aluminum piston is cold-revved in a relatively dry cylinder on start-up or during abnormally high operating temperatures. The biggest drawback of adding silicon to pistons is that the piston becomes more brittle as the ratio of silicon to aluminum is increased. This makes the piston more susceptible to cracking if the engine experiences pre-ignition or detonation. Performance replacement alloys When auto enthusiasts want to increase the power of the engine, they may add some type of forced induction. By compressing more air and fuel into each intake cycle, the power of the engine can be dramatically increased. This also increases the heat and pressure in the cylinder. The normal temperature of gasoline engine exhaust is approximately . This is also approximately the melting point of most aluminum alloys and it is only the constant influx of ambient air that prevents the piston from deforming and failing. Forced induction increases the operating temperatures while "under boost", and if the excess heat is added faster than the engine can shed it, the elevated cylinder temperatures will cause the air and fuel mix to auto-ignite on the compression stroke before the spark event. This is one type of engine knocking that causes a sudden shockwave and pressure spike, which can result in failure of the piston due to shock-induced surface fatigue, which eats away the surface of the piston. The "4032" performance piston alloy has a silicon content of approximately 11%. This means that it expands less than a piston with no silicon, but since the silicon is fully alloyed on a molecular level (eutectic), the alloy is less brittle and more flexible than a stock hypereutectic "smog" (low compression) piston. These pistons can survive mild detonation with less damage than stock pistons. 4032 and hypereutectic alloys have a low coefficient of thermal expansion, allowing tighter piston to cylinder bore fit at assembly temperature. The "2618" performance piston alloy has less than 2% silicon, and could be described as hypo (under) eutectic. This alloy is capable of experiencing the most detonation and abuse while suffering the least amount of damage. Pistons made of this alloy are also typically made thicker and heavier because of their most common applications in commercial diesel engines. Both because of the higher than normal temperatures that these pistons experience in their usual application, and the higher coefficient of thermal expansion due to low-silicon content causing greater thermal growth,