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DETONATION . AND PREIGNITION
More racing engines sustain power losses, are crippled by 4) Fractured spark plug firing ends may indicate detonation
detonation, and are destroyed by preignition than by any is occurring.
other engine abnormality. 5) When a cylinder goes into preignition or detonation ...
Detonation is not detected in practice or competition be- _ whatever the primary cause... the spark plug is usually
cause its audible sound (“ping”) is drowned out by the roar destroyed. Consequently, other components or tuning con
of open exhausts. A racing engine will not perform at peak ditions may have initiated this state.
power when in detonation. A racing engine cannot survive Preignition destroys engines by heat not by mechanical
many races even in a state of mild detonation, as the physi shock. The searing increase in heat occurs on a piston and
cal and thermal stresses inflict progressive damage to parts cylinder wall when ignition occurs too early in the cycle.
... piston, spark plug, rod and bearing failures, and, in many Although preignition may occur in only one cylinder* of
cases, blown head gaskets can be traced to the engine being a 4-, 6-, or 8-cylinder engine, the unaffected cylinders will
in a state of detonation.
continue to reciprocate at high speed dragging the preignit
Spark occurs —Combustion begins ----- continues ------------ > detonation ing piston up and down. The intense heat can cause the
piston to seize, break, and “blow” the entire engine.
Providing plug range is correct, a piston is the most
vulnerable part of a racing engine. It takes heat over the
------- . full surface of its crown but can pass heat only at its cir
cumference ... this must be transferred by the piston rings
J to the cylinder walls using two oil films as the conducting
agent... the oil on the cylinder wall and the oil on the
A* piston.
DETONATION
*Hot spot, difference in engine timing among cylinders,
Detonation is an uncontrolled burning of the fuel. A too hot a spark plug, extremely lean mixture due to
smooth flame front does not occur. Detonation is a violent unequal manifolding, ring flutter, ring breakage.
collision of flame fronts within the cylinder occurring after
the spark plug has fired.
DETONATION
T.
I T.D.C.
'I
. G. N. duces a general rise in tem u
I perature, with resultant u
J weakening and removal of u
I
I metal from the piston crown. J
I Severe detonation is most
Sustained detonation pro Limn.
I I likely to hammer a hole
through the piston. The hole
--------------------------- VOLUME----------------------------► --------------------------VOLUME-------------------------- ► is usually sharp-edged. Radial
cracks and a depressed area
NORMAL COMBUSTION DETONATION ("KNOCK-PING1') may be found adjacent to the
Detonation can lead to preignition. When preignition oc actual break. Evidence of ex
curs, a spark is not required to ignite the fuel. A part within cessive temperature can some
the combustion chamber has reached the point of incan times be seen in pitting of
descence, or the fuel (due to its instability under pressure metal on the top surface of t piston. Broken ring lands
and heat) simply ignites without spark. As no orderly firing or cracks in the piston wall m also be in evidence. X
sequence occurs, the engine or the cylinder “goes out of
time.”
PREIGNITION
Ignited by regular ignites flame fronts
hot deposit ignition spark remaining fuel collide The extreme temperatures
associated with preignition
generally result in melting of
the piston. The edges of the
breakthrough indicate a typi
cal thermal failure unlike the
mechanical failure caused by
detonation. Spark plugs ex
P
posed to sustained preignition
PREIGNITION
temperatures are likely to
Here are some important points to remember about deto have their center electrodes
nation and preignition: melted away. In the extreme
1) Sustained detonation can cause temperatures to rise to cases, the ceramic insulator
the point where preignition takes over. tip may appear fused. Broken
2) All fuels (gasoline, methanol, “nitro”) have preignition or split spark plug firing en are usually indicative of
limits... all are different. The amount of pressure, the detonation.
degree of temperature the mixture strength determine
these limits. If detonation is suspected, richen up or retard spark ...
3) If equal cooling and uniform mixtures are not existent or both. These “emergency measures” may prevent engine
in all cylinders, the lean (or “hungriest”) cylinder will damage until the intrinsic cause is located. In some cases,
detonate or preignite first. detonation can be “seen” on a spark plug (page 43).
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