RD ,i think you are confusing compression ratio with compression pressure. The ratio is fixed mechanically by the piston stroke swept volume and the volume left in the head/piston at TDC. Compression pressure is a function of compression ratio in a good engine but drops over time with age/wear whereas compression ratio will never change. Sorry to be pedantic

 

Our colleague is confused on this and so many other points that I don't know where to start.

Hopefully the OP's car is operating correctly again.

 

Have to chime in here. Knock occurs after the spark plug fires, never before. All of the cures in a particular engine sample for knocking involve modifying ignition advance through the initial setting of advance timing or adjusting the mechanisms or electronics that advance timing as the engine RPM increases. All modern gasoline engines fire the spark plug before the piston is at Top Dead Center. This is advance timing. The cure in older engines to knock was to reduce the initial setting of the timing advance by degrees in so doing the spark plug fired when the piston was closer to the top of the chamber.

The fuel burn is not an explosion, rather a propagation. The engineering magic is the design of the chamber to make this as efficient as possible. When the fuel has a pre ignition issue it is due to the heat in the chamber causing a portion of the air/fuel mass to ignite before the propagating combustion started by the spark plug reaches it. This is caused by a heat issue. The way to reduce this heat is to start the combustion process later by reducing the timing advance. The reason that I did not mention pressure as in compression ratio is that pressure creates heat. More pressure equals an increase in heat. The only heat control variable for all practical purposes that the designers and mechanics have control of in a manufactured sample is the advance timing because when you start combustion by firing the spark plug you get more heat through the combustion itself and the increased pressure of the propagation.

Knock is a sound that echos through the piston rods because it is an explosion as opposed to the desired orderly propagation. The forces created by it are multi directional shock waves that damage components.

At least in our engines and most that I am aware of, the VVT is not used to modify compression ratio.