Detonation by Bill Pedersen Detonation is not the same as pre-ignition. Pre-ignition is ignition of the fuel/air charge by a hot spot before the spark plug fires to initiate normal combustion. Many times it is caused from the overheated center wire of a spark plug whose heat range was too hot for the application, a sharp edge on a piston/combustion chamber that gets very hot, carbon buildup, a spark plug ground strap that has been sharpened to a point, etc. A plug that is too long will have threads exposed in the combustion chamber. The threads fill up with carbon which damages threads, or the carbon deposited in them, may become a hot spot precipitating pre-ignition. Pre-ignition soon provokes detonation, so confusion between the two is understandable. Detonation occurs when a portion of the fuel/air mixture, usually the end gases, begin to burn spontaneously after normal ignition takes place and has burned part of the charge. The flame front created by this condition eventually collides with the flame initiated by the spark plug. This causes a rapid and violent pressure build-up. The end gases at the very outer limits of the combustion chamber self-ignite and cause detonation. The most common conditions that lead to detonation are high fuel/air mixture density, high compression ratio, high inlet charge temperature, and excessive spark advance. To help prevent detonation, the end gases need to be kept cool and the time required for the combustion flame to reach the end gases needs to be reduced. The combustion flame will reach the end gases in a small combustion space more quickly than a larger camber. A central located spark plug reduces flame travel, but is certainly not the only choice. An offset combustion chamber has a lot of benefits as does a dual plug setup mentioned in one of the other posts. If we move the combustion chamber down as close to the piston crown as possible by reducing the squish band clearance, combustion should not occur around the edges of the chamber until the piston has traveled well past TDC. This large surface area acts as a heat sink and conducts heat away from the end gases, preventing self-ignition and faster combustion. . By using the right combustion chamber design and squish clearances with race gas you can make the two-stroke motors live with high compression. On the GP race bikes they want to see slight detonation accrue. This is where you make the most power, but everything has to be perfect or you will run into major problems. To achieve maximum power a high compression ratio is necessary. The drag race Pro Stock motors are now running around 16:1 compression ratios with the new combustion chamber/piston design and the digital ignition systems.. That is about the top of the range for a gas motor and they can only do it for a quarter mile. I havent tried it but would sure think you would run into major problems trying to run 18:1-20:1 ratios on a circle track motor regardless of what ignition system is used. That is the range that a lot of diesel motors run and that is igniting diesel fuel, not gasoline I am not familiar with the piston fire ignition systems used for racing. If there is no ground strap on the spark plug, where does the piston get its ground? Does it go thru the rings/cylinder wall/oil film or crank/bearings/oil film???? If you have enough power to fire a plug with a wide gap, what are the advantages of the piston fire? Can you measure a difference in HP/torque? One of the gains in performance a few years ago on the supercharged drag race motors came from the new high powered MSD magneto. A few years ago I used to use a Mallory super mag on a blown alcohol drag race motor. We would watch the mag closely and when the amps would fall below 3.5, we would have Mallory recharge the magnets. A good mag would put out 3.7-4.0 amps. If the amps fell below 3.5, the car would slow down a slight amount. MSD came out with a new design of "rare earth magnet" that no longer needed to be recharged and had tremendous power. By changing to the MSD 12 amp mag you could see another slight improvement. It had 300 milliJoules of spark energy. (a lot of the stock motorcycle ignitions produce about 20-30 milliJoules) MSD also had a 44 amp mag for the nitro burning Top Fuel dragsters and Funny Cars. It had 1000 milliJoules of spark energy, however you couldnt measure any increase in power over the 12 amp mag. Depending on how good your system is for your conditions will determine what kind of an increase in measurable power. On motors that use a magneto, could we see an advantage by putting rare earth magnets in the motorcycle mag? This might give you the power to run a .o8o plug gap. A lot of power gains on two-strokes are being found by varying ignition timing with programmable boxes from Vortex and Wolf..... Bill Pedersen