The air-cooled 911 combustion chamber is far from ideal, its geometry compromised by both air cooling and an off center plug location so combustion remains incomplete at most engine speeds. This means that some portion of the mixture in the cylinder is not burned, resulting in roughness, wasted fuel and lost horsepower. Porsche typically ran a single plug design on all turbos that were production street cars. Only the Porsche MotorSport cars received this option and it was on every car they typically produced. Most still think that the 94 C2 turbo and the 993TT were twin plugged and in fact they were not. Only the naturally aspirated Porsches starting in 1989.5 were twin-plugged due to higher compression. The theory of twin plugging is really very simple. It can best be explained as firing both halves of the piston. The single plug in a motor is set to fire so many degrees Before Top Dead Center (BTDC) this amount of advance is based on one thing only, how long it will take to ignite the cylinder. As the fuel enters the cylinder and air is pulled in the spark must start to ignite the fire wave. This wave will carry around the cylinder. Most CIS cars will create not a single flash as you would think, rather it will swirl around the edge of the piston until it is at full combustion. The amount of timing is based on the time it takes to create the most complete combustion. Often timing is advanced so that more of the combustion occurs earlier using more of the fuel and air and not creating as much waste. The only way to accurately know whether or not you have achieved as much advance as the motor can take is to place the car on a dyno. The dyno then will steady a certain speed under full load in a given RPM band, once max TQ is achieved you have max timing. You are asking how will I know…Well, lets say you were at 29 degrees BTDC and the motor made 505TQ. You went to 30 degrees BTDC and the motor was still at 505TQ then the motor had reached it max timing at 29 degrees. This is where you will want to set your timing. HP is an equation of TQ (HP = RPM x TQ / 5252) therefore you will also arrive at your max HP at the same time. Where twin plugging comes into play is when you need to lower timing value and create a more efficient burn based on added CR or higher boost levels. The more wasted fuel and the larger the advance is the more one risks detonation. This will effectively destroy a motor by causing metal in the chamber to melt, spark plugs ignite from heat even though they are not being fired and cause the chamber to light when it should not. By twin plugging you place a spark plug on both sides of the chamber in the head. As the car comes up on stroke two plugs ignite causing the chamber to light simultaneously. When this occurs the need to have 29 degrees BTDC is not needed. It might take 18 degrees to create the same effect, but with two plugs in the chamber you actually create a much more efficient burn therefore creating less stress on the bottom end and rods from resistance starting early from pre-firing, you burn more fuel and create a more efficient environment for combustion.
Twin plugging can come in many forms. The two most popular is using a distributor with two edges on the rotor and double the amount of cylinders on the distributor cap. It also requires to CDIs, two coils and double the plug wires. The second system which is more tunable is a crank fire or sequential digital system. This means that there is a sensor reading the crank rotation and telling a computer where that sensor in comparison to the stroke to send a signal to a set of coils. This in turn provides spark to the motor. 6 coils will typically be used with two ends on each coil. These systems are some times more desirable because the computer taking in the signal and sending the command to the coils can often be tuned for specific RPM bands, boost levels and or knock, etc. Both can be costly if the dist is not something in production or something that has to be custom made due to fabrication cost. For example a RSR dist cap for any 911 or 930 is apx $1200.00. This is the cap alone! The cost of the entire Electromotive system is $1200.00!
This is what a twin plug system looks like on a piston
This is from a single plug piston. If you look at the center you can see an incomplete pattern in the center and how it fires to one side over the other
This is a piston that had too much boost and was not twin plugged. Notice the half that is lighter than the other. The pits are from metal melting off of the piston and chamber.
This is a HPX system that is mounted on a self contained system then mounted to the back of the engine bay.
Dist based twin plug setting. Everything times two!