Thread: Distributors?

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Can someone explain the differences in distributors, and what is best for different applications?

HEI, electronic, points, mechanical advance, vacuum advance?

Andy,
Not sure how complicated you want to get, so I'll just toss out some basics, then you can ask for expansion if necessary.

The distributor's fuction is to "distribute" spark to the individual engine cylinders to ignite the fuel/air mixture. The "old style" points distributors and the "newer style" electronics, work fundimentally the same. Both have a center shaft that is generally turned by gearing from the cam shaft. At the top of the shaft is a "rotor" that sends the spark signal to each cylinder through the wire attached to the distributor cap. The rotor location is "timed" in such a way as to send the spark to the spark plug at each cylinder at the correct moment when there is a fuel/air charge, and the cylinder is nearly at the top of it's travel so that the expanding, burnt gases can push the piston down. That timing is very important. Too early, or too late, maximum force is not generated when appropriate. Bad things will happen to the engine if timing is significantly off. As the engine turns faster, the spark signal needs to be sent earlier in the engines rotation in order to give the spark time to get to the cylinder at the appropriate moment. To achieve that, the rotor "pivots" on the shaft so that it can be "moved" forward and send the signal just a bit earlier. This is accomplished with weights and centrifugal force to achieve what's called mechanical advance (advancing the timing of the spark signal). The mechanical advance has some limitations due to size and space vs time to react, so in some cases, mostly where more controlled variability was deemed necessary, a vacuum "motor" (basically an arm attached to a diaphragm) is used to help rotate the rotor to the advanced position. The points are basically a switch that "releases" the built up energy from the ignition coil to send sufficiently high voltage spark to the spark plug in order to ignite the fuel/air mixture. The coils job is to increase battery voltage (say 12v) to something higher (say 50kv). The points are opened and closed by a cam that is machined into the distributor center shaft. Since the points are opened and closed by a rubbing block that rides on that cam, and since the points have some mass, the rubbing block is prone to wear, and the effects of inertia affect the rate of closure of the points. While a point distributor was a relatively dependable, and inexpensive to manufacture, device, it is not a "fine tuned" method of spark distribution with a lot of precision.

As engine design progressed and more accurate timing was necessary, along with a need for a more powerful spark, the market moved to "electronic" distributors. The basic design of a center shaft, turned by gears from the cam shaft, with a rotor to distribute the spark to the appropriate cylinder at the correct time, remained essentially the same. What became the electronic part was the "switch" that trigered the release of the coil's energy. Instead of using the mechanical opening and closing of the points, an electronic switch is employed, usually triggered by an electro/mechanical device. Usually it is a "wheel" in pretty much the same location as the former cam lobes that activated the points set, that has magnetic "strips" on it to trigger the switch. Optical (using light) triggers have been used also. These electronic switches proved more reliable, and precise, than the former mechanical points switches. Since the rest of the distributor architechure is mostly the same, the advance mechanisms work pretty much the same as above.

HEI is GM's name for "electronic" ignition. This of course comes from a period before the ever increasing role of computers in the control of engine function management. With the progression of on board computer power, the distributor became less of a "player" in the spark timing game. The computer is vastly more reliable and "tunable" for precision. In many cases today, there no longer is a distributor in the classic sense. Often, there are a set of coils, attached directly to the spark plugs, and the computer gets a signal directly from the crankshaft, which in turn allows the computer to send the "fire" signal directly to the coil at "precisely" the correct moment for operating conditions.

I understand the basic function of the distributor. I was just a little confused about the different types.

So, I guess most people just use the standard electronic version. I was looking at the Mallory Unilite in particular.

I also expect to go with the vacuum advance, but the canister is just an eyesore.