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What is Detonation?


Illustration of Detonation
Detonation is spontaneous, almost instantaneous combustion of the unburned air-fuel mixture in the combustion chamber. Normally, the spark plug starts the combustion and it progresses across the chamber at a somewhat consistent pace. Cylinder pressure rises at an acceptable rate, typically 20-30 psi/deg.

This combustion process does require some time, and is why you must fire the spark BEFORE top dead center. During this combustion process, the unburned mixture ahead of the flame front is being heated and compressed. A somewhat obscure, chemical process occurs in this unburned A/F mixture, and if enough time elapses, the remaining, unburned mixture will spontaneously combust. If this happens, cylinder pressure rises sharply, perhaps 60-100 psi/deg. This causes an audible noise, which we call Knock or Ping, and produces undue stress on the piston and other components.


Detonation - Image courtesy of thor-racing.co.uk
The likelihood of detonation to occur can be reduced increasing the fuel's octane rating, reducing air inlet temperature and pressure, increasing engine RPM, etc. Many things which improve an engine's performance will also increase cylinder pressures and temperatures, and therefore increase the likelihood of detonation. Therefore, race engine builders must pay close attention to factors which affect detonation. An engine can tolerate slight detonation for a short period of time. However, severe detonation will destroy an engine very quickly and must be avoided.


Piston Damage due to Detonation - Image courtesy of thor-racing.co.uk

Programs that take Detonation into account

We have 3 programs that work with Detonation when conducting Engine SimulationEngine Analyzer v3.4, Engine Analyzer Plus v3.4. And Engine Analyzer Pro v3.9 also uses Cylinder Volume in Engine Simulation.

And we have 5 other programs that take Cylinder Volume into account when making calculations. Compression Ratio Calculator v2.3, Compression Ratio Calculator Plus v2.3 Port Flow Analyzer v3.5 with Valve Opener and Engine Log Book v1.1.