This uneven, spontaneous burn produces amplified pressure waves in the combustion chamber. Raise the compression ratio too far and a spontaneous auto-ignition occurs towards the outer edges of the combustion chamber, instigated by the sudden rise in cylinder pressure when the spark plug ignites, instead of burning outwards from a central point. In an ideal world the compressed fuel/air mix ignites from a central point in the combustion chamber (where the spark plug is), expanding out evenly in all directions and pushing the piston down (because that's the only bit that can move at this point). This is known as detonation or 'knocking', and it's this that prevents engines running very high compression ratios.Ĭompressing a gas raises its temperature, so as the piston rises and the fuel/air mix is compressed, it heats up. The trouble with petrol is that if it is compressed too much it starts igniting of its own accord, rather than when you want it to. But there are limits to how high the compression ratio can go. By way of an example, a standard CBR600RR has a 12:1 compression ratio, while an FW Developments supersport-tuned RR will run at around 15:1. Modern bike engines tend to run compression ratios in the 10:1 to 12:1 region. High compression pistons are, according to FW Development's Ian Park, "bolt-on horsepower". Whichever way we go about it boosting the compression ratio is an easy route to more power. By increasing the engine's displacement we've changed the ratio of compressed to uncompressed gas volumes, and increased the compression ratio. Total cylinder volume is up to 570cc, reduced once again to 50cc at TDC. But if we give the engine a 2mm overbore and make no other modifications, its displacement is now 520cc. Its compression ratio, therefore, is 550 divided by 50, or 11:1. With the piston at TDC, the volume is reduced to 50cc. With the piston at BDC, the total volume of the cylinder is 550cc. Let's also say its cylinder has a combustion chamber volume of, for the sake of easy calculation, 50cc. Say we have a 500cc single-cylinder engine with a bore of 100mm and a stroke of 63.66mm. Any modification that reduces the volume of the combustion chamber will raise compression, but it is also affected in other ways too.īoring out a motor or increasing its stroke gives more power not only by increasing capacity, but also by boosting compression. 'Skimming' the cylinder head by machining away its bottom face where it mounts flat against the cylinder barrel (which effectively reduces the combustion chamber's volume) or fitting a thinner head gasket will increase the compression ratio, as will fitting 'high compression' pistons with a raised crown. Apparently small changes to an engine will make a significant difference.
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