The Difference between Ignition Systems

Different Porsche ignition componants layed out

Single coil, coil over, DIS or CDI are all types of ignition systems that can be used on your Porsche. What is the difference between them and why would you use one over the other. In this week’s article I look at the difference between each system and its pros and cons.

Single coil Ignition Systems

The single coil ignition system, also known as the Kettering Ignition system, (named after its inventor), is one of the simplest systems. It uses a switch in the distributor, (ignition points), to allow current to flow through the coil. As the current flows through the coil it produces a magnetic field. When the current stops, (switch opens), the magnetic field collapses through the secondary side, further amplifying the charge and delivering a spark.

Kettering Ignition system
Kettering ignition system layout

This system was the first ignition system to be used and has the most amount of limitations. The biggest of the limitations is the available charge time of the coil. The charge time is limited to the amount of points dwell, or closed time. The longer the points are closed the more charge the coil can generate. However, as cylinders are added, (4 – 6 – 8, Etc.), the amount of charge time is reduced. Also, as the engine RPM is increased the amount of charge time is reduced.

Charge Time

The effects of the reduced charge time results in lower spark energy at the plug. To combat this issue many manufacturers experimented with different coil construction methods to create a coil that could charge faster. Therefore, you will see coils that require a ballast resistor on the primary circuit and coils will have different internal resistance numbers. All of this was to increase the available ignition power.

Primary Coil Charging diagram
Primary Coil Charging

Even if the coil can get its optimal charge time and is at its highest strength the amount of actual spark energy produced is relatively low. The most amount of current produced by a single coil is usually around 30 to 50 mJ of energy. Though hot enough to ignite the fuel mixtures at low speeds this energy cannot be maintained as RPM increases.

DIS Ignition Systems

DIS ignition systems, (Direct Ignition System or Waste Spark ignition systems), are when two cylinders are connected to the same coil. The connected cylinders are both at TDC at the same time though one will be at overlap and the other will be at compression. On a 911 for example cylinders 1 – 4, 6 – 3, and 2-5 will share a coil. The cylinder that is under compression load will draw the highest amount of spark energy. While the cylinder that is in overlap will still have a small spark across the plug, (the waste spark).

DIS ignition System
6 Cylinder DIS Coil Pack

This is because the compression pressure in the cylinder makes it harder for the spark to jump the plug gap. Therefore, it will use more of the available energy produced by the coil. The coils in this type of system are charged using a module and an ignition control unit. The control unit looks at engine speed and timing requirements, then sends a signal to the module. The module is a form of switch that controls the charging of the coils. This is the same as the original Kettering style system, just computer controlled.

DIS ignition system wiring
Waste Spark Ignition

Drawbacks and limitations

Though the charge time is significantly increased by sharing the coils it still decreases as RPM increases. The amount of energy that the coils can produce is still limited by the type of coil construction and charge time. The system is also more expensive to install as now you will need a control unit, trigger wheel and sensors to monitor engine speed and position. Though clearly more efficient than the single coil system is still a compromised system.

Coil Over Ignition Systems

Coil over systems are an ignition system that has a coil on every spark plug. By having a coil on every spark plug the charge time of the coil can be approximately 700 degrees of crankshaft rotation. That way the coil can get the maximum possible charge because it only has to fire one spark plug.  The coils charging is controlled by an engine computer. The engine computer will monitor engine speed and position and will fire the plug at the right time and then recharge the coil.

Coil over ignition system
Coil over plug Ignition Coil

As far as inductive coils go this type of system is the most efficient. However, the amount of spark energy put into the plug is still going to be relatively low. This is due to the limitations of the size of the coil and its design.

CDI ignition Systems

CDI ignition systems, (Capacitor Discharge System), are a very powerful and fast ignition system. They work by taking battery voltage, (12 volts), and step it up to 450 – 500 volts. They then charge up the capacitor with approximately 150 -175 mJ of energy. This happens in approximately 1mS, in most cases before the points can close. When the energy is delivered to the spark plug it runs through a secondary transformer, this in turn steps the voltage up another 100 times to around 45,000 to 50,000 volts.

