Once we get the proper air/fuel charge into the cylinder, the next most important item is how and when to ignite this mixture. This process is controlled, as we all know, by the ignition system. Simply stated, the ignition system creates a spark which starts the rapid burning process in the cylinder. This rapid burn is what pushes the piston down and propels the vehicle. However, this equation is greatly complicated due to the fact that load and RPM are constantly changing.

To deal with these variables of optimum output we have both centrifugal and vacuum controls housed in the ignition distributor. The vacuum controls are mainly for emission controls and driveabilty and usually don't even come into play during full load operation, which is what we are going to focus on here.

In your personal travels you have most likely experienced a condition referred to as pinging or detonation. This happens under full load acceleration when the fuel ignites before its optimally designed time. Not only does this sound terrible, but it also robs your engine of large amounts of horsepower. What happens is that the air/fuel mixture explodes very quickly. This causes a flash of pressure to the top of the piston. It's like hitting it with a hammer, which is where the noise comes from. What we want to happen is for the fuel to burn slowly and firmly push the piston down to create maximum power.

When many cars are tuned up they have their timing set to a factory spec. Then the car is test driven to check for detonation. If the car pings it may get the timing retarded. This is a fine practice for the average car, but we are looking for more than average. We are looking for the optimal set up.

Let's say, for instance, that a 1980 911SC is test driven and found to detonate severely at 4000 RPM but not at any other engine speed. This tells us the engine is receiving excessive timing at that particular engine speed. So we retard the timing and the pinging stops. End of story, right! No, not end of story because when you retarded the timing you retard the timing at all RPM, including the maximum timing at 6000 RPM. What you've done is given up high speed power to solve a midrange detonation problem.

The right solution is to repair the exact problem (at 4000 RPM) and not just cover it up. There are two sets of advance weights inside the ignition distributor. One set is for low speed operation and the other for high. This is how we get a two stage advance curve (see figure 1 below).

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The problem with the 911SC in question is more than likely caused because the primary weights are over advancing. This condition would cause an over advance at the midrange speed but no problem at redline. To repair this situation the distributor needs to be disassembled and repaired. The cause might be as simple as a loose or broken spring or worn advance weight stops. After obtaining the optimum advance curve for your car you should be able to feel a noticeable improvement in performance.

This is essentially what a high performance computer chip does for later model cars. In this case we are making the changes with weights and springs instead of resistors and microchips.

GOOD LUCK