Detonation and Pinging
                     "Listen my children and you shall hear..."
                                by George Brown


Actually you don't want to hear either. There are two conditions you don't want 
to occur:pre-ignition and detonation. There always seems to be confusion as to 
what these two very different conditions are.

Pre-ignition is exactly what it sounds like. During the compression stroke the 
mixture ignites before the spark plug fires. This is due to a "hot spot" on the 
piston crown or in the combustion chamber. A hot spot is caused by a very thin 
metallic edge or a carbon deposit which may actually begin to glow (like a starting 
glow plug in a diesel engine or a glow plug in a model airplane engine). Usually 
what you will hear is a metallic "pinging" sound. Compression ratio and octane 
ratings have little to no effect on pinging.

To eliminate hot spots you'll either have to decarbonize piston crowns and combustion
chambers, or slightly radius any sharp edges on piston valve reliefs or combustion 
chamber edges (and/or spark plug threads in the combustion chamber).

Detonation is a botched mixture burn. When the spark plug ignites the mixture, 
the burning process should be smooth and relatively progressive, the pressure variance 
(rise and fall) looking something like a standard bell curve. For best power generation, 
max pressure of the burning fuel/air mixture should normally occur at about 10 degrees 
after TDC of the ignition stroke (which also determines the amount of ignition advance 
for your engine, hmmm?).

If the gasoline doesn't burn evenly and completely (the "flame front" doesn't propogate 
smoothly) pressure variations can occur. The pressure variations can be extreme, 
like hitting the piston crown with a hammer; causing shock loads to piston crown, 
rings, wrist pin, and rod bearing.

Detonation is almost always octane related, in that higher octane gasolines have 
additives to make them burn more slowly, evenly, and completely (yeah, regular burns 
faster than premium);but it may also occur when mixtures become too lean or ignition 
timing is set too advanced. Detonation is also compression ratio related; the higher 
the CR, the higher the octane rating requirement to prevent detonation (this has to 
do with an endothermic rise, as pressure increases the temperature goes up; this is 
Boyle's law again from high school physics: PVT = P'V'T').

With currently available 94 octane (not motor rated) the highest practical ratio 
with a fairly large displacement V8 is 9.5:1 with cast iron heads and 10:1 with 
aluminum heads (depending on your specific heads, spark plug placement, etc., you 
might go slightly higher). This is because heat plays a role, too, and aluminum 
dissipates heat better than cast iron. Note that smaller displacement engines, like 
a 1.8 liter 4 cyl, 16 valve DOHC rice burner can run a higher CR because combustion 
chamber volume and bore diameter are also variables in the burn rate cha-cha (smaller is better).

Detonation usually manifests itself as severe stumbling or stuttering under heavy load
accelleration. If detonation occurs, try retarding the ignition in small increments 
until it goes away. Obviously you wouldn't want to retard much more than 6 to 8 degrees 
from max factory specifications; if detonation still occurs try a slightly richer mixture 
(maybe raise the carb float level 1/16" and/or go up one size on the main jets, and/or go 
up one step richer on the Power Valve, and/or improve the accellerator pump shot). 
If you still have the problem you'll probably need to run gasoline additives (octane boosters), 
or for non-smog cars, mix CAM II (or other racing fuels) with pump gas. Some filling stations 
in larger cities have 100 octane unleaded gasoline.

Pinging is not good for an engine and should be corrected. Detonation can be deadly - it can
destroy an engine - and MUST be corrected.

To set ignition timing correctly using today's fuels (assuming everything else is OK), 
warm the engine up completely, then slowly rotate the distributor until you get the highest 
vacuum reading (or highest RPM; both will occur at the same position), then lock down the clamp. 
Use your timing light only to make sure the advance unit is working properly. If you get detonation, 
back off about 2 degrees on the crank (about a thin pencil line width at the distributor base) 
and try again.

Good luck, George