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Doug Prince, Mr. Fuel Injection in the So. Cal. Chapter, deviated somewhat into the subject of properly firing that fuel once it enters the engine, Ignition Timing. Doug’s presentation can be summarized by discussing the “Total Timing” or measuring the “Total Advance” of the C1’s. He did not limit his discussion with ignition timing at idle speed but discussed the total advanced curve associated component parts of the vacuum and centrifugal advance .
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Doug’s discussion began by describing the original engine distributors components used on our C1’s. A center shaft extends all the way through the distributor assembly. The bottom end of this shaft has a spiral gear which engages a gear on the engine camshaft and at the top end is a “T-flange” which is located just below the distributor cap. 
In between, about a 1/3rd of the way from the top is a cam-type surface with lobes. This entire shaft turns as the engine runs and the lobes turn to open and close the ignition points within the distributor. The T-flange on the top end connects via springs to weights which also spin and exert force against the springs. All these components will be further discussed in relationship to the distributor performance. Following the identification of distributor components, the purpose for the distributor was discussed and the “timing” of fuel ignition with respect to mechanical piston location. As a piston travels up and down in the cylinder, fuel enters the combustion chamber and ignites, transferring force onto the piston (rapid expansion of gases, etc.). The fuel ignition timing is critical and this timing depends upon the rotational speed of the engine (RPM), and the amount of “advance firing” needed to provide a optimum maximum force onto the piston. The higher the RPM, the more advanced firing time, before top dead center (BTDC), is needed. It follows, that a variable firing time NEEDS to be used to maintain the fuel optimum firing time and this variable timing must react to the engine RPM. Furthermore, some distributors also sense the engine vacuum and participate in the ignition timing. |
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| The ignition points opening and closing cause ignition (spark plug firing) to occur. The cam lobes on the distributor center shaft mechanically cause the ignition points to open and close. Point gap (maximum space the points open) is a designed value because it establishes how long the points open/close during the distributor shaft rotation. To small or to large of a gap does not result in a optimum spark at the plug. Another method for setting this gap is using a meter to measure the electronic Dwell, or how long the points are closed (26 to 32 degrees for C1 V8’s). Once set, this Dwell should remain constant for any RPM. The location that the ignition points open is determined where the cam lobe is located with relation to the points. The plate for mounting the ignition point does rotate and the rotational position is mechanically controlled by the centrifugal and vacuum advance mechanisms. The plate rotational position is how the firing location is allowed to Advance or change as RPM and vacuum change.See the above which illustrate this concept. The illustrations are for a 6 cylinder engine so the degree numbers and number of lobes on the distributor cam are not representative of a V8. The concept is the same however. |
| As indicated earlier, two mechanisms are incorporated to control the variable advance firing time, centrifugal and vacuum advances (some distributors only have centrifugal advance control). Centrifugal advance relates to the engine RPM, i.e., the higher the RPM, the faster the distributor shaft turns and (through springs to weights) creates a mechanical control on the location where the spark firing occurs. Most distributors contain a second device which uses the engine vacuum to control engine firing. The vacuum advance is also mechanically coupled to the ignition mounting plate. The ignition point mounting plate rotates as a direct relationship to engine vacuum changes producing an additional spark advance effect. RPM and engine vacuum are interrelated and the design combinations of the centrifugal and vacuum advance components contribute to the Total Advance Curve. That purpose again relates to maintaining the optimum firing and power of an engine throughout all the driving conditions. It should be noted, that not all C1 Corvettes have both vacuum and centrifugal advance mechanisms, some engines rely only on centrifugal advances. |
| Whichever engine design is used in your car and whichever distributor is installed, a “Total Advance Requirement” design is needed for optimum performance. The total advance on our C1’s (V8’s) is 36 to 42 degrees. Below is a generic curve for the degrees advance versus RPM. Since the engine crankshaft is rotational and turns through 360 degree (one revolution) the amount of advance firing within a cylinder is talked about in degrees. If a piston is at the highest point in the cylinder, this is 0-degrees and Top Dead Center (TDC). Advance firing occurs prior to reaching TDC and this degree number depends upon the RPM.
