Timing

Distributor Advance Mechanisms

If your car has a distributor, it has two mechanisms that advance the timing based on manifold vacuum, and rpm. Let's start with the vacuum advance. There are two vacuum pots on the side of your distributor. One for leading, and one for trailing. Each of the pots is referenced to manifold vacuum through a vacuum line or hose.

When the throttle plates are closed, or just partially open, there is high vacuum in the manifold. The reason is that the engine, which is simply a pump, is spinning away trying to draw in air, but since the throttle is blocking airflow, a great deal of suction or vacuum is present in the manifold. This is the equivalent of plugging the hose of your shop vac with the palm of your hand. The motor is spinning, trying to move some air, but since you are resisting the airflow, a vacuum, or more specifically, a low pressure is created.

This vacuum, actuated through the vacuum advance pot will advance the timing based on the intensity of the vacuum. As you open the throttle, the vacuum lessens, and the timing advance decreases. At full throttle (Butterflies all the way open.) the vacuum advance has no effect on the timing, because there is no manifold vacuum.

Centrifugal, or "Mechanical" Advance
In case you were wondering, centrifugal advance, and mechanical advance are two different terms for the same mechanism. The centrifugal advance will advance the timing based on engine rpm. It consists of two weights held in place by springs which are attached to the distributor shaft. At low rpm, when the shaft is spinning slowly, the weights are held in place by the springs. As the rpm increases, the centrifugal force of the spinning weights overcomes the spring tension, and allows the weights to move outward. These weights are attached to the advance mechanism in such a manner that as they move outward, the timing of both leading and trailing will be advanced. This advance will begin at about 1,500 rpm, reacing maximum or "total" advance of approximately 20 degrees by 4,000 rpm, at which point no further advance will result from increasing rpm.

Why All the Complexity?
The timing requirements of the engine vary, based on charge density, rpm, and..here's the biggy...emissions requirements! It is common to eliminate both advance mechanisms so that the engine will be at full advance at all times. You will have a few less things that can fail, and setting the timing will be less of a hassle. It is manily for these reasons that race cars run "locked" timing.

Setting Your Timing For Best Power
Now for the fun stuff. Most stock or mildly ported 12A's will make best power with total timing of 24 degrees before top dead center leading, and 16 degrees before top dead center trailing. 13B's (GSL-SE.) Will normally run best at 26 leading, and 16 trailing. The timing for best power will vary slightly from one engine to another, but a leading/trailing split of 8 degrees works best in almost all 12A applications, and 10 degrees for a 13B. Since it is the total advance that we are concerned with, this creates a problem because your stock pulley is marked at top dead center, and twenty degrees after top dead center which is the factory recommended setting at idle.

$60 Pulley?
You don't have to buy an expensive aftermarket pulley just to have the appropriate timing marks. It is quite simple to make additional marks on the stock pulley.

The first step is to remove the stock pulley. This comes off easily by removing the four 10mm. screws that hold it in place. Some of the stock pulleys are indexed, and will only go on one way. Others can be placed in any of four positions, (90 degrees apart.) so I recommend spinning the engine to top dead center before removing the pulley so that you don't put it on wrong.

You will need a pair of vernier calipers, a small file or hack saw blade, a roll of masking tape, a calculator, and some brightly colored paint. If you do not own a pair of calipers, this is just the excuse you need to buy a set. Most retail tool supply stores carry them. They range in price from $25 to over $200. Luckily, the $25 calipers are more than accurate enough for this application, and once you have a set, you will be amazed at how often you use them.

Do not buy a set of plastic calipers from Sears or Home Depot. These are absolute crap, and most of them are colored to look like stainless steel so you will buy them, only to find that they are pieces of shit once you get them out of the package. A good set will have a small dial gauge, and come in a protective plastic box. Note: Amtos is a good inexpensive brand.

Step by Step
1. Measure the diameter (Distance across.) of the pulley. This should be approximately 4.5 inches.

2. Take the diameter, and multiply it by 3.14. This will give you the circumference (Distance around.) of the pulley

3. A full circle is divided into 360 degrees, so if you divide the circumference by 360, the answer will be the distance around that equals one degree. For this application, 5 degree increments will be fine, so divide the circumference by 72. The answer will be the distance around the pulley that is equal to 5 degrees.

Example: If the pulley is 4.5" in diamter, 4.5 times 3.14=14.13. 14.13 divided by 72=.196. so .196 = 5 degrees.

4. Tear off a piece of making tape approximately 3 inches long, and stick it to a smooth flat surface. Make a mark at the far left side on the edge nearest you, and label it TDC, for top dead center. Using your shiny new calipers, set them at 5 degrees, which using the above example is .196 inches. Make a mark .196 inches to the right of the TDC mark. This is your 5 degree mark. Now multiply .196 by two (.393). Set your calipers to this number, and make another mark .393 inches to the right of the top dead center mark. This is your 10 degree mark. Now simply continue to multiply the 5 degree number by 3,4,5, and 6 to get your 15, 20, 25, and 30 degree marks.

You are probably thinking screw that, I'll just set the calipers at .196 (5 degrees) and measure from my last mark. Doing it that way will multiply your errors as you get further from TDC. If for instance your mark is off by .005", and you make the same error every time, by the time you get to 30 degrees, you could be off by several degrees! Take your time and make it accurate!!!

5. Stick the tape to the pulley lining up your TDC mark exactly with the TDC mark on the pulley. Looking at the pulley from the front, with TDC on top, your tape marks should be to the right of TDC, and the factory 20 degrees after TDC mark (Trailing.) should be to the left.

6. Now take a small square, or triangular file and make a notch on the pulley at each of your marks. Make sure that your notch is exactly centered on your pencil mark.

7. Dab some brightly colored paint into each of the notches. If you are using a spray can, you can spray the notches, and wipe off the excess with some solvent and a rag. If you lightly wipe the excess, the paint will remain in the low spots that you filed into the pulley.

8. Put the pulley back on the engine and you are done.

Setting the Timing to Total Advance.
Now that you have marks on the pulley, rev the engine above 4,000 rpm, and set the timing at the appropriate spot. I mentioned 24, and 16 which of course is not marked on the pulley. It is easy to eyeball this and arrive at any number you wish. If you are concerned about accuracy, you can add as many marks as you want to the pulley, but I have found that marks every five degrees are sufficient.

Final note: Turning the distributor changes the timing of the leading and tailing by equal amounts. You should set the leading first, and then adjust the vacuum pot to set the trailing.

Don't be afraid to try different timing settings. Every combination is a bit different, and detonation is not a concern in a naturally aspirated engine. If you do experience detonation, and the leading is less than thirty degrees advanced, you probably have excessive carbon buildup inside of the engine. Redline injector cleaner will do a good job of reducing this buildup, although it may take several tanks to completely clean the engine.



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