Distributor Timing & Tuning by Steve Lothridge (geebjen@frontiernet.net) INTRODUCTION The intent of this article is to give you a complete understanding of distributor timing, how to set it and alter it, and what settings are a good starting point for most Street or Street/Strip Pontiac V8 Engines. This is an area that is very, very important to the performance of your engine, and is so inexpensive or even free to alter, yet is so often misunderstood. Do yourself a favor, and take the time to understand this subject, and you will thank yourself again and again. DISCLAIMERS I am not a professional Ignition Guru or Engine Rebuilder. I am just a hobbyist trying to help others out. While I have done my best to ensure that the information in this article is correct, you may find errors or areas that need additional information. If you do, please e-mail me and let me know, and I'll be sure to make the corrections and/or updates. It would be helpful to me if you provide documentation along with your corrections, so I can verify the information you give. THEORY If you don't care WHY, then skip this section. I'm not particularly an expert in this area anyway. But I know enough to help beginners at least understand why you alter timing at all. The reason that timing is advanced from TDC (Top Dead Center) and altered with load and RPM is really pretty simple. The Piston is moving upward toward TDC with some velocity and therefore takes a certain amount of time to reach TDC. At the same time, the explosion of the fuel mixture (flame propagation) takes some amount of time. If you had the spark occur at TDC, then the force of the explosion would occur at a later time, when the piston is already on its way down, and would provide less power. So you advance the spark a few degrees, which give the explosion time to develop as the piston is still on its way up, with the full force of the explosion occurring just as the piston has passed TDC. Now when you speed up the engine RPM, you are also changing the speed that the piston is moving. BUT, the time the explosion takes to develop is the same. So you need to advance the spark even further if you still want the explosion to happen just past TDC. Think of it as a race if you want. The flame propagation doesn't get faster, but the piston does, so you have to give the explosion a "head start". This is why you have a mechanical advance. As far as Vacuum Advance, this is why you need/want it. High vacuum translates to low cylinder pressure, because the throttle blades are limiting the density of air fuel mixture that can enter the cylinder. The low cylinder pressure slows the flame propagation. There are two reasons for this. First, low cylinder pressure lowers the charge density, which causes the oxygen and gasoline molecules to be farther apart. With the molecules farther apart, they take longer to re-combine in combustion. The second is low mixture temperature. Because heat is required to combine the molecules, the lower temperature slows the process down. For these reasons, a Vacuum Advance is used to advance the spark as vacuum is increased. It should be noted that Vacuum Advance is primarily for Fuel Economy, and has no affect on your performance at Wide Open Throttle (WOT) since you have almost no vacuum present. ** Thanks to Steve Chin for providing the technical details in ** ** the Vacuum Advance section ** INITIAL TIMING There really isn't a lot to say about initial timing, but here are the basics. In order to measure this you must do two things. First, you must disconnect your vacuum advance and plug the port at the carb. Secondly, you need to be sure that your Mechanical Advance has not yet begun to advance your timing. This might happen if you have very light advance springs or your idle speed is too high. In order to set initial timing, you only have to loosen the distributor hold down bolt and rotate the distributor. There is nothing that affects this setting other than the rotation of the distributor. Well, I shouldn't say nothing. If you have a points distributor then the point dwell (or gap) will change your timing also. Any time you adjust the point dwell (or gap) or change the points, be sure to re-set your timing. Lastly, you should know that Initial Timing is relatively unimportant to your car's performance at WOT. If it is too low or too high your car will not start and may not idle well. Other than that, it really doesn't matter much. (Remember, I didn't say "At All", I said "much"). Also, beware setting too high and thinking it is OK because it started fine, it may not re-start when it is hot. A safe range for initial timing