Q:  What is detonation?  What does detonation sound like in a rotary engine?

Detonation is defined by any uncontrolled ignition of the combustion charge, usually due to pre-ignition, uncontrolled timing, and/or lower than acceptable fuel octane.  The words "detonation" and "pre-ignition" usually go hand-in-hand.  Pre-ignition is defined by the uncontrolled ignition of the combustion charge other than triggered by the spark plugs through the stock ignition system.  Pre-ignition can be triggered by localized hot spots within the combustion chamber and/or lower than acceptable fuel octane that ignites spontaneously within the combustion chamber.  Octane is also directly related to the entire mix, as octane is defined by a fuel's resistance to pre-ignition.

Detonation in a rotary engine is bad!  Unlike piston engines which have piston rings what are buried down in the piston skirt (this is the reason why piston engines can tolerate a little more abuse from detonation), the rotor apex seals are basically wide-open to the combustion chamber.  Damage from detonation is actually caused by the higher than normal combustion pressures.  Compared to normal, controlled combustion (about 1,000psi), detonation can exceed normal combustion pressures by ten times - up to around 10,000psi!  This elevated pressure is what causes engine damage; the rotary engine's exposed apex seals take the brunt of the detonation force, which ends up failing by shattering into little tiny pieces.  Having little tiny piece of steel flying around in a rotor chamber with a spinning rotor is a recipe for a chewed rotor housing.

So what does detonation in a rotary engine sound like?  The best analogy I've heard is "popping popcorn."  Mild detonation sounds like a Jiffy popcorn popping on the stove.  It sounds close to a low, muffled rumble.  Power should significantly decrease due to the uncontrolled ignition of the combustion charge.  Serious detonation sounds like a metal hammer on a metal pipe; it has a very easily distinguishable metallic "tink" that is very high pitched in frequency.  This is basically the sound of the rotor trying to reverse direction and spin the opposite way!  Bearings and gears take a tremoundous pounding under serious detonation; you can always tell if a motor has undergone serious detonation by inspecting the rotor bearings, rotor gears' teeth, and stationary gears teeth for unusual wear.

As a side note, serious detonation can cause curious indentations on a rotor face, on opposite ends of the "dish" in the center.  Kouki FC3S and FD3S rotors tend to do this pretty easily due to their lighter weight and, therefore, less mass and metal thickness at the rotor face surface.  Zenki FC3S rotors seem to be more resistant to this depression damage from serious detonation.  The Japan tuners have seem to have caught onto this characteristic, as demand for Zenki FC3S rotors (even with the lower compression of 8.5:1 versus 9.0:1 for Kouki FC3S and FD3S rotors) have skyrocketed for drag racing applications where insane boost levels are seen, typically over 25psi!

Questions?  Comments?  Send email to:  reted@fc3spro.com