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Ford Distributors & Ignition Systems
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Few things about an engine build seem more mysterious and magical than the ignition system.
There have been a lot of advances in ignition technology through the years. Not even 30 yeaars
ago we were driving vehicles with crude point-triggered ignitions and conventional distributors.
Keeping spark timing in sync with engine RPM has been a great challenge with conventional
ignitions. With high-performance engines, keeping the spark alive at high revs under great
combustion pressures has been asking a lot of those dated point-triggered ignitions.
Time and technology have brought us better ignition systems — even distributorless ignition, which
is as precise as it gets. Because you’re building a budget V-8, distributorless ignition probably isn’t
in the cards. So we have to concentrate on how to achieve precision ignition via the distributor.
First, let’s look at the basics.
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POINT-TRIGGERED IGNITION
Conventional ignition systems consist of a primary and secondary circuit. The primary circuit
consists of the battery, ignition switch, and the primary side of the ignition coil (positive terminal).
The secondary circuit consists of the secondary side of the ignition coil (negative terminal), ignition
harness, and spark plugs. Think of it this way: The primary circuit gets power to the ignition coil and
the secondary circuit gets processed power to the spark plugs. We call it processed power because
we are changing it from 12 volts to 10,000 to 30,000 volts to feed the spark plugs. A high-voltage
spark is needed to bridge the spark plug gap.
To process and distribute power, we need an ignition coil, distributor, points, and condenser. Think
of ignition points as a switch that is open and closed in time with the distributor. When the points
are closed (switch on), the primary circuit side of the ignition coil is energized. When this happens,
current flows from the ignition switch (via a resistor wire or ballast resistor) through the coil to the
ignition points to ground. The points are a switch to ground. When current passes through the
primary windings of the coil, a magnetic field builds up around these windings. As the distributor
cam turns, the points open (switch off) and current stops flowing through the primary side of the coil
windings. When this happens, the magnetic field collapses in the primary windings inside the coil. It
then finds a path through the secondary windings inside the coil to create a very high electrical
current that is routed to the spark plugs via the distributor rotor, cap, and wires. The condenser’s
job is to limit the amount of current passing through the points, which prevents pitting and burning.
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This is a simple single-point Autolite
distributor with dual advance. The
dual advance, which first saw use in
1968, both advances and retards the
spark for improved emissions. Earlier
Autolite distributors only advance the
spark. All vacuum advances are
adjustable either through shims
(Ford) or with an Allen wrench
(aftermarket).
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Think of the ignition system as a voltage management-and-distribution operation. It has a tough
job, and that is to keep the release of current to the spark plugs in perfect time with engine speed,
load, and throttle position. At idle, the ignition system has an easy job. Contrary to what you might
have been taught, fuel does not explode in the combustion chamber. It ignites in a “quick fire” just
like your gas or oil burning furnace lights on a cold night. Because the fuel/air mix ignites in a quick
fire, not an explosion, the spark must occur a given number of degrees of crank rotation before the
piston reaches top-dead-center. Why? Because time passes from the time the spark plug fires until
there is a productive mixture light-off with working heat and pressure. If we ignite the mixture with
the piston at top-dead-center (TDC), we waste most of the charge because actual light-off doesn’t
occur in that scenario until after top-dead-center (ATDC). The engine doesn’t make power in this
case; it just burns and wastes fuel.
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Ford’s Autolite dual-point distributor
was installed on most factory high-
performance V-8s throughout the
1960s. The use of dual points spread
the load over two sets of ignition
points, providing a more precise spark
at high revs. Most Ford dual-point
distributors did not have a vacuum
advance unit because these
distributors did their best work at high
RPM. Beginning in 1969, Ford fitted
the dual-point with a dual-advance unit
for improved emissions.
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If our specifications call for static timing of six degrees before top-dead-center (BTDC), this means
it is going to take six degrees of crank rotation and piston travel before the ignited mixture will work
for us. When the spark plug fires at six degrees BTDC, mixture light-off happens before the piston
gets to the top of the bore. By the time the piston reaches the top of the bore, light-off is well
underway and we have a useful charge to drive the piston down the bore, which turns the
crankshaft and makes power along with seven other cylinders on the same timed mission.
As engine RPM and power demands increase, the spark must occur earlier in the cycle. This is
where two forms of spark advance come in — vacuum and mechanical. In vacuum-advance
distributors, both vacuum and centrifugal advance units work together in seamless harmony for the
solid application of engine torque. The vacuum advance advances the spark during engine
acceleration. Leaning on the gas advances the spark via a vacuum advance diaphragm unit. This
helps the engine produce torque under acceleration. The vacuum advance should smoothly give
way to the centrifugal advance as RPMs increase — that is, if timing is where it should be.
