Changing the Advance Curve on HEI Distributors
By Jim Hand

[This article and accompanying photo and sketches previously appeared in Vol. 1 No. 4 (July/August) of Pontiac Enthusiast. It appears on this site courtesy of the author and the magazine.]

In order to obtain optimum performance, the mechanical advance for any distributor should be checked and revised as needed after the compression ratio of the engine has been changed, after a camshaft with different timing has been installed, whenever a fuel with a different octane rating is used, or if the starter has a hard time turning the engine at the factory-specified timing setting.

In general, automatic-equipped cars with stock-type or slightly loose torque converters respond favorably to more mechanical advance in the lower rpm range. Manual trans vehicles and vehicles with converters that stall to 3000 rpm and up are not as sensitive to such changes because the engine is not usually loaded as the lower rpms.

What is meant by a timing advance curve? The advance springs and weights are designed to advance the timing as the engine speed increases. If measured at discrete increments of rpm, the timing figures generated by the weights and springs as they move during engine acceleration define a curve when the valves are plotted on a graph. The factory curves are engineered to provided acceptable performance and emissions control over the life of the engine. However, during the conditions described in the first paragraph, varying the curve will usually yield improved engine operation.

We have found an excellent combination of mechanical advance components to use in our street/strip engine HEIs. This combo consists of #139 weights, #398 center plate, and moderate-rate springs (similar to the mid-range springs found in most aftermarket HEI curve kits). This combination provides the timing numbers shown in Figure 1, below.

Mechanical advance begins at 900 rpm and increases as follows:
 rpm Degrees rpm Degrees rpm Degrees
1000 6 2000 13 3000 17
 1200  9  2200  14  3200  18
 1400  11  2400  15  3400  18
 1600  12  2600  16  3500  19
 1800  12  2800  16    

Figure 1: Results using a #398 center plate, #139 weights, and moderate springs.

Check your advance curve with a bright timing light. A light with a built-in timing dial will simplify the job. Place a 30 degree advance mark on the harmonic balancer.  The 30 degree mark will be located clockwise (viewing the balancer from the front of the engine) from the existing 0 mark, which is also 1/12th the circumference of the balancer from the 0 mark. For the large balancer (1968 and later) 1.75" is close enough, and earlier, smaller units should be marked at 1 3/8 inches. All other style balancers should be measured and marked at 1/12th of the circumference. With the vacuum advance disconnected, increase the engine speed incrementally and read the timing from a combination of the old and new balancer marks and the existing scale on the timing cover. It is not necessary to check each 200 rpm interval, but the significant points of interest are where the advance begins, where it ends, and what it measures at 1800 rpm.  Good engine response in the 1800 rpm range will be felt each time you accelerate from a stop with a normal converter.

There are several methods for obtaining a quicker curve. Unfortunately, most aftermarket curve kits of weight and springs, and a center plate are not very accurate and my provide a poorer curve than what exists. (The medium and heavy springs in these kits can be used with stock weights.) Heavier weights bring in the curve quicker, as do lighter springs. However, very light springs will fatigue quickly, and the weights will stick more toward the open position, causing hard starting and poor performance. The center plate also has an effect on both the curve and the maximum advance. Its shape defines the advance rate and the maximum advance. Modifying the shape of the center plate is easy and yields significant results.  Figure 3 shows how a #398 center plate was reshaped; installing it with the stock #139 weights and a set of medium springs yielded the timing results shown in Figure 2.

Mechanical advance begins at 900 rpm and increases as follows:
 rpm Degrees rpm Degrees rpm Degrees
1000 6 2000 23 3000 28
 1200  8  2200  24  3200  29
 1400  17  2400  25  3400  29
 1600  21  2600  26    
 1800  22  2800  27    

Figure 2: Results using a #398 center plate reshaped as shown
in Figure 3, below, stock #139 weights and medium springs

The very quick and extended curve was installed to allow the initial advance to be set as a low 4 degrees (33 total mechanical advance) to prevent kick back against the starter on a relatively high compression engine. Vacuum advance connected to full vacuum is used, and this provides a smooth idle even with initial timing set at only 4 degrees. It is important that the mechanical advance not start at or below normal idle speed, or the idle speed will vary unpredictably as the mechanical advance intermittently engages.

The vacuum advance may need some limiting as a result of the faster rate of increase or additional mechanical timing. Limiting may be accomplished by shortening the travel of the vacuum advance pull rod by brazing, in stalling a small bolt and nut, or installing a metal sleeve. Aftermarket adjustable vacuum advance units work well.  Remember that mechanical advance controls full throttle performance and mileage, while vacuum advance affects only mileage and light-throttle response.

The HEI ignition is an excellent system and will provide adequate ignition functions for any engine to 5800 or 6000 rpm. The stock module and coil are more than adequate and will work as well as any aftermarket units, provided the stock units are in good factory specified condition. We have seen cases where aftermarket add-on coils caused arcing and crossfire problems and had to be removed.

GM used several different ignition modules, but the module used in most larger V8s has a number series of "990" in the part number. It is the preferred module to use in all applications. some of the aftermarket components may fill a need in very high rpm applications but will not show a performance increase in properly calibrated HEI units operated below 6000 rpm.

Figure 3 shows actual-size sketches of the #139 weights, stock and modified versions of the #398 center plate, and an overlay that shows the modifications (shaded area), plus the before and after orientation of the center plate/rotor assembly as a result of the modification.

Weight sketches 
Figure 3

Other combinations of weights and plates can be used, and the plate can be modified with a different curve or slope. The plate is installed with the the stamped part number down. The assembled weights and center plate are sketched as they are located on the distributor.  The weights pivot on pins attached to the distributor shaft, and the center plate is attached to the movable assembly that holds the rotor. The springs are connected between the shaft posts and the rotor posts. The springs try to hold the two weights together, but during operation the centrifugal force generated by the rotation of the weights pull the weights outward, which advances the rotor--and therefore ignition timing--at the rate and amount dictated by the shape of the intersecting points of the weights and the center plate. Note that the modified center plate allows the center plate/rotor assembly to rotate clockwise an amount equal to 5 distributor degrees, or 10 crankshaft degrees, in relation to the distributor shaft.  During operation, the rotor moves this 10 crank degrees in addition to the original 19 degrees advance provided by the weights and springs, for a total of 29 degrees of mechanical crank advance. 

The modification described here was prepared using a distributor machine. However, similar changes can be made without such a machine, and accurate measurement can be made with a timing light and a helper to set the engine rpm.

Correct timing is relative to the compression ratio and camshaft timing of an engine, the mechanical condition of the engine, and the quality of the fuel used. The original factory-specified timing settings are generally not accurate for our traditional Pontiac V8s on today's pump fuels, nor are they optimal for any modified cars.

Jim's Addendum Comments

February, 1999.

The above article was prepared a number of years ago, and although it is still accurate and useful, the term "balancer" is used to describe what is more accurately called the "damper". The harmonic damper does not balance any part of the engine - rather, it is designed to absorb and mitigate the twisting action of the crank due to each cylinder firing. By limiting/absorbing some of the twist and shock, the damper allows the crank to last it's design life.

An example of running 29 degrees in the distributor was used, and we did run that setup for some time. This article shows that the distributor can be easily modified to obtain almost any desired mechanical advance. However, the engine will idle better and be more responsive at very low rpm with more timing in the initial setting, and less in the mechanical advance. For full throttle power past idle, the engine does not care which provides the timing as long as the total is optimum for the engine.--Jim Hand


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