Many high end distributors,...
Many high end distributors, such as this unit from MSD, feature a moveable collar which helps you dial in the correct engagement between the distributor gear and the drive gear on the camshaft. This is an extreme example, but a good one nevertheless. Notice how worn the brass distributor gear is on this unit. Although it’s way past time that this gear should be swapped out, it still did its job. The brass gear is designed to sacrifice itself instead of wearing the drive gear on the camshaft. This brass gear can be swapped out quickly and without much cost, meanwhile the drive gear on the much more expensive steel camshaft is still in good shape.
There are two black wires extending from the distributor; one with an orange stripe and one with a purple stripe. They connect to the two wires that lead to the ignition box, but they aren't marked the same way. Instead, these are green and violet. (It just looks like a slightly different shade of purple.) When connected correctly, the orange-striped wire connects to the violet wire and the purple-striped wire connects to the green wire. As you can guess, if you aren't paying attention when clipping the wires from the stock connectors, it can be easy to mistakenly think that the wire with the purple stripe connects to the "purple" wire on the other ends.
But if this happens it causes problems with the system, actually advancing the proper timing by 30 degrees or more. Normally, the ignition doesn't send power to the spark plug until the trailing edge of the reluctor in the distributor cap leaves the sensor. When the wires are hooked up backwards, the system fires the spark plug when the leading edge of the reluctor reaches the sensor. The difference in width from the rear of the reluctor to the leading edge is about 15 degrees at the distributor.
The cast distributor gear...
The cast distributor gear on the left is good when used with cast camshafts (normally found in flat tappet, low lift valvetrains). But if you’re using a steel camshaft, you will need to swap it out for a brass gear like you see on the right to prevent premature wear. These also happen to be for a Ford distributor which uses a small roll pin to hold the gear to the distributor shaft. While swapping them out, it’s also a good idea to drill out the hole in the distributor shaft to take a beefier roll pin (center) which is used in Chevrolet distributors. This reduces the chances of the roll pin shearing off and leaving you without spark (or a functioning oil pump).
That may not sound bad, but remember, because the distributor spins half as fast as the crankshaft the actual advance in timing is around 30 degrees. When the combustion process is kicked off by the spark plug firing so early you wind up with a huge spike in cylinder pressure while the piston is still trying to move up the cylinder bore. It results in big-time detonation and if it isn't caught quickly it almost always ends up with engine damage.
When the wires are reversed it's possible to retard the timing through the distributor enough to get it back where it was originally supposed to be, but it still won't be as good as if everything is wired correctly. By retarding the timing manually by turning the distributor housing everything will look OK with the timing light, but the rotor phasing will still be off. The reluctor is still firing the ignition when the leading edge reaches the sensor. This creates a larger gap that the spark has to cross and you wind up with less power reaching the spark plug—and it normally shows up as a mysterious high-speed miss that gets blamed on the engine builder.
What About HEI?
These MSD 6AL boxes are mounted...
These MSD 6AL boxes are mounted to the wall in an engine dyno cell, but the same concept holds true in your race car. MSD’s “brain boxes” have been known to work flawlessly for years without problems, but it is always a good idea to isolate them from the vibrations your race car produces by using the rubber mounting pads.
While a capacitive ignition system like the MSD with a 6AL box is popular in high end racing classes, many Street Stock-level classes require a stock-style ignition system. In almost every case, this means an HEI ignition where the coil and "brains" are all contained within the distributor. Stock HEIs can rarely handle the stresses of racing, but companies such as Performance Distributors have produced racing-specific HEI ignitions that use an upgraded coil and module capable of powering a racing engine to 8,500 rpm and beyond.
An HEI is an inductive discharge system, which means the coil is the sole source of energy storage until the energy is released to fire the spark plug. A performance coil, such as the one Performance Distributors uses in its high-performance DUI line of ignitions, features updated materials and windings that offer less resistance to the electrical charge. This means the coil charges faster and transfers more of that energy to the spark plugs. More energy at the plugs means you can open up the spark plug gaps to help with power production in high-compression, high-rpm race motors.
Fire it Up
Once the distributor gear and all the wiring is checked, actually installing the distributor and setting the timing is the easiest part of the deal. First, spin the engine until the No. 1 cylinder is at approximately your timing mark on the compression stroke. If you don't know where your optimum timing should be for maximum power, a good rule of thumb is 30 to 32 degrees advanced for Ford N-style heads (N351 is a popular head for Late Model Stock classes) and 38 to 39 degrees advanced for Chevy 23-degree heads. As a rule, the more efficient the combustion chamber design, the less timing you need.