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Originally Posted by SaabJohan

So when using high compression ratios in turbocharged engines this often means that the ignition must be retarded and the fuel mixture must be rich to supress knock. This causes temperature problems due to the later ignition and also an increased fuel consumption during boost due to the ignition and richer fuel mixture. Most modern roadgoing turbocharged engines are built this way which mean that they can run with a better efficiency of boost while still have the power that is needed sometimes.

Are you saying that as long as the ignition is retarded and the the fuel mixture is more rich, I can make the compression as high as I want to?

Turbocharged racing cars are often built with a low compression ratio, this allows the to use high boost pressures, a leaner mixture when boosted and a more advanced ignition timing. This allows the cars to run more efficient during boost with increased power and a lower fuel consumption. The downside is that the won't run that well when off boost.

So by lowering compression, I can run leaner and advance the ignition timing? Isn't running lean a bad thing? Does it not cause detonation?

But wait, there is more to it. In an engine bore, stroke, con-rod length, squish zones and so on affect the engine in many ways when thay are changed. For example, if we use a thicker head gasket to lower the compression ratio this will also increase the clearence between the piston and cylinder head. Those surfaces that are near eachother at TDC are called squish zones and the cause turbulence and also help to cool the air fuel mixture between them, this can affect combustion speed, engine knock and so on. Usually a little more distance between the surfaces are wanted for full throttle (racing engines) but a smaller distance are wanted for part throttle (road cars), so there are many things to think of when something is altered.

I thought that lowering compression with thicker gaskets, and bigger piston rods were bad? Why would having a thicker gasket be better for full throttle?

Superchargers can both use blow off valves and pop off valves. The blow off valve is used like on a turbocharged engine and it's only used if the supercharger is placed before the throttle. A pop off valve will open when a certain boost pressure has been reached, and it can therby limit the boost pressure.

So a popoff valve will open when there's too much pressure, and the blow-off valve opens up when there's too much pressure, but keeps the supercharger spinning? Now I am confused .