Ignition timing is typically split into three types. The first is initial timing—that is, the amount of spark lead that is set using a timing light (aimed at the harmonic balancer) with the engine at idle. The second type is mechanical advance, determined by the distributor’s mechanical advance system. The final type is vacuum advance, determined by the vacuum canister attached to the distributor. All three combine to produce the total amount of ignition timing in your engine at any given load and/or rpm.Since a mechanical distributor is still the most popular form of ignition, we’ll focus on it. But before we set these values, it’s important to understand how these events interact. Initial advance is set by moving the distributor in the engine. Once initial timing is set, it’s complemented by the mechanical and vacuum advance curves.Mechanical advance adds additional timing to the engine and is based strictly on rpm. The mechanical advance weights and springs inside the distributor combine to produce a curve that begins with initial timing (with the engine at idle) and increases to a certain rpm, but generally peaks below 3,000 rpm. This is often referred to as the mechanical ignition curve.
Vacuum advance also adds timing to the engine, but it is based on engine load and is controlled by intake manifold vacuum. When the throttle is partially open (highway cruising), manifold vacuum is high. This vacuum signal is routed to the distributor and “pulls” on the vacuum advance canister to create more ignition timing. As the throttle opening increases, load increases and manifold vacuum decreases. At wide open throttle (WOT), manifold vacuum is at or near 0 and the vacuum advance produces no additional ignition timing.
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Now that we know how these three systems work, let’s look at a real-world example. Our fictitious engine is a small-block V-8, any V-8, but these variables interact the same with all engines. Let’s give the motor 8 degrees of initial timing, as observed with a timing light. A test on the distributor machine reveals 28 crankshaft degrees of total mechanical advance (at 2,800 rpm) and another 14 degrees of vacuum advance. By adding all three values, we can see that at 2,800 rpm at light throttle (14 inches or more of manifold vacuum), the engine has 50 degrees of total ignition advance. At WOT at 2,800 rpm, however, the engine only has 36 degrees of total timing (50 degrees minus 14 of vacuum advance).
1 The timing light is a simple device, a neon stroboscope triggered each time the spark plug fires. To hook up the timing light, connect the power leads to the positive and negative leads on the car battery and attach the inductive lead to the Number One spark plug wire. The light will flash when the engine is running.
2 To check initial timing, disconnect the vacuum advance line from the distributor. Point the light at the timing tag next to the harmonic balancer. At idle, typical initial timing figures are 4-10 degrees before top dead center (BTDC). Timing tags often are marked with an “A” (Advanced), an “R” (Retard), a “B” (Before), or an “A” (After). To set timing, loosen the hold-down and move the distributor housing, like so.
The photos illustrate how to go about moving the distributor and setting the initial timing. By applying a timing tape from MSD (PN 8985) to the harmonic balancer, for example, you can set initial timing with a standard timing light and check total timing as well. To check only initial plus mechanical advance, disconnect the vacuum advance.
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3 Wondering which way to turn the distributor to advance or retard the timing? A quick way to determine this is to lay your hand on top ot a distributor with your fingers aligned with the vacuum advance canister. Curl your fingers and they’ll point in the direction of rotation. The Chevy distributor (left) rotates clockwise while the Ford distributor rotates counter-clockwise.
4 The Chevy distributor makes it easy to modify the mechanical advance because the mechanism is located directly beneath the rotor and is fully accessible—Ford and Chrysler distributors locate the advance mechanism inside the distributor body. On either type, total mechanical advance is limited by a slot-and-pin system. The slot can be elongated to increase advance, or brazed up to decrease advance.
5 MSD adhesive timing tapes attach directly to the harmonic balancer. With a timing light, you can check mechanical advance in 500-rpm increments from idle on up. Usually, the curve is all in by 3,000 rpm, but check to make sure. Record the event at each increment and you create a timing curve similar to that generated by a distributor machine. Don’t forget that you’re readingboththe initial and the mechanical advance at each rpm level.
6 Here you see a typical mechanical advance curve for a mild street engine. Total advance should all be in by 3,000 rpm and will be the sum of the mechanical plus the initial timing as set at the distributor.
Total timing requirements are dependent upon many variables. A typical street engine with a compression ratio of 9.0:1 runs very well with around 36 degrees of timing (initial plus mechanical) for a rough total of 15-55 degrees of timing at light throttle when vacuum advance is employed. If the engine detonates or pings under WOT, reduce the timing. If it surges or pings at part throttle, reduce the amount of vacuum advance.
Experimentation with timing can help performance, driveability, and mileage by customizing the ignition curve to your particular application. Don’t be afraid to make changes.
TOOLS
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Timing light
Screwdrivers
SOURCE
Autotronic Controls Corp. (MSD)
Dept. HR04,1490 Henry Brennan
Dr., El Paso, TX 79936,
915/857-5200
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