When talking about high energy systems there are a few basic concepts that need to be understood so it all makes sense.
First, the voltage required to jump the spark plug gap is primarily determined by the number of molecules of fuel mixture between the spark plug electrodes. If you widen the spark plug gap there will be more molecules of fuel mixture between the electrodes and it will require a higher voltage to jump the spark plug gap.
If you increase the fuel mixture density there will be more molecules of fuel mixture between the electrode and it will require a high voltage to jump the spark plug gap. Increasing the compression ratio, turbocharging, or otherwise increasing the volumetric efficiency will increase the fuel mixture density and increase the voltage required to jump the spark plug gap.
Using less restrictive air cleaners, larger carbs, head porting, larger valves, and a freer flowing exhaust system will all increase the volumetric efficiency and increase the fuel mixture density, which will raise the voltage required to jump the spark plug gap.
Retarding the spark timing closer to TDC increases the cylinder pressure and fuel mixture density and requires a higher voltage to jump the spark plug gap. Using a hotter heat range spark plug increases temperature inside the spark plug gap and lowers the density of the fuel mixture inside the spark plug gap and lowers the voltage required to jump the spark plug gap.
Sharp electrode edges reduce the voltage required to jump the spark plug gap. When spark plug electrodes become worn and rounded, it requires more voltage to jump the spark plug gap, which is why your engine may begin to miss when the spark plug electrodes become worn and rounded.
Within a limited range, richer fuel mixtures are easier to ignite. However the richer fuel mixtures are not optimum for combustion. With a weak ignition system, some people richen their fuel mixtures to reduce missing, when in fact they should have either reduced the size of the spark plug gap, or better still improved the ignition system. Once the mixture becomes too rich, it becomes more difficult to ignite.
The conclusion is that nearly anything that you to to improve the performance of your engine will increase the voltage required to jump the spark plug gap, which either requires an improved ignition system capable of delivering higher voltage, or you must REDUCE your spark plug gap.
Wider spark plug gaps with a high energy ignition system will improve the performance and gas mileage of ANY engine. Improving your ignition system is the one modification that you can do that doesn't have any serious tradeoffs like most other performance improvements have.
Spark timing and ignition timing are not the same. There is a lag time between when the spark is applied and when the fuel has been ignited to the point that the ignition system no longer has any control over the burn. The amount of lag time varies with different engine conditions and CANNOT be compensated for. The result is that under some engine conditions your ignition timing will be ideal, but under all other engine conditions your timing will be retarded from optimum and your performance and fuel mileage will suffer as a result.
However, you can BYPASS much of this ignition lag time by using a high energy ignition system with wider spark plug gaps. The result is that your ignition timing will be closer to ideal under all engine conditions, and your horsepower and gas mileage will improve at all throttle positions.
Using a weak stock ignition system is like using a single match in the wind to try to light kindling on fire. You can't predict how long it will take, or even if you will be able to get the kindling lit.
Using a wider spark plug gap is like using 3 matches at once. You expose more of the kindling to the flame at one time and are more likely to light the kindling and light it faster.
Using a high energy ignition system is like using 3 propane torches to light the kindling. Not only are you assured to ignite the fuel mixture, but you will ignite it much faster.
There is no downside to using a higher energy ignition system. It gives you both improved perfomance and fuel mileage. By reducing or eliminating missing, there is no raw fuel to wash down the cylinder walls and contaminate your oil. Your piston rings will last longer. Your engine will run more smoothly which will also reduce bearing wear.
Of course there are real world limitations on ignition systems. The spark plug will have a maximum voltage limitation. There will be a limitation on the largest spark plug gap that you can have before the spark jumps to the spark plug body instead of the outer electrode.
The ignition wires will have a voltage limitation. However thicker insulation may be used to withstand a higher voltage. Good quality ignition wire is not the weakest link.
The spacings between the distributor cap terminals and between the distributor cap terminals and the distributor body will determine the maximum voltage that you can use. The voltage at the spark plugs can be affected by many factors as previously outlined. The practical method of limiting the voltage inside the distributor to a safe level is by adjusting the size of the spark plug gap so that the spark plug fires before the voltage arcs to the wrong terminal inside the distributor cap or to the distributor body.
The ultimate ignition system would consist of a distributorless ignition system with a separate ignition coil for each spark plug and and a high energy ignition system capable of driving the ignition coils. But that's another discussion.
