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.

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

Comments or ideas--email me
at [email-masked]

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