General Discussion of Ignition Systems

• First Step - As the piston starts going down, the inlet valve is mechanically opened by the turning of the crank-shaft. A mixture of air and gas is sucked in. As it reaches the bottom, the valve closes.
• Second Step - Then the piston starts going up, caused by the force of the turning crankshaft. When it reaches the top, the air/gas mixture reaches a certain compression which is highly explosive.
• Third Step - At this point, the electric circuit connected to the spark plug is turned on (driven mechanically by the position of the crank shaft. The spark plug causes a spark in the dense mixture of air and gas. The spark starts the explosion in the cylinder on top of the piston. This expansion caused by the explosion pushes the piston down. This force turns the crank-shaft around.
• Fourth Step - As the piston goes up again, the outlet valve opens (being driven mechanically. The used air (smoke) from the explosion escapes through the outlet valve. As the piston reaches the top, the outlet valve closes. The cycle is then repeated.

It is important to understand the four-stroke design of the aircooled VW engine when adjusting the valves and when adjusting the ignition timing. To both valve adjustment and timing it is very important to note that you will turn the crankshaft pulley completely around twice during the process of adjusting the valves – you will start with cylinder #1 at the TDC mark, #2 180 degrees round, #3 back to the TDC mark, #4 another 180 degrees around, and back to #1 the TDC mark for the second time. Before you start, though, you must make sure that cylinder #1 is at TDC and not cylinder #3.

This point is also very important when timing the spark. You must make absolutely sure that it is Cylinder #1 that is at TDC and not cylinder #3. If in doubt run our Find TDC procedure.

With that understanding, then we proceed with a discussion of ignition timing.

Ignition timing in an internal combustion engine is the process of setting the time that a spark will occur in the combustion chamber (during the power stroke) relative to piston position and crankshaft angular velocity. Setting the correct ignition timing is crucial in the performance of an engine. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines (other than aircooled VW engines) are generally controlled by an engine control unit that uses a computer to control the timing throughout the engine's RPM range. Older engines, like those we’re dealing with in our Volkswagens, use mechanical spark distributors that rely on inertia (by using rotating weights and springs) or manifold vacuum to set and advance the ignition timing throughout the engine's RPM range, or a combination of both mechanical and vacuum advance.

There are many factors that influence ignition timing. These include which type of ignition system is used, engine speed and load, which components are used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.

Setting the Ignition Timing

“Timing advance” refers to the number of degrees before top dead center (BTDC) that the spark will ignite the air-fuel mixture in the combustion chamber during the compression stroke. “Timing retard” refers to the number of degrees that ignition is delayed after the point that would have resulted in generating maximum power.

Timing advance is required because it takes time to burn the air-fuel mixture. Igniting the mixture before the piston reaches top dead center (TDC) will allow the mixture to become fully burnt soon after the piston reaches TDC. If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC, allowing the ignited mixture to push the piston down the cylinder. This will utilize the engine's power producing potential. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually the pistons against the direction of rotation which will of course, loos power. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston is already traveling too far down the cylinder. This results in lost power, high emissions, and unburned fuel.

The ignition timing needs to become increasingly advanced (relative to TDC) as the engine speed increases so that the air/fuel mixture has the correct amount of time to fully burn. Poor volumetric efficiency at lower engine speeds also requires increased advancement of ignition timing. The correct timing advance for a given engine speed will allow for maximum cylinder pressure to be achieved at the correct crankshaft angular position.

Mechanical Ignition Systems

Mechanical ignition systems use a mechanical spark distributor (the “009” distributor in VW jargon) to distribute a high voltage current to the correct spark plug at the correct time. In order to set an initial timing advance or timing retard for an engine, the engine is allowed to idle and the distributor is adjusted to achieve the best ignition timing for the engine at idle speed. This process is called "setting the base advance." There are two methods of increasing timing advance past the base advance. The advances achieved by these methods are added to the base advance number in order to achieve a total timing advance number at any given idle speed.

Mechanical Timing Advance

An increasing mechanical advancement of the timing takes place with increasing engine speed. This is possible by using the law of inertia. Weights and springs inside the distributor rotate and affect the timing advance according to engine speed by altering the angular position of the points plate with respect to the actual engine position. This type of timing advance is also referred to as centrifugal timing advance. The amount of mechanical advance is dependent solely on the speed at which the distributor is rotating. In a 2-stroke engine, this is the same as engine RPM. In a 4-stroke engine however (which our VW engines are), this is half the engine RPM. The relationship between advance in degrees and engine RPM can be drawn as a simple two-dimensional graph called an “advance curve.”