There are two basic types of high energy ignition systems that we are concerned with. First there is the inductive switcher. It operates in similar manner to the conventional Kettering ignition system. A transistor inside the ignition system conducts current to the ignition coil just like ignition points do. However, it has the capability of adjusting the dwell time of the ignition coil to maintain the output voltage as the RPM increases. The stock Kettering ignition system will continue to lose output voltage as the RPM increases.
The second type of high energy ignition system is a capacitive discharge ignition system. This ignition system boosts the +12 VDC to over 400 VDC and stores the charge in a capacitor. Then at the appropriate time the capacitor is switched to the ignition coil and the energy stored inside the capacitor is transferred to the ignition coil which produces the high voltage for the spark plug.
Inductive switchers have longer duration sparks which work very well at lower RPM and are claimed to help the engine run more smoothly at lower RPM.
Capacitive discharge ignition systems have shorter duration but higher current sparks. They are claimed to run better at higher RPM but not as smoothly at lower RPM. The higher current spark is better able to fire spark plugs during fouling conditions such as flooding and over rich conditions. Capacitive discharge ignition systems often make up for the shorter duration sparks by using multiple sparks.
Also extremely important is the ignition coil design. For maximum performance you should use an ignition coil with an air gapped core without any extra ballast resistance.
The air gapped core will vary the spark energy as the spark plug needs it, and will deliver more current during fouling conditions to keep the spark plugs running clean.
I measured a voltage loss of 16% when I added a ballast resistor to a Jacobs ignition coil to equal the 3.0 ohms that a Bosch Blue coil has, when used with a capacitive discharge ignition system. Most high energy ignition systems internally limit the ignition coil current to a safe level so a ballast resistor is usually not required.
The terminal spacing inside the distributor caps, of most distributors available for air cooled VWs, is the weakest link in the ignition system. Most high energy inductive switching and capacitive discharge ignition systems are capable of outputting more voltage than the distributor caps can handle.
A small Bosch distributor cap can only handle a maximum of about 28,000 volts before crossfiring or arcing to the distributor body occurs. The small cap Mallory distributor can handle about 32,500 volts before crossfiring occurs, which is about 16% more voltage than the Bosch distributor caps. The Pertronix and MSD distributor caps appear to be similar to the Mallory in size and are not likely to handle much more voltage.
Left: Bosch small cap . . . . . . . . . . . Middle Mallory Comp 9000 . . . . . . . . . . . . Right: Mallory small cap
Mallory Comp 9000 modified for VW
They only distributor that can handle the voltage that most of the high energy ignition systems are capable of outputting is the Mallory Comp 9000, which must be modified to fit a VW engine. It will not fit a Type 3 or stock Type 4 engine, or engine with the 911 style fan shroud. It may fit an upright type 4 conversion. The Comp 9000 will fit with the stock mechanical fuel pump. With the Comp 9000 distributor and a high energy ignition system, there is not likely to be a significant improvement in spark quality using the available distributorless ignition systems.
As the rest of the world phases in distributorless ignition systems, people are selling their high energy ignition systems rather cheaply and great deals can be had. With the exception of a person that wants to maintain a period correct VW, there really isn't a good reason not to use a high energy ignition system.
A high energy ignition system draws so little current through the distributor points that pitting of the points no longer occurs. The rubbing block will probably wear out before the contacts do. Only periodic adjustment will be required to compensate for rubbing block wear. The gap in the points may also be reduced to increase the higher RPM capability.
Also required is a non-resistor rotor as a high energy ignition system will burn it out.
Non-resistor spark copper core plugs should be used for best performance. Fine wire platinum and iridium only improve weak ignition systems, as they will fire at a lower voltage. However, there will also be less power in the spark than with a copper core spark plug.
Low loss spiral wound magnetic suppression ignition wires should be used. The OEM ignition wires and resistor rotor will cause the voltage inside the distributor cap to be higher and force you to use a smaller spark plug gap. Between the OEM stock ignition wires with the resistor ends and the resistor rotor, there is enough voltage loss, that by switching to low loss ignition wire and a non-resistor rotor that the voltage losses will be reduced enough that you can open your spark plug gaps by around 0.005".
In general, I recommend an inductive switching ignition system for most street driven air cooled VWs. However, a turbocharged, or race driven, or off road driven VW can benefit from the increased fouling resistance of a capacitive discharge ignition system. But you can also use a capacitive discharge ignition system on a street driven VW with excellent results.