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Sometimes we have too much timing (too much spark advance), causing the spark to occur too
early. When this happens, we have pre-ignition, or detonation, meaning the fuel ignites
prematurely. The combustion shock against the piston dome creates a rapping or rattling we hear
as “pinging” under acceleration. It is a popular misconception that this sound is caused by valves
knocking or by rod bearing noise. It is neither. Rather, it is piston wrist pin and skirt noise caused by
the abnormal combustion shock that goes with detonation/pre-ignition.
The centrifugal advance advances the spark at a slower rate and more at higher revs. Flyweights
and springs tied to the distributor shaft work together and advance the rotor position according to
RPM. The more RPM we have, the more the flyweights advance the spark. Spring tension retards
the spark as RPM decreases.
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For cohesive spark advance operation, you need to program both the vacuum and centrifugal
advances to advance the spark smoothly as the need for power comes into play. If both forms of
advance are in perfect tune, the vacuum advance will hand off its duties to the centrifugal advance
as engine speed increases. Too much vacuum advance too early will cause the engine to ping or
misfire. Too little and the engine will fail to produce sufficient torque under acceleration. It will fall on
its face. The same goes for the centrifugal advance at higher RPM. Too much spring tension and it
will not advance. Too little and it will advance too early.
A distributor’s advance process must be tuned in steps. First, you program the centrifugal advance
to come in fully by the time an engine reaches a given RPM range. For street engines, full advance
should come in around 3500 rpm. Full advance should be roughly 36-40 degrees BTDC. You dial
in centrifugal advance with the right combination of flyweight weight and spring tension. As stated
earlier, the greater the spring tension, the more slowly the distributor will advance. Remember, the
spring’s job is to retard the spark. The flyweight’s job is to advance the spark. The object is to get
these guys working together smoothly.
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Programming the vacuum advance takes the right combination of vacuum and spring pressure
inside the advance. The more spring pressure you have, the more slowly the vacuum advance will
come into play. Spring pressure fights the vacuum. Vacuum fights the spring pressure. Vacuum
advances the spark. Spring pressure retards the spark. You want a balance here where vacuum
advances the spark smoothly while you are leaning on the throttle.
Older Autolite distributors have adjustable vacuum-advance units that use shims against spring
pressure. The more shims you add, the greater the spring pressure and the slower the advance
rate. When you remove shims, you decrease spring pressure and increase the rate of advance. If
your Autolite distributor is vacuum-advancing too quickly (which will cause misfire and rough
operation), you need to add shims one at a time. If it doesn’t advance quickly enough (resulting in
sluggish performance), then you need to remove shims one at a time.
Replacement vacuum-advance units for Autolite and Motorcraft distributors are adjustable using an
Allen wrench through the vacuum hose port. Turn the Allen wrench clockwise to increase spring
pressure, or counterclockwise to decrease spring pressure.
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This brings us to Autolite/Motorcraft dual-point performance distributors. A dual-point ignition allows
for a more positive saturation of the ignition coil at high revs, which is why we have two sets of
ignition points tied together in series. Prior to 1972, Ford used dual-point, mechanical-advance
distributors in high-performance applications like the 390, 406, and 427 High Performance, the 289
High Performance, the Boss 302, 351, and 429 V-8s. In earlier applications like the 289, 390, 406,
and 427 High Performance V-8s, the C5OZ-12127-E dual-point distributor was a
mechanical-advance only for high-RPM use. Because these engines come on strong at 6000 rpm,
this is where a mechanical (centrifugal) advance does its best work. Ignition timing at idle with this
distributor is 12 degrees BTDC. Based on a lot of engine-tuning experience, you can push this
distributor to 16 degrees BTDC at idle. The main thing to remember is not to allow total advance to
go beyond 36 to 40 degrees BTDC at 3500 rpm.
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The 1969-70 Boss 302
dual-point distributor had a
dual-advance unit that both
advanced and retarded the
spark. Under acceleration,
the spark was advanced.
During deceleration, the
spark was retarded to reduce
exhaust emissions.
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Like the conventional single-point Autolite/Motorcraft distributor, the dual-point’s centrifugal
advance comes into play based on distributor speed, flyweight weight, and spring tension. The
greater the spring tension, the later the centrifugal advance comes on-line. Likewise, the heavier
the flyweights or weaker the springs, the earlier the spark advance comes into play. This is
something you have to play with either in the engine with a road test or in a distributor test fixture.
