As luck would have it, I found this disertation I posted here a while back.
I hope it helps
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To understand how aftermarket ignitions work, you need to understand how a stock-type distributor and coil ignition works. (point or electronic distributor) We will start with the ignition coil. The ignition coil has primary and secondary windings. (There are many more secondary windings than primary) When the switch inside your distributor is closed, the points, for instance, ~12 volts is sent to the primary side of the coil by your distributor. (This voltage is from your battery/alternator) The voltage passing thru the primary windings slowly generates a magnetic field. (This same magnetic field passes thru the secondary windings) When the 12 volts is turned off by the distributor, the magnetic field collapses near the speed of light, and when the collapsing magnetic field passes thru the secondary windings, the voltage is increased because there are more windings on the secondary side. The stepped-up voltage from the secondary windings is sent to your spark plugs via the cap/rotor.
As engine rpm's increase, the amount of time for the magnetic field to build-up decreases. This means less energy is sent to the spark plugs to ignite your air/fuel mixture. Factory ignition systems are designed for specific operational parameters. Altering the ignition timing and/or increasing the cylinder pressure and rpm's above those parameters make it more difficult for the ignition to ignite the air/fuel mixture.
If your factory type ignition system is able to properly burn your cylinders air/fuel mixture across your engine's entire operating RPM's, then you may not see a performance increase by adding an ignition control box like a MSD.
So, why by one?
To answer that, you need to know how the capacitive discharge ignition control boxes like the MSD work. The distributor no longer sends a signal to the primary side of the coil. Instead, it sends a signal to the MSD to fire its capacitors. These capacitors when they 'dump' their voltage, send app. 400 volts to the primary side of the coil. (instead of the 12 volts as before) The coil now steps up the voltage from a expanding magnetic field rather than a collapsing one as in a standard (non-MSD) ignition. The increase in voltage to the primary side of the coil result is more energy at the spark plugs to potentially ignite the air/fuel mixture. This system of using a capacitor and a expanding magnetic field means the amount of energy produced at the spark plug will NOT decrease as RPM's increase.(to a point)
What about the multi-spark feature?
Capacitive discharge based ignition systems can charge and discharge very rapidly. As RPM's increase, there is less time to charge and discharge the capacitors. This means MSD ignitions do stop multi-firing above a certain number RPM's. This is not important on a race car.
Why?
When the spark jumps the plug gap, it ignites the air/fuel mixture. The flame front begins to travel away from the point of ignition. Another spark firing after the first one will do little to nothing. The flame front has already moved away from that area of the combustion chamber. There is little/no air/fuel left to burn in that area.
Also, as RPM's increase, the piston is near TDC for less and less time. This means that if the first spark did not ignite the mixture, a second one won't do you any good because the piston is already on its way back down the bore.
What else should I know?
1) As RPM's increase, all Capacitive discharge boxes like the MSD increase their current draw. Sometimes, it can be a substantial amount at 8000 plus RPM's.
2) The more 'off' your air/fuel mixture is, (too rich or too lean) the bigger increase in performance you will see from a ignition box. Cars that have a properly dialed in fuel system will see less increase. Late model, computerized, fuel injected cars in particular.
3) Different boxes have different uses. Just because a MSD 7 box puts out more energy to the spark plug that the MSD 6 does not mean it's the best box for your ITC.
4) In most cases, you can use a stock coil with a ignition box. Coils designed for CD systems do work better, but if you need to bypass the box and go back to stock ignition because the box fails for some reason. A stock ignition cannot fire a coil designed for a CD system.
5) If your car has an magnetic pick-up type of ignition system (most do), you can by-pass the factory ignition module and wire the CD box directly to the magnetic pickup.
This eliminates a weak point in some vehicles were the factory module is known to fail.
6) If you are worried about a ignition box failing, you can set up your car just like NASCAR does, Two boxes and coils, with a switch to go from one box to the other. They also use 'potted' boxes that are vibration resistant.
7) Coil mounting is important. Oil-filled coils can only be mounted in certain positions. Incorrect mounting can cause the coil to overheat and fail. (MSD offers canister type epoxy-filled ignition coils for CD systems that can be mounted on their side)
8) You can and should mount the box inside the passenger compartment. This removes it from a potential failure source, heat.
9) Many factory tachometers will not work even when connected to the 'tach out' on a CD box. MSD, Malloy, and others make a fuel injection/tach adapter that allows use of factory electronics. It is best to use a high-quality aftermarket tachometer, such as Autometer SportComp or ProComp. 'Cheap' tachs will not work in most cases. You can destroy a factory tachometer left hooked to the negative side of the coil after you install your CD system.
To Summarize:
With a aftermarket CD system like the MSD, you will get:
Increased spark energy that does not taper off throughout the entire RPM range.
Increased reliability by installing the box away from heat sources.
Yes, I run a MSD 6 and epoxy filled coil on my 83 RX7, (leading ignition side, but that's another post)
Email me if you have any other questions.
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Rodney Williamson
#93 IT7
www.titaniummotorsports.com
