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Old 09-11-2012, 10:05 PM
ram50v8 ram50v8 is offline
Motorized Bicycle Newbie
Join Date: Jul 2011
Location: Central Illinois USA
Posts: 8
Default Re: What exactly is Detonation

Originally Posted by Moto View Post
Ok so I am a bit embarrassed to ask such a noob question when Ive built a few of these bikes now, but what is detonation? I hear people talking about it all the time and I have gathered it has to do with heat and the head? Fill me in... MOTO
Moto, don't be embarrassed about this! Detonation and Pre-Ignition are 2 of the most mis-understood events that happen in a cylinder. In simple terms I will explain what detonation is. Detonations direct cause is when the spark fires and as the flame front moves across the cylinder the pressure rises causing a rise in temperature which at the hottest spot where un-burned Air/Fuel mixture resides causes the A/F mixture to ignite on its own. The resulting flame moves across the cylinder and meets the original flame front and that is detonation aka spark knock.

The Other is pre-ignition where the flame front starts before it is desired (before the spark plug fires) and is caused by the combustion chamber temperature exceeding the ignition temp of the F/A mixture. This can be due to overheating of the engine or by a localized hot spot, or carbon build up that has started to glow in the cylinder and becomes a "glow plug"

There are many overlaying factors that cloud these problems and cause the confusion. Overheating, fuel quality, Air/Fuel ratio, carbon build up (glowing carbon) and ignition timing. If you let it go and allow them to do their damage you can see the differences in damage. Pre-ignition tends to damage the center of the piston whereas detonation does its damage at the rings/piston edges. However with today's fuels and leaner mixtures the two often are in concert with each other.

Now before some one starts jumping in, remember this was the layman's version. There are many many factors involved that can make it one way over the other. Some are mechanical like static compression. Others are timing related verses temp and fuel quality. And then there is the fuel itself and it's ability to perform with maximum efficiency without igniting too soon or burning too fast. So many things involved but hope this answers your question. With today's modern engines (not these lol) we want to run right at the edge for maximum power and efficiency.

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