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 haven’t 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 couldn’t
                    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