Mikuni American Corporation

5: Detonation ("Spark Knock")

Detonation, often called pinging, is nothing less than a series of small explosions that take place within an engine's combustion chambers. It can be extremely destructive, breaking pistons, rod bearings and anything else from the pistons down that a large hammer could damage. It is best avoided.

Pinging is a descriptive name for detonation. Pinging is that high pitch ringing sound that an engine sometimes makes when the throttle is opened with the engine under load. It sounds as though the cooling fins are ringing as they do when you quickly run your finger nail over their edges.

Pinging indicates trouble. Trouble that does damage. That damage can be quick and catastrophic but usually isn't. Most often, detonation occurances are small in energy and the engine is able to absorb the punishment, at least temporarily. However, over time, even light detonation does harm; weakening pistons and overheating the top piston rings.

Severe detonation can destroy an engine literally in a heart beat.

HOW IT HAPPENS

After a spark ignites the air/fuel mixture in an engine's combustion chamber, the flame front travels across the chamber at a rate of about 5000 feet per second. That's right, one mile per second.

Flame front travel for detonation is closer to 19,000 to 25,000 feet per second; the same rate as in dynamite. The difference between normal combustion and detonation is the rate at which the burning takes place and therefore the rate of pressure rise in the chamber. The hammer like blows of detonation literally ring the metal structures of the motor and that is what you hear as pinging.

Detonation occurs when the air/fuel mixture ignites before it should. Normal burning has the flame front traveling from the spark plug(s) across the chamber in a predictable way. Peak chamber pressure occurs at about 12 degrees after top dead center and the piston gets pushed down the bore.

Sometimes and for various reasons a second flame front starts across the chamber from the original source of ignition. The chamber pressure then rises too rapidly for piston movement to relieve it. The pressure and temperature become so great that all the mixture in the chamber explodes. If the force of that explosion is great enough --- the engine breaks.

WHAT CAUSES IT

Anytime the combustion chamber pressures become high enough, detonation occurs. Anything that creates such pressure is the cause of detonation.

Here is a list of possible causes, it may not be complete:

* Timing - if the spark happens too soon, the chamber pressure may rise too high and detonation results.
* Gasoline - if the gasoline burns to quickly (a too-low octane rating), high pressure and detonation are likely.
* Glowing objects - a piece of carbon, a too hot spark plug or other glowing object can start burning too soon. Pressure rises too high and detonation can happen.
* Cranking pressure - Any given combustion chamber has a maximum pressure (before the spark is struck) beyond which detonation is likely.
* High engine tempertures - High chamber temperatures raise cranking pressure and promote detonation.
* Lean jetting - Weak air/fuel mixtures can result in very uneven mixtures within the chamber, uneven burning, pressure spikes and detonation.

Note that each of these possible causes are relative. That is, there is no absolute timing, mixture strength or ignition timing that is going to guarantee detonation. Equally, there are no absolute settings that guarantee that detonation does not occur.

Motorcycle manufacturers, Harley-Davidson included, spend a great deal of time and money fine tuning their engines to eliminate or nearly eliminate detonation. When we change the engine design in the direction of detonation by, say, raising the compression pressure with domed pistons or milled heads, we increase the chance of detonation actually occurring.

Gasoline quality helps determine whether or not an engine is going to detonate. The higher the octane rating, the lower the chance of detonation.

Modified engines often have had several engine design changes that, combined, increase the likelyhood of detonation. High compression pistons, thin head gaskets, some alternative ignitions, some exhaust system designs, etc.

Stock street bike carburetion is very lean for emissions purposes. When the air cleaner and/or exhaust system are replaced by less restrictive components, this stock jetting becomes impossibly lean. The engine does not run well and detonation is likely at some throttle settings. Re-jetting or wholesale carburetor replacement (Mikuni!) is the cure for this particular problem.

If one fits high compression ratio pistons together with an early closing (mild) cam, the cranking pressure may become high enough that serious, engine-deadly detonation is likely. How much is too much you ask?

Well (Rule of Thumb here), Evolution engines are fairly safe against detonation if the cranking pressure remains at 180 psi or less. The TC88 motor can dodge detonation if the pressures remain at 190 psi or less. Keep in mind that these maximums are for fairly stock engines; no porting, no chamber work and no squish areas.

A well shaped combustion chamber with squish effect is much less likely to detonate than most stock examples. The main reason the TC88 engine can withstand higher cranking pressures than the Evo is its better chamber design.

Cranking pressure here refers to the number one gets by conducting a normal compression test. This test is done by removing the spark plugs and fitting a compression gage in one of the spark plug holes. The throttle is then held open and the engine cranked with the starter until the gage needle stops climbing. The resulting number is the cranking pressure.

Ignition systems are important. If the spark plugs fire too soon, the combustion pressure may rise too quickly bringing on detonation. The main reason for having an advance curve built into an ignition system is to avoid detonation. The correct timing for any given engine design (and state of tune) varies with rpm and throttle setting.

Hot spots is more than a night club. If your engine has been running rich or burning oil, it may have thick bits of burned-on carbon. This carbon build-up can literally glow and, under the pressure of compression, start burning before the spark is struck. This leads to severe pressure excursions and, often, detonation.

Lean carburetion can lead to detonation. Uneven combustion in over-lean air/fuel mixtures can escalate pressures and bring about sudden explosive burning. Also, lean mixtures elevate chamber temperatures which, as you now know, can lead to dreaded detonation.

If all this leads you to think that your engine is in imminent peril, then we have succeeded. Detonation is a terrible thing to happen to your expensive Harley engine. The pressures of those explosive events can be enough to hammer rod bearings, pistons and rings into useless junk.

If you hear the tell-tale ringing of detonation next time you open the throttle on a hot day or at low rpm or after a tank of questionable gasoline, back off the throttle and ride carefully until you can find and render harmless this demon visiting destruction upon your motor.