it's nominal, right where it ought to be, try richening the mixture a bit by going up one size on the jet the carb is using when the ping takes place. If that doesn't work, you may have to retard the timing a couple degrees.

Don't cheat yourself by buying cut-rate fuel. If your handbook calls for premium, use premium. If you have a built engine, and know the compression ratio, be conservative in your fuel choice. An 8 to 1 compression ration can get away with regular gas. 9 to 1 can get away with regular as long as all other factors, like temperature and load are OK, but it can be pushed into deto pretty easily. If you're running 9.5 or above you'd better be using high-test all the time, no bull, or be prepared to replace your pistons and other stuff on short, noisy, notice.


Fuel Problems
Something else to think about when gassing up is this: more and more stations have one hose serving all three grades of fuel from the pump.
That means that if the guy who filled up just before you put low-octane regular in his pickup, and you come alone and want the best premium hi-test you can get, you'll be mixing in at least a couple quarts, and maybe as much as a gallon (depending on how the pump is plumbed), of regular along with your premium gas. That reduces the overall octane of what you wind up with in your tank.
Will it make a difference? Most times, probably not. But a combination of the wrong circumstances ("high perf motor, hot day, etc) and it sure can.

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