TheSpeedRacer

Bottom line, use the proper octane to prevent knocking. Even though the ecu can retard the timing when knocking occurs, the knocking has to first occur.

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A Honda knock sensor is a modified microphone that only has enough sensitivity to detect PRE-ignition or knock up to 4000 rpm. After this point, it can't distinguish between regular engine noise as the motor revs from knocking and so it only acts as an idle/partial throttle detector and then instructs the ECU to retard the base ignition timing map program at these rpms BEFORE it can get ugly up in the higher rpms. Once detonation occurs, it is too late: the horse is out of the barn.

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Detonation; Knock; Ping; Pre-igintion. You hear these terms mentioned all the time, so we might as well straighten them out.

Let's get pre-ignition out of the way first. Nothing mysterious about it. The A/F mixture (intake charge) explodes before the spark plug fires. You would figure the intake charge would have to get pretty hot to do that, and you would be right. The pressure from a high compression engine is enough to generate that kind of heat. (In fact, diesel engines are designed to fire on the heat from compression alone.) Higher octane fuel is the antidote, so in general, a higher compression engine will need higher octane fuel.

Cramming more intake charge into the combustion chamber has the same effect as raising compression, so in general, the higher your boost, the higher the octane requirement to avoid pre-ignition. Finally, premature inflagration occurs more easily if the intake charge is hot when it enters the engine. This is why larger intercoolers add a margin of safety in forced induction engines--at least until you turn up the boost.

Another cause of pre-ignition is a hot spot in the engine. Maybe some of those carbon deposits are glowing red hot. Maybe the spark plug itself is hot enough to ignite the mixture before firing. This is almost certainly the case if you have ever experienced a car that kept trying to run after you turned the key off.

The more tricky term is "knock." Although most of us prefer to talk about "detonation," it turns out that "knock" is the correct term as used in automotive texts.

"Detonation" is actually slang, and "ping" is not a well defined term at all. That having been said, I will stick with the term "detonation" for this discussion.

"Detonation" differs from pre-ignition in that it occurs AFTER the mixture starts to burn. Normal burning involves a flame front--a relatively slow, controlled explosion--which marches along in a calculated fashion. As you would expect, normal burning raises the pressure in the combustion chamber. Sometimes this is enough to get the last bit of intake charge (called the "end gas") so excited it explodes before it is supposed to. It is a very hot explosion, on the order of ten times the heat of controlled combustion.

But there is more to it than that.

If you graph the amount of pressure in a combustion chamber during normal burning, it shows a relatively smooth event. The occurrence of detonation shows up as a sharp spike on the graph--a sudden shock wave if you will, with pressures on the order of several thousand psi. The duration and strength of the explosion is too fast to contribute to the rotational output of the engine. Like a slap in the face, the full impact must be absorbed within the combustion chamber itself. Damage is most likely to occur at the weakest points--namely the piston pin and piston crown. Piston engines designed for high stress situations can have the piston rings further away from the crown of the piston. The shock of repeated detonation will eventually weaken anything it can, and the heat generated will take care of the rest.