Bosch 3 pin CDI box
Bosch 3 Pin CDI Box

The CDI ignition system is not affected by engine RPM. Because the system can operate so quickly to recharge the capacitor the spark plug will receive the same amount of energy at 1000 RPM as it will at 7000 RPM. The plug burn will be hotter and longer when using a CDI system. This will result in a more complete ignition and less plug fouling.

6 Pin CDI wiring schmatic
6 Pin CDI Wiring Schmatic

Draw backs of the CDI System

The biggest issue of the CDI system is also its greatest strength, its power. One of the by-products of producing a very high energy ignition is that it also creates a lot of electrical noise. Every time the spark energy is pushed down the spark plug wire it creates a magnetic wave of voltage around the wire. Now a car that is using carburetors or MFI or CIS fuel injection is not an issue. However, a car that uses electronic Fuel injection is a different story.

A car that uses electronic fuel injection also uses an array of engine sensors. Most of these sensors run on a 5-volt reference voltage. It does not take much for the CDI system to impart a voltage signal greater than 5 volts into the sensor circuits. When this happens, it will confuse the engine computer, which in turn commands the incorrect fuel and spark requirements for the engine.

Therefore, when cars started switching to DME control systems, (Digital Motor Electronics), they had to take a step back in ignition quality. Even though the CDI is a much more efficient and powerful ignition it cannot be used with the electronic systems.

Complete Electronic Control

As the complete engine controls progressed so did the ignition systems. At first Porsche went to the TCI single coil system, which is module driven computer-controlled ignition system. As time progressed it was not long until all cars were fitted with coil over ignition systems. This is all to restore as much spark energy as possible without interfering with the engine electronics.

Bosch DME control unit
Bosch DME control unit used in the Porsche 1984-89 911 models

Retrofitting an older car

A lot of people seem to think that installing a coil over system or a DIS system on an older car that uses a CDI system is an upgrade. It is not, unless you are switching to an electronic engine management system, that controls both spark and fuel all you are doing is reducing the amount of available spark energy. On a car that uses either a carburetor or MFI fuel injection you will run the risk of increased plug fouling.

My advice is stick to the system that best matches your entire engine control package. Don’t get sucked into the hype of just because it is computer controlled it must be better.


7 thoughts on “The Difference between Ignition Systems”

  1. Jack Bramwell

    I need a coil for an old stationary engine that has a points and magneto ignition. Bt the magneto coil has blown so want to convert it to 12v battery, points, condensor and coil system. I found a Bosch ignition coil at the wreckers, its a TCI coil. It has 3 Ohms of resistance. Do i nead a ballast resistor? thanks

    1. Jack
      A TCI coil is designed to be used by computer controlled system and a module to control the charging of the coil. Hence the name Transistor Controlled Ignition or TCI for short. You would be better off using a Bosch blue coil, these are a common and inexpensive unit that can be purchased just about anywhere. The part number is, these coils do not need a ballast resistor and can be wired directly to the +15 circuit.

  2. Hi Kurt

    Some times things just take longer than expected and we are finally at a point where the engine can be installed in the car. With that I’d like to ask which spark plugs you recommend together with the Partsklassik CDI boxes and coils?

    My engine is naturally aspirated, 3L, high compression, high revving, twin plug, carburetors.

    Hope all is well!


    1. Johan,
      Spark plug selection is going to be dependent on what your actual compression ratio is. If you are less than 9.5:1 I would use a Bosch W4CS and if above that I would use Bosch W3CS or the Bosch W3DPO platinum plugs. Do not use a plug that has a resistor built into it as it will take away from the spark energy. Plug wires should be solid core and total system resistance should be 6000 ohms per spark plug with 1000 ohms at the plug and 5000 ohms at the rotor. If the rotor does not have a 5000-ohm resistor built into it then you must use a 5000-ohm resistor on the coil wire. Failure to not do so will result in coil and or CDI box failure in a short amount of time.

  3. Thanks Kurt!

    I’m using the Partsklassik wires you put together for me and compression is around 11.

    Should the gap be 0.7-0.8?

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