One cause for detonation, or just overheating of some parts, and other types of engine damage, is called LUGGING. Lugging does not have to lead to piston detonation. It can. Lugging does not have to lead to pre-ignition. It can.

There is no absolutely totally agreed-upon definition of lugging. I will give you definitions and descriptions that are generally accepted.

You increase the throttle, the rpm and speed does not increase, or does so very sluggishly; or even Decreases. Some might add to that idea that you cannot reach yellow or red line rpm in that gear, no matter how much throttle. Manufacturer's almost always have gearing such that you can NOT reach red line rpm in top gear, even at sea level. Thus, the top gear may sometimes be thought of as 'an overdrive'.

Lugging is not usually involved in the situation where you are maintaining a reasonable speed on a highway with light to moderate throttle, and neither gaining or loosing speed.

BAD lugging generally occurs at lower rpm.....an example might be your trying to ride your bike up a hill at 2000 pm, using very heavy throttle; and, bad lugging also can occur at higher rpm, and THAT is generally where it is seen the most....perhaps 3500 or even more. Generally, though, lugging is thought of as a lower rpm situation, with heavy throttle.

The problem with 'lugging' is that cylinder pressures go quite higher, perhaps extremely high, and a lot of heat can be, and usually is, developed. A hundred or so degrees of extra heat on an exhaust valve is actually a fair amount, and two hundred+ or so is A LOT. The pressure and heat is increased, and the length of time that these two are increased is lengthened, thus the cooling provided by the exhaust valve closing and thereby transferring heat from valve to exhaust seat, is REDUCED. The head heats up, the exhaust valve heats up, and even the intake valve heats up more. Not only are the valves affected, but lugging is hard on the rod bearings. If the lugging is done at rather low rpm, the bearings will be, in essence, starved for lubrication. Bearings need proper rpm and oil flow for lubrication protection. A treatise on how lubrication REALLY works is on this website.


TWO other types of ""lugging"" needs to be discussed here. This first one is probably hardly ever described as lugging. It is when a fair amount of throttle (usually, but sometimes only quite light) is being used at quite modest rpm, and one hears a metallic pinging noise from the engine. Americans call this PINGING, the Brits call it PINKING. It is commonly the result of too low octane in the fuel for whatever the engine tuning and condition happens to be. That can be thought of in another way, meaning that an engine that runs fine if in proper adjustment, can make such noises if the engine adjustments are wrongly made or compression ratio too high for the gasoline quality, etc. Most Airhead engines have a pinging "zone" from idle rpm to maybe 4,000 rpm (at the very most). Pinging/pinking is quite affected by the ignition timing and piston speed, and is one of the several reasons that engines generally have an automatic ignition timing device. For the Airheads, the ATU (Automatic Timing Unit) is located inside the ignition control area on the front of the engine. Except for the earliest /5 Airheads, our Airheads ATU's start advancing the ignition at about 1500 rpm and quit at about 3000 rpm. Occasional and transient pinging/pinking is not very dangerous to the engine's health, but it DOES accumulate its effects over long periods of time. If the pinging is relatively constant, for a period of time, that is BAD for the engine, and BAD things can happen, quickly.

The final type of lugging is very serious. This effect is called detonation, or piston detonation, and is not an explosion in the way the word detonation is typically used in life (although if the piston collapses it will look like it). This piston detonation is caused by extreme pressures in the cylinder. It is NOT a metallic sound, but more of a brighter thudding. Detonation can happen from a faulty ignition system, but that is not something too likely. Detonation is a sudden burning of the fuel, typically caused by the compression itself, like in a diesel engine. It can happen so early in piston movement that the pressures are ENORMOUS on the piston top and rod, and if prolonged, the piston can collapse or disintegrate.