You know the advance is programmed right when there is a solid application of torque as RPMs
increase. If there is pinging (detonation), the advance is coming on too soon. Then it’s a matter of
calibration with the use of different springs primarily. To slow the rate of advance, use a stronger
spring.
Beginning in 1969, Ford used a dual-point Autolite distributor with a vacuum dual-advance unit for
improved emissions. The dual-point ignition allowed for better performance at high revs. The dual-
advance unit was designed both to advance under acceleration and retard the spark during
deceleration. Retarding the spark during deceleration reduces hydrocarbon emissions. Vacuum
switching based on engine coolant temperature is what determines whether or not you get spark
advance or retard. Suffice it to say that the dual-advance unit was one of Ford’s first steps toward
more in-depth engine emissions management.
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ELECTRONIC IGNITION
In 1974, Ford began using an electronic ignition system known as Duraspark. The Duraspark
ignition system consists of a Motorcraft distributor with a magnetic pick-up and an ignition amplifier
that mounts on the inner fender or firewall. The Duraspark ignition system is simple to install and
use. Since the Duraspark system has been installed in millions of vehicles since 1974, cores are
plentiful from salvage yards. What’s more, it is a proven ignition system with an excellent track
record. You can even purchase a remanufactured Duraspark distributor and a new aftermarket
ignition amplifier from your local auto parts store. Duraspark ignitions are available for all Ford V-8s
except the Y-Block and MEL-series V-8s. Both of these engines were discontinued for more than a
decade prior to the availability of Duraspark.
The Duraspark ignition system can be installed with the amplifier visible or invisible. Invisible, you
can hide it in the fender well (shielded properly from dirt and water) or inside the firewall under the
dashboard. All the amplifier needs is adequate airflow for cooling purposes.
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There are three types of Duraspark ignition systems. Duraspark I is the first generation, with a very
simple distributor and amplifier package common during the 1970s. Duraspark II is an advanced
form of Duraspark I, more tied to engine function. Duraspark III has a crankshaft sensor that makes
it more an electronic engine control system. For your budget V-8, we suggest the original
Duraspark I system.
Two types of distributors were used on early 5.0L high-output V-8s. Carbureted versions were fitted
with a magnetic-trigger Motorcraft Duraspark distributor from 1979-85. Those fitted with CFI or SEFI
(fuel injection) were fitted with a different type of Motorcraft distributor that works on the Hall Effect
concept.
Magnetic trigger is a conventional electronic ignition (Duraspark) that uses a pick-up module and
an eight-point reluctor attached to the distributor shaft. The armature whirls around with the shaft in
time with the engine’s camshaft. The pick-up module senses a disruption in the magnetic field,
which switches the ignition coil on and off, just like ignition points and condenser did in the old days.
For the most part, the mag-trigger distributor is trouble-free. It works on conventional ignition
principles with both a vacuum and centrifugal advance like an older Ford V-8 engine. The last year
Ford used this distributor was 1985 on the 5.0L-4V HighOutput V-8.
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Hall Effect distributors are computer controlled, part of a complete engine-control package called
EEC-IV (Electronic Engine Control, Fourth Generation). EEC-IV first saw use on the Thunderbird
Turbo Coupe, Cougar XR-7 Turbo, and the Mustang SVO. It was used with Duraspark III. The 2.3L
Turbo OHC four that powered these cars needed a precision system that would allow smooth, safe
operation. EEC-IV did a good job of controlling spark, fuel, and boost curves for smooth operation.
Ford applied EEC-IV to CFI first in 1984, then SEFI in 1986 when port fuel injection came to the V-8
arena. EEC-IV is a programmable system, in which you can adjust fuel and spark curves with a
laptop computer.
The Hall Effect Motorcraft distributor is a conventional distributor fitted with a TFI (thick film
integrated) module, profile ignition pickup (PIP), trigger wheel, and a Hall Effect switch. The trigger
wheel rotates on the distributor shaft, interrupting a stationary magnet and a current carrying Hall
Effect semi-conductor. As the trigger rotates between these two elements, it provides a signal for
the EEC-IV system. It also allows for coil saturation and the release of high-voltage electricity to the
spark plugs. A crankshaft sensor takes care of the rest of the EEC-IV triggering system.
The TFI module does what a vacuum and centrifugal advance used to do. It also controls coil
discharge and dwell timing. This means the TFI module does the same job that points, condenser,
and advance units used to do. Doing all of this work makes the TFI module run hot. It has a
reputation for failure due to heat issues. One solution is to use dielectric grease between the TFI
module and the distributor during installation. One other solution is to retrofit your 5.0L HighOutput
V-8 with a 1994-95 Motorcraft distributor where the TFI module has been moved out of the
distributor to the inner fender for cooler operation.