Lighter weights or heavier springs can be used to delay the timing advance until the engine revs increase. Heavier weights or lighter springs can be used to advance the timing at lower engine rpm's. Usually, at some point in the engine's rpm range, these weights contact their travel limits, and the amount of centrifugal ignition advance is then fixed above that rpm.

Vacuum Timing Advance

The second method used to advance the ignition timing is called vacuum timing advance. This method is usually used in addition to mechanical timing advance (but not always – the “009” distributor is solely mechanical advance – it uses no vacuum advance at all). Vacuum advance generally increases fuel economy and driveability, particularly at lean mixtures. Vacuum advance works by using a vacuum source to advance the timing at low to mid-engine load conditions by rotating the points mounting plate in the distributor with respect to the distributor shaft. Vaccum advance changes with both engine revs (more air therefore more vacuum through the carburettor) and throttle position (a partly open throttle will increase the vacuum signal from the carburettor, and a rapid throttle openeing will temporarily reduce the amount of vacuum signal from the carburettor, until the rpms and volume of the air travelling through the carburettor, catch up to the new throttle position. This is called "load sensing". Vacuum and vacuum-plus-mechanical distributors can "load sense" but the 009 mechnical-only distributor cannot load sense since it has no vacuum connectio - it advances according to rpms only.

One source for vacuum advance is a small opening located in the wall of the throttle body of the carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle. The small opening in the wall of the throttle plate connects to a port on the left side of the carburetor.

(The following discussion is adapted from "Choosing the Right Distributor" by John Connolly (Aircooled.Net)

Up through the late 60s, VW supplied their engines with several different vacuum-advance distributors. These were not concerned with smog controls - emissions had not become an importnt issue. These vacuum advance distributors do their job well, and all are dependent on a vacuum signal from the carburetor, (with the exception of some early type 2s, which were chronically under-powered and used a centrifugal only distributor). Stock units work VERY well when installed in stock or near-stock engines with 28 or 30 series carburetors.

In 1971 (in the USA) the VW engines were shipped with a "smog" distributor, which had a vacuum retard in addition to the vacuum advance. The engines were also changed to the dual port configuration, along with a change in carburetors from the 30 PICT series to the 34 PICT series, which had the additional port for the vacuum retard. These new carburetors were also LEAN in their operation, since they had to conform to the new tailpipe standards.

The mechanical advance "009" distributors (no vacuum advance) have been sold by the ton. Early VW engines (pre-71') could work with these distributors reasonably ok, but the smog engines when equipped with the 009/010 distributors had a pronounced and annoying "flat spot". A "flat spot" is a hesitation just off idle, and can range from being almost unnoticeable to getting broad-sided or rear-ended by approaching cars! (Many 009/34 equipped owners mistake this hesitation for POWER. They don't notice the hesitation, but they DO notice the kick in the back of the seat once the engine catches and it starts accelerating! They mistake this for "more power" since there is such a difference between the stumble and actually operating properly.)

Unfortunately, writers like Muir (How to keep your Vokswagen Alive) were under the mistaken impression that the mecanical-only distributors like the 009 were the "bees knees" and encouraged VW owners to ditch the original vacuum units in favour of these "racing" distributors, aparently not realising that "racing", "cruising down the highway", and "accelerating smoothly away from the lights", are entirely different engine conditions, and one-size-fits-all distributors are NOT ideal for road use.

The vacuum advance distributors do not have this hesitation since they advance the timing when the throttle is opened as part of their operation (load sensing). Obviously, since the 009 distributor is only rpm based, and there is no vacuum advance. When a 009 distributor, which only STARTS to advance from about 1200rpm onwards, is combined with the lean SMOG operation of the 34 series carburetor, the flat spot is the result. "Solutions" to this problem that most people are currently using are all modifications to the carburetor. These modifications richen up the fuel delivery in various forms, whether it's the idle circuit, the main jet, and/or the accelerator pump circuit.

The error with these "fixes" is that they are curing a symptom, not the problem. The problem is the lack of additional advance just off idle, not lean operation. The stock distributor/34 carburetors working wityh a vacuum distributor didn't have a hesitation!

The SVDA Distributor

Enter the SVDA distributor. A production VW distributor with the 009 advance curve (close enough), but with added vacuum advance. There is an additional advantage to the vacuum advance (on engines that have the proper vacuum port - 34 PICT series carburetors), and that is the gain of 4 mpg improvement over the 009 distributor! Same performance, plus 4 mpg improved mileage, and no flat spot. The vacuum line for the single-vacuum advance must be disconnected and plugged when adjusting the initial advance setting, which is usually specified in a workshop manual.

