by Alain Hoffmann
Noise - Spark Knock
This is only for petrol burners so what are you diesel guys doing here?
Spark knock comes up under heavy acceleration and sounds like marbles being bounced around in the engine. A well defined and frightening noise. Stops as soon as you step from the 'Go'-pedal.
Spark knock or pre-ignition can have several causes. Your engine needs leaded fuel (on older Rovers but they have such low a compression knock is unlikely to occur), bad fuel quality (a problem with high-output engines as the Vitesse versions, they have an 10.5 compression ratio) or too much advance. If you intend to buy a car which makes noises like that and the owners pretends "Don't worry, it always made this sort of noise when goin uphill" run away as far as you can. Driving for any lenght of time with spark knock harms every engine. VW edited some 20 years ago an pamphlet on the famous van where they discribed the phenomen and gave a figure of 1:1000 for wear. This means for every mile you drive with spark knock your engine wears as much as in 1000 miles of normal travel. Those figures, according to all my teachers, are even way on the low side.
Why is this so? In normal conditions The piston travels upward without much resistance (1). Then you have an flame front in your cylinder building up just in time the piston reaches it's top travel (2, TDC). The expanding gases now gently push the piston down until the lowest point of it's travel (3).
In case of preignition the gases are already expanding BEFORE the cylinder reaches its top point (4). The expanding gases now hit the piston which MUST still go up a bit as it's forced by the crank. This gives the noise as the piston gets hammered on the head, transmitting the hit to it's pin, rod and bearing to the crank and the main bearings. All of them get a heavy blow. The extremely hot gases are now still trying to expand but get further compressed by the pistons stroke. This increases the temperature of the gases by several hundred degrees. (5) Probability of a burning valve, wrapped or burned gasket is increased also by 1000 times. On it's way down the piston and the walls still have to cope with the much hotter gases (6).