The other technique is a little more subtle but usable if attention is paid to EGT (Exhaust Gas Temperature). Detonation will actually cause EGTs to drop. This behavior has fooled a lot of people because they will watch the EGT and think that it is in a low enough range to be safe, the only reason it is low is because the engine is detonating.
The only way you know what is actually happening is to be very familiar with your specific engine EGT readings as calibrations and probe locations vary. If, for example, you normally run 1500 degrees at a given MAP setting and you suddenly see 1125 after picking up a fresh load of fuel you should be alert to possible or incipient detonation. Any drop from normal EGT should be reason for concern. Using the "Tin Ear" during the early test stage and watching the EGT very carefully, other than just plain listening with your ear without any augmentation, is the only way to identify detonation.
That leads us to pre-ignition. Pre-ignition is the initiation of the flame front BEFORE the planned spark ignition event. It is a normal flame front, just started too soon. As opposed to detonation where EGT is little changed, pre-ignition results in rapidly falling EGTs and rapidly rising CHTs. The Peak Internal Cylinder Pressures can get very high, very quickly and this can destroy an engine in seconds.
jsmcortina wrote:EGT safety has tripped on my with a lean nitrous setting.
Why do you think it was designed for det prevention?
The fact is, EGT is not a good reflection of combustion temperature at all. It is more an indication of when the combustion occurs relative to top dead center or valve opening. In fact, if you lean enough, to something around 150ºF LOP, just before combustion ceases entirely, the EGT will rise briefly, as the very slowly-burning combustion event continues until the exhaust valve starts to open! This little oddity drove us nuts when we first saw it. (As an aside, diesel engines have very high peak pressures and temperatures, but tend to have lower EGTs than spark-fired engines.)
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