is 8-14 degrees, though some later year low compression engines may be OK with even more. MECHANICAL ADVANCE There are two key components of Mechanical Advance. I'd like to explain each one separately. First is the AMOUNT of Mechanical Advance. For points distributors, this is completely controlled by the length of the advance slot and the size of the bushing (if any) on the pin. If you don't know where these are, open up your distributor, take off the rotor, and look at the under side of the advance plate. If you don't see what I mean now, then force the weights out and look for where you see the pin moving in the slot. Once you have seen this, it is pretty easy to understand how to change the amount of advance. To get less, add a bushing (or larger bushing) on the pin OR braze the slot so it is shorter. If you want more advance, then get a smaller bushing or lengthen the slot. For HEI distributors, the amount of advance is controlled by the shape of the center advance plate (or cam). You can try to re-work the shape of the cam, or buy different cam part numbers in order to change it. (See HEI Recurve article below under Internet Resources). The second component of Mechanical Advance is the Advance Curve. The Advance Curve is controlled by the shape and weight of the weights, shape of the cam (in between the weights) and the tension in the springs. If you install lighter springs, the weights will be able to extend out at a lower RPM. If you install heavier springs, it will take a higher RPM for the weights to overcome the force of the springs. Below are a few examples to illustrate. In each case, the initial timing is set to 0, for ease of discussion. If you were to change the initial timing, you would simply add that number to the others in the curve. ----------------------------------------------- | RPM | Curve 1 | Curve 2 | Curve 3 | ----------------------------------------------- | 500 | 0 | 0 | 0 | ----------------------------------------------- | 1000 | 2 | 4 | 4 | ----------------------------------------------- | 1500 | 8 | 12 | 12 | ----------------------------------------------- | 2000 | 14 | 18 | 16 | ----------------------------------------------- | 2500 | 20 | 22 | 18 | ----------------------------------------------- | 3000 | 22 | 24 | 18 | ----------------------------------------------- | 3500 | 24 | 24 | 18 | ----------------------------------------------- | 4000 | 24 | 24 | 18 | ----------------------------------------------- Now, for each example in the chart, here are some characteristics we can attribute to them. Curve 1: The advance is said to be "all in" by 3500 RPM because there is no further advance after that. This is a relatively slow curve. The Mechanical Advance is 24 degrees. Curve 2: The advance is "all in" by 3000 RPM. This is a fairly typical curve. The AMOUNT of Mechanical Advance is the same as Curve 1, 24 degrees. Curve 3: The advance is "all in" by 2500 RPM. This is a somewhat quick curve. The amount of advance is only 18 degrees in this case, which is less than Curve 1 or Curve 2. TOTAL MECHANICAL ADVANCE Total Mechanical Advance, is really just the sum of the Initial Advance and the Mechanical Advance. For further example, here is the table from above with some initial timing values added. ----------------------------------------------- | RPM | Curve 1 | Curve 2 | Curve 3 | ----------------------------------------------- | 500 | 12 | 12 | 8 | ----------------------------------------------- | 1000 | 14 | 16 | 12 | ----------------------------------------------- | 1500 | 20 | 24 | 20 | ----------------------------------------------- | 2000 | 26 | 30 | 24 | ----------------------------------------------- | 2500 | 32 | 34 | 26 | ----------------------------------------------- | 3000 | 34 | 36 | 26 | ----------------------------------------------- | 3500 | 36 | 36 | 26 | ----------------------------------------------- | 4000 | 36 | 36 | 26 | ----------------------------------------------- Now for some analysis of these curves. Curve 1: This is a pretty decent curve. 12 Initial and 36 Total. But it comes in a little slow, this can be changed with lighter springs. Curve 2: Again 12 Initial and 36 Total but all in by 3000. This is a pretty good curve to start with. Curve 3: This curve has problems. 