Frequently Asked Question:
Q: What type of logs should I take to monitor for detonation/knock?
A: The default logging list on the AccessPORT will include the relevant monitors, so that is a great place to start. In general, detonation and knock will be most dangerous when cylinder pressures are highest, which will usually occur during full power (WOT) operation. In general, a log over a singular gear through the full RPM range (from 2000 RPM to redline) is sufficient for a street car; for a race car you should consider logging longer sessions that includes gear shifts. Making sure to keep your logs short (IE, start and end the logging session when appropriate) will help prevent from having to filter through large
quantities of irrelevant data.
Q: What is detonation or knock? How is it detected by the ECU?
A: “When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of
fuel/air mixture outside of the flame front. A local shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits.”i From a more succinct technical perspective, “Knock is the explosive spontaneous ignition of fuel/air mixture ahead of the normal propagating flame
and the subsequent cylinder pressure oscillations.”ii Audible detonation is often referred to as “pinging” due to the sound created; knock takes its namesake from the audible sound created by the event (much like a fist knocking on a door) and is sometimes also known is pre-ignition. To facilitate detonation
detection, a Piezoelectric microphone sensor is hard-mounted to the engine block and relays detected noises to the ECU, where they are further filtered to hone in on the specific frequencies generated by detonation in an attempt to accurately determine their source and relevance. The timing of these events
relative to the measured crank shaft angle is used to estimate the source cylinder(s) by more advanced recent implementations.
Q: What causes detonation and/or knock?
A: In short, any number of conditions or factors can induce detonation. The sources are highly variable but in general, increased cylinder pressures and/or temperatures or reductions in effective octane level are the common “causes”. The factors that lead into those are again varied, but from a tuning perspective, detonation and knock occur:
• When the engine speed is low and the manifold absolute pressure (MAP) is high
• When combustion duration is long
• When temperatures are high (ambient, coolant, combustion chamber surface)
• When the charge dilution is low
• With high carbon deposits
• When the spark advance (or injection timing) is high
As can be seen, the “big three” of boost, ignition timing and air-fuel can all play an integral part in causing or contributing to detonation/knock.