page6

PRIMARY DETONATION DAMAGE

Detonation is generally thought of as spontaneous ignition of the end gases ahead of the flame front advancing frora the spark pług. This leads to severe pressure fluctuations or "shock waves" within the combustion chamber causing chamber walls and other parts to vibrate in sympathy. This phenomenon produces the familiar knock or ping which may or may not be audible to the operator under actual driv-ing conditions.

While sustained detonation produces a generał rise in temperatures, with re-sułtant weakening and removal of metal frcm the piston crown, severe detonation is most likely to hammer a sharp-edged hole through the piston...shown at the right. Radial cracks and a depressed area may be found adjacent to the actual break.

Ihe high frequency vibrations are likely to split the spark pług insulator and flex ground electrodes until fracture occurs at the bend. Photo at left shows a pług damagcd in this fashion will have the electrode stub securely welded to the shell.

When primary detonation damage is indicated, check the following: a) basie ignition timing settings, b] automatic spark advance, c) leaking intake manifold, and d) cross-fire.

PRIMARY PREIGNITION DAMAGE

Preignition or surface ignition is considered to occur when glowing deposits on an exhaust valve or overheated spark pług ignites the fuel charge before the timed spark discharge. This leads to extremely high temperatures and pressures within the combustion chamber which may, in tum, incite detonation. The extreme temperatures associated with preignition generally result in melting of piston...shown at the left below.

Spark pługs exposed to sustained preignition temperatures are likely to have their center electrodes melted away. In the extreme cases, the ceramic insulator tip itself may appear fused...photo at right below.

When primary preignition damage is indicated, check the following:    a) valve tappet

adjustment, b) amount o f combustion chamber deposit ,

c)    spark pług heat rangę,

d)    condition of cooling system, and e) ignition timing