8 Initial is OK. But 26 total is unlikely to give you the best performance. In order to try Total Timing numbers higher than 32, you may have to set your initial too high for reliable re-starting. I would advise expanding the Mechanical Advance beyond 18 degrees. It should be noted that many of the factory settings for mechanical advance, particularly in the later HEI years, are completely inadequate for hi-performance applications. Their advance curves may be lacking both in the Amount of advance AND the Advance Curve, due to the spring/weight/cam combinations. VACUUM ADVANCE Vacuum advance is not necessary at all for full throttle performance, since there is no vacuum at WOT. However, Vacuum Advance will greatly increase fuel economy, which is its primary purpose. In addition, Vacuum Advance can have a positive affect on idle quality and engine temperature. There isn't any adjustment to factory Vacuum Advance units, and they came in many combinations of Amount and Rate of advance. While you CAN tell the Amount of advance of a factory unit, since they are stamped with that number (or easily tested), you can't easily tell the Rate. If you want a real adjustable unit, buy the Crane adjustable vacuum advance can. However, if you just want to limit the Amount of Vacuum Advance, you can find a way (braze) to shorten the slot in the in the Vacuum Advance unit. Be aware that doing this will also change your Initial Timing, and you will need to reset it after this change. The primary area of debate with Vacuum Advance seems to be what type of vacuum to use. If you use Manifold Vacuum, you will effectively add your whole Vacuum Advance to the timing at idle. Most engines seem to like this, and respond by increasing idle speed. Simply use the idle speed screw to return to normal idle RPM. You should also see your engine run cooler at idle. The other school of thought is to use Ported Vacuum as the source. Supporters of this method site better response under certain throttle conditions. I suggest you try both and see which works best for you. SETTING YOUR TIMING CURVE The descriptions above should give you all the basic knowledge you need to set your curve. Still, here are some tips. * Before anything else, get a correct timing tape on your dampener, or at least make a mark at 36 degrees Before TDC so that you have a good reference. By the way, 34 degrees on a 6 3/4 Pontiac Dampener is very close to 2.00 inches and each 2 degrees with be approx 0.125 (1/8 inch) from there. * Disconnect your Vacuum Advance during all tests and measurement of Initial and Mechanical Advance. * If at any point you start to get Ping, reduce the portion of the timing you are working on until the Ping goes away. If you can't get the Ping to go away without really hampering performance, then you need to either buy Race gas, or reduce your compression ratio. * Try to get your Mechanical Advance set to as near to 24 degrees as possible. Why? Because that will allow you to vary your Total Mechanical Timing between 32 and 38 degrees as you change your Initial timing from 8-14 degrees. NOTE: There is no guarantee that this will be the best setting for your car, but it is a good starting point, and will probably work for most Classic Ponchos out there. * Experiment with the Total Mechanical Timing until you get the best performance. Results should be taken at a track (best), with a GTech/Pro or similar (Good), by stopwatch (if you have to), or by feel (worst). Remember, MPH is a better indicator of your power than E.T. * Experiment with Advance Curve, by changing springs, until you get the best performance. * Reconnect the Vacuum Advance and set it for the most advance you can without getting Ping. Internet Resources: An excellent article by Jim Hand on re-curving HEI Distributors. http://www.boyleworks.com/ta400/psp/distcurve.html More good information in the IGNITION section of this tune up guide by Jim Hand. http://www.classicfirebird.com/hand/jhand3.html DEFINITIONS Initial Timing - The timing setting when the engine is at idle, with Vacuum Advance Disconnected. Mechanical Advance - The mechanism that advances the timing as the engine RPM increases. Centrifugal Advance - Same as Mechanical Advance Vacuum Advance - The mechanism that advances the timing based on the amount of vacuum present. Total Mechanical Advance - Generally refers to Initial plus maximum Mechanical Advance, with Vacuum Advance disconnected. Also referred to sometimes as just Total Advance, even though this is not completely correct on cars using Vacuum Advance. Advance Curve - Describes the relationship between RPM and Mechanical Advance. In other words, the rate at which the Mechanical Advance "comes in." Manifold Vacuum - Present at its maximum at idle, and also present to some degree throughout the throttle range, except WOT where it is essentially zero. Ported Vacuum - In every way like Manifold Vacuum, EXCEPT it is NOT present at idle. TDC - Top Dead Center WOT - Wide Open Throttle