[Dave-R]

SPARK AMPLIFIERS AND COMPUTERS

The simplest form of spark amplifier is the capacitor discharge (C.D.) system. These can be used with points or electronic ignitions but work in an opposite way to normal.

In these systems the battery voltage is amplified to a much higher level, depending on model, from 250 to 600 volts. This is stored in a capacitor. When a spark is required the points open and the capacitor discharges all this power into the primary of the ignition coil. The sudden increase in current and magnetic field induces a huge voltage in the secondary winding which goes to the spark plug. Remember that in a normal ignition it is the sudden collapse of the magnetic field that induces the spark.

If a normal coil is used, with a typical 100 to 1 ratio, the voltage in the secondary winding will be 25,000 to 60,000 volts with an C.D. ignition amplifier!

The spark current in a normal 12 volt inductive system is in the order of 0.02 to 0.06 of an amp but in the C.D. system current is only limited by parasitic effects in the coil such as leakage inductance and resistance. These are much higher then when the same coil is used in a normal system. When special coils designed for C.D. systems are used, peak primary winding currents in excess of 100 amps and spark currents of over 1 amp can be generated. With this kind of spark power it is almost impossible to foul a plug and they will even fire with the gap submerged in water!

The problem with these high spark currents is that they can easily lead to a condition called cavitation where the air and fuel molecules are pushed away by the current and causes a misfire. You can have too much spark power.
Other drawbacks are that these systems can drastically shorten plug life nor do they work well with resistor plugs or wires so that radio suppression can be a problem.

The kind of spark these systems generate is ideal for high rpm, heavy load conditions, such as drag racing, which need a short duration / high current spark. Unfortunately under low rpm, light load conditions you need a long duration / low current spark which makes these systems unsuitable for normal street or motorway driving.

The manufacturers try to get around this lack of spark duration by creating multiple sparks at low rpm. This is sold to the public as a good thing even though it is because the system falls short of ideal for street cars. MSD is one company that produced equipment like this although they now (along with others) advertise units with a long duration single spark.

'COMPUTER' IGNITIONS

Note: In this section I am referring only to ignition computers and not engine management computers.

The 'computer' ignition is the modern version of the old C.D. / multiple spark ignitions. They can be clever little systems.

They start by taking 12 volts from the battery and storing it as high voltage A.C. When the distributor tells it to produce a spark it will start applying voltage to the primary coil. This voltage increases until the computer senses that arc-over at the plug gap has occurred. The voltage on the primary coil can be anything up to 600 volts if needed.

Computers can sense when ignition has occurred by monitoring the resistance at the spark plug gap. When a spark is jumping the gap and current is flowing, there is a drop in resistance across the gap. The computer senses this drop and knows that the spark is now burning but ignition of the air / fuel may not have occurred yet.

It then starts to adjust the spark current while continuing to monitor the resistance. When ignition of the fuel starts there is a second drop in resistance. The computer will continue to adjust spark current until this second drop is measured. It is so fast that it can make up to 33 adjustments in only 1.5 degrees of engine rotation but usually only 11 - 18 are necessary.

The spark is left to burn a little longer, just to be sure, and then the current and voltage are reversed for the same amount of time in order to re-deposit metal onto the plug centre electrode. This extends plug life dramatically.

It is amazing that these things can do all this in such a small fraction of a second.

The computer does not use all of the stored power availible to it, only enough to achieve ignition, and so avoids problems of cavitation and short plug life.

Fuel economy improvement claims go as high as 18%

Manufacturers of these systems include Accel, Jacobs, MSD and Mallory but some of these companies still sell the old style multiple spark systems so check what you are buying.

[Tim Ellison (Tim)]

Dave,

Following on from my question on the MMA board, am I right in thinking the MSD 6AL is the 'old style' multiple spark system, as described above, whereas the Jacobs systems you recommended is a pukka Computer Ignition (I'm thinking of their Pro Street kit)? The technical literature, even from the manufacturers themselves, ropes in CD, digital and multi- spark for both units, making it a bit confusing knowing what you're paying for. If the Jacobs is a proper Computer Ignition, as you describe above, its worth the extra cost (though I'll have to sell one of the kids to slave traders, which I was hoping to avoid till the nitrous needed paying for).

Thanks for any light you can shed.

[Dave-R (Roppa440)]

MSD and Accel both make units very close to the Jacobs unit now. They are all CD types. I have run a Jacobs unit for 10 years and it is still a great ignition even though their new units are much better. The other manufacturers didn't have anything like it at the time. Plus seeing as everyone was automatically using the MSD unit I had to use something else. Everyone else was buying 350 Camaros as well.

Get as much infomation as you can about each of the systems and try tio make an informed choice. At least what i have written here gives you some of the things to look out for.

[Tim Ellison (Tim)]

Thanks for pointing me in the right direction Dave. Getting the benefit of experience is saving me a lot of time and money, where I might have gone for unsuitable/ mismatched/second rate components.

I owe you a very large drink (or three).