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We are sometimes asked which is the best aftermarket ignition system for a budget 5.0L/5.8L SEFI
V-8. The truth is, the stock Motorcraft EEC-IV system provides all of the ignition power your budget
V-8 will ever need. It packs a wallop at nearly 40,000 volts at the spark plugs. So save your money
and invest it in what is really important inside the budget engine.
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AFTERMARKET IGNITIONS
Stock Ford distributors have one major shortcoming: their shaft bushings don’t receive sufficient
lubrication for long life. They wear out quickly. As a result, excessive shaft-side play causes
irregularities in point dwell and gap. The aftermarket offers alternatives to the stock Ford ignition
system.
If you are attached to your stock Ford distributor, Pertronix has fast answers with its Ignitor and
Ignitor II retrofit ignition modules. The Ignitor module installs in 15 minutes and never requires
service for the life of the module. Remove the ignition points and condenser. Then fasten the
Ignitor to the breaker plate and connect the leads, just like you would a set of points. Then set the
air gap and install the shutter wheel. Pop on the rotor and distributor cap, and the job is finished.
The beauty of the Ignitor is its simplicity. No periodic maintenance required and no one knows it’s
there but you. It’s perfect for the stock, original driver that you would like to keep stock in
appearance. One more thing about the Ignitor. Should it fail, you can reinstall the points and
condenser, which will get you back on the road in short order. The Ignitor has an excellent
reputation for reliability, so failure is uncommon.
If when you install the Ignitor the engine will not start, or operation is erratic, check the ground first.
There should be a ground strap between the breaker plate and the distributor housing, just like
Ford did it from the factory. If the ground strip is missing, the engine likely will not start, or it will
mysteriously shut down and not restart.
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A solid supporter of stock Ford ignition systems is Performance Distributors in Memphis,
Tennessee. Two types of Motorcraft Duraspark distributors are available from Performance
Distributors. The basic Duraspark distributor offers a heavy-duty ignition module, high-energy
distributor cap, and custom curving applicable to your application. The DUI (Davis Unified Ignition)
coil-in-cap distributor from Performance Distributors gives you the benefits of GM’s HEI ignition in a
Ford-specific distributor. This custom-curved system eliminates the external ignition coil and steps
up the power to your spark plugs.
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Previous | Next
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This has been a sample page from
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How to Build Max Performance
Ford V-8s on a Budget
By George Reid
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Low-cost formulas for building serious horsepower!
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This book addresses high-performance V-8
engines such as the 289, 302, 351ci small-blocks
found in Mustangs, as well as the FE series of
big-blocks. Emphasis throughout is a budget
approach to building high performance powerplants
through the use of over-the-counter factory
components and selected aftermarket pieces.
Includes realistic, low-cost formulas for building
serious horsepower in Ford V-8 engines. Read the
sample pages to learn more!
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Click below to view sample
pages from several chapters
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1 - Engine Building Basics
2 - Making Power
3 - Engine Block
4 - Crankshaft, Rods & Pistons
5 - Cylinder Heads
6 - Camshaft & Valvetrain
7 - Headers and Exhaust
8 - Ford Ignition Systems
9 - Engine Build Ups
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Softbound
8-3/8 x 10-7/8
128 pages
300+ b/w photos
Item #SA69
Price: $18.95
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Click here to buy now!
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If you're serious about building a powerful
Ford V-8 you need this book!
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Other items you might be interested in
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How to Rebuild the Small-Block Ford
In this 144 page book you're walked step by step through a rebuild,
including: Planning your Rebuild; Disassembly and Inspection;
Choosing the Right Parts; Machine Work; Assembling your Engine;
and First Firing and Break-In. Includes 289, 302, 351W, 351C, 351M
and 400.
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Price:
$18.95
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The Step by Step Guide to Engine Blueprinting
Practical methods for racing and rebuilding, selecting and preparing
parts, and how to buy machine shop work. This completely revised and
updated version containing an additional 32 pages is simply the best
book you can buy on engine preparation for street or racing! Engine
Blueprinting shows the reader how to use precision measuring tools,
calculate compression ratios, degree a camshaft, and much more!
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Price:
$18.95
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How to Build Big-Inch Ford Small Blocks
Thoroughly explains how to build a stroker, with information that will
help you to better tailor your heads, cam, intake manifold, carburetor &
exhaust system to get the most of the extra cubes. Also included is a
complete guide to head and block castings so you can choose exactly
the right parts for your project.
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Price:
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