Knocking (also called pinking or pinging)—technically detonation—in internal combustion engines occurs when fuel/air mixture in the cylinder has been ignited by the spark plug and the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it. Combusting stops suddenly, because of the explosion, before the optimum moment of the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber and pressures increase catastrophically, creating a characteristic metallic "pinging" sound.
The fuel/air mixture is normally ignited slightly before the point of maximum compression to allow a small time for the flame-front of the burning fuel to expand throughout the mixture so that maximum pressure occurs at the optimum point. The flame-front moves at roughly 33.5 m/second (110 feet/second) during normal combustion. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the detonation occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond design limits. If allowed to persist detonation will damage or destroy engine parts.
Detonation can be prevented by:
The use of a fuel with higher octane rating
The addition of octane-increasing "lead," isooctane, or other fuel additives.
Increasing the amount of fuel injected/inducted (resulting in lower Air to Fuel Ratio)
Reduction of cylinder pressure by increasing the engine revolutions (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination.
Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction).
Retardation of spark plug ignition.
Improved combustion chamber design that concentrates mixture near the spark plug and generates high turbulence to promote fast even burning.
Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation
As you can see above......
The leading causes of detonation are : incorrect timing (too advanced), Lean mixture (carburation), Over loading, and incorrect Spark plug (too cold a heat range).
When detonation occurs temps and pressures in the cylinder rise dramatically. This causes hot spots to develope on the piston. The aluminum piston does not require much heating beyond its normal operating range before the metallurgy of the piston is affected.
When this happens the piston loses its "temper" and will melt. Aluminum pistons are manufactured from alloyed and heat treated materials and once you over heat them this is lost.
Riding two up at highway speed (60-70mph) up and down hills on a 350 sounds to me like a sure way to overwork the engine leading to detonation.