30 Series carbs can not use the SVDA distributor because the vacuum signal is not correct; it won't pull the advance in on the small canned SVDA (you will notice the early distributors use larger vacuum cans than later distributors because the vacuum signal is different).

There is an additional ignition topic that is important to consider. The ignition is separated by primary and secondary parts. The primary ignition is the low voltage side; points, condenser, etc. The secondary ignition is the high voltage side; coil, cap, rotor, wires, and plugs. The primary side of the ignition is responsible for the triggering of the spark, and the secondary side is responsible for making the spark, amd sending it to the right spark plug.

Primary Ignition

Most mechanics agree that points are obsolete. You would be well advised to replace the points with some sort of magnetic triggering device (Compufire, Pertronix, etc.). These do not increase the spark quality (as some claim) compared to a properly operating points triggered ignition. However, they do not deteriorate like conventional points/condenser ignitions. They are exceptionally reliable, and they provide rock steady timing at all engine speeds, which can not be said for points. Basically, the magnetic pickup assures optimum triggering all the time. Simply buy the magnetic pickup points replacement unit and throw the points in the glove box in case you have a problem with the magnetic unit.

Secondary Ignition

This is another place where large improvements in engine operation can be gained. The stock Bosch coil is only adequate to about 2500 RPMs, and the spark quality deteriorates from this point and up. The problem is with the coil's primary voltage (12-14V). As engine speed increases, there is less time for the primary voltage at the coil to create the required magnetic field that generates the high voltage spark we need at the plugs. There are two solutions: 1) increase coil current. The problem with this is that you will burn out points (if equipped) much faster, since the coil's current is passing through the points. 2) increase the coil's primary voltage. This is the solution that Jacob's, Universal, MSD, and other companies have decided to use. There is no drawback to this solution other than cost.

Capacitive Discharge Ignition (CDI)

The conventional induction ignition creates a spark by applying electric potential (12 volts) to the primary side of the coil. The coil steps the primary potential up to as much as 18,000 volts and delivers this high voltage to the spark plugs. However, this "step up" process is relatively slow, and as crank speed (rpm) increases, the secondary voltage declines dramatically.

This limitation is partially solved by the development of capacitive-discharge ignition (CDI) systems. Instead of applying 12 volts to the coil, a CD ignition increases the primary current by storing it in a kind of miniature battery called a capacitor. When this higher primary current is applied to the coil, the secondary voltage is dramatically increased.

The principal advantage of a CDI system is the ability to present a superior spark to the air/fuel mixture inside the combustion chamber, thus maximizing burn efficiency. The easiest way to get a bigger spark is to increase the spark plug gap size. However, increasing the gap distance also increases the voltage necessary to ionize the air/fuel mixture. And the resistance of the air/fuel mixture increases as the mixture is pressurized in the cylinder, requiring even higher voltage to spark across a plug. A CDI system provides the higher voltage allowing an increased spark plug gap size, thus providing very intense spark.

A CDI ignition system can create spark potential as high as 37,000 volts. Most engines only need about 20,000 volts for reliable ignition. The stock system begins to 'droop' as the rpm goes up. At highway speeds, the spark voltage becomes more and more marginal, dropping below the preferred 18,000 volts. With a CDI system, the step up process is very fast compared to a conventional 12-volt induction. This assures a more consistent spark delivery across the plug gap, even at very high crank speeds (rpm).

Note: The consistently higher voltage will cause neoprene-type insulation to break down rather rapidly. You need sparkplug wires with better insulation, such as silicone or non-metallic.

The carbon conductor in the non-metallic leads will produce less radio interference and won't corrode like metallic leads. Electrically, if you've got 37,000 volts available, the ignition system can't tell the difference between metallic & non-metallic leads.

Note from Bob Hoover regarding CDI - The CDI module doesn't claim to be anything special, just a good basic ignition system. If all you want is a good, reliable ride, it's the best idea since beer in cans. It will save you money, help clean up the environment and make your car run better, all at the same time.

Once the secondary ignition is improved with a CDI system, the spark plug gap can be increased to 0.040-0.045". (See our Tune-Up Procedures for more detail regarding spark plug removal and reinstallation.

You will find smooth running at all temperatures and instant starting, with a 10-15% increase in gas mileage. This savings in fuel will pay for the CDI quickly!