An upper advance value on the indicated graph is 36 to 42 degrees for the C1 V8’s. This includes the contribution of vacuum and centrifugal advance (for those distributors using both advance mechanisms). It is also evident from the chart that at lower engine speeds, idle, there is a “setting” which is a specific advance number. This lower value occurs when the vacuum is disconnected from the vacuum advance and the RPM is low enough to have the centrifugal advance springs not affecting the advance. This is the normal setting everyone sets from manufactures specifications using a timing light on the crankshaft counterweight (defined by a line) and the tab containing numbers attached to the engine timing cover. The tab numbers refer to 0 (TDC) degrees and as the numbers increase each number will represent degree of advance firing. Normally, our C1’s at idle speed use a number near 4 to 6 (degrees). This is the beginning point on the chart. It is important that this number be measured with the vacuum disconnected from the advance mechanism because the vacuum will have an affect on this setting. |
| The “Line” connecting the two advance degrees levels on the above chart is what drag racing, engine modifications etc. is all about. Many variations exist in vacuum advances, centrifugal weights and associated spring tension to make the curve look different. However, there always is a starting point and an ending value. Measuring and assuring this upper Total Advance level is the problem. A problem vacuum advance, stuck weights or bad springs can always impact the engine performance so it is important to determine and verify the upper value. |
| A timing light is a high speed “strobe” light which will produce a bright light flash when a electrical short burst of energy is connected. This short burst of energy originates when the light is connected to a cylinder spark plug using some manner of connection. Newer timing lights use inductive coupling where a loose clamp senses the electric energy burst occurring in the spark plug wire and fires the light. This is the basic concept for “timing” your engine for seeing the timing mark. Connecting your timing light to #1 cylinder lets you see the timing mark when spark occurs in #1 cylinder. The mark appears stationary as the engine operates as it will only flash when a spark occurs. |
| Viewing the timing mark using a timing light, the mark will begin to move as the RPM is increased. Very quickly, the mark will move away from the idle speed tab setting and to to higher numbers. This means that the centrifugal advance is being energized and advancing the firing. Higher RPM’s and there are no numbers, to provide spark advance readings so there needs to be another method used to measure these values. |
| Another timing light is now available and contains the flexibility to measure the degrees advance at various RPM settings. This timing light is known as a “dial-back timing light” and they are available from various manufacturers. This light contains the same strobe-type light but there is a dial feature where when turned can delay the strobe flash sufficient to “dial back” the crankshaft timing mark to a position on the timing Tab. The timing light dial also has numbers corresponding to the dial pointer. This dial-back feature will now allow to measure our Total Advance end value.The method to measure your total advance is to connect your vacuum advance and increase the RPM to 3 to 4K and dial-back the timing mark to the zero on the timing cover tab. The Total Advance is now the value pointed to on the timing light knob. IF, the amount of centrifugal advance and vacuum advance contribution is desired, disconnect the vacuum advance from the vacuum source and the at the higher RPM you can dial-back to measure the centrifugal advance value. Reconnecting the vacuum will also verify that contributions of both components are operational producing a higher value. Additionally, an entire curve can be plotted by a 2nd person raising the RPM to different values and determining the advance values. When done at several RPM’s a curve can be plotted. |
| Further comments on this Topic:1. A friend of mine (editor) who does a lot of racing at Willow Springs has used the Total Advance value to set the timing in the cars to a desired value since during his racing he operated at the higher RPM’s. He was not concerned with idle RPM.
2. One note on this discussion was many Corvette distributors do not have vacuum advances and rely on centrifugal only. The discussion is the same but disregard the comments on vacuum advances
3. Several other items about distributor adjustment was not part of this discussion. Dwell, the setting for the amount of time the ignition points are closed was not discussed. The dwell is essentially setting the point gap electronically and once set is suppose to remain constant through the RPM range. If not, bushings in the distributor may be worn and you get wobble in the center distributor shaft.
4. A question was asked about dual points in Corvettes and the advantage. Doug said advantage with dual points was the sequencing of the opening and closing of the points and ignition coil saturation was better. Editor note: I have to read up on the behavior to fully understand. |
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