Aftermarket Ignitions: Additional Info
I would like to contribute some extra information to the following article:
FIX FOR AFTERMARKET IGNITION AMPLIFIER WITH K-JET
by Jon Glommen
Jon has hit upon an important fact to consider when installing aftermarket ignitions into fuel-injected (mostly foreign) cars. Many on-board computers of foreign design (Bosch, Mazda, etc.) require a special signal from the coil (-) terminal to notify the computer that the ignition is operating. It is easy to confirm that the signal is more than just 12 volts to Zero volts, with a scope. When the stock ignition amp grounds the coil (-) you DO have 0 volts, but when it releases the ground, (plug fires), there is an inductive kick from the coil that produces a quick peak of 150 volts or more at the coil (-) terminal. The large peak voltage is what the on-board computer looks for to confirm that the coil is working. In many cases, your stock tach is going to be looking for that same voltage spike as an input signal.
When adding a performance ignition, you will usually be delivering 500 volts or so to fire the aftermarket coil, so no other connections to the coil primary are allowed! MSD manufactures a part called a "tach adaptor" to allow these systems to work properly. The "tach adaptor" is really a dummy coil placed at the output of the stock ignition amp where your original coil used to be. These coils run hot because they have a fairly high current running through them. The dummy coil supplies the original inductive kick to the on-board computer and stock tach. Jon has pointed out that using a real spark coil - you can use your original stock coil! - might be more reliable because of the high heat generated during normal operation. However be aware that you are also generating 20 - 30 KV at the coil output and it has no place to go. Usually, the high voltage DOES eventually find a place to go, and it will start burning a path along the outside of the coil to ground.
Now, here is an even better idea: Jacobs Electronics performance ignitions are available as a "system" that contains computer ignition, performance coil matched to the electronics, AND a method of making your on-board computer happy by using your stock coil to fire the aftermarket ignition. Jacobs supplies a "load resistor" in the ignition kit that provides a safe place for your stock coil's spark to go. Essentially, the stock coil continues to function as usual, but it is not called upon to do as much work as before. The coil "load resistor" is tapped to provide a trigger signal to the Jacobs. Another advantage to the Jacobs is that the original ignition is available for use at any time, should the performance ignition fail for some reason. (Just keep the original coil wire in your glove box.) As of Jan. '05, the Jacobs CD ignitions have been redesigned to accept a more or less standard trigger system that resembles what a set of breaker points would produce. The same type of trigger is found on MSD and Crane CD ignitions. A hard ground (100 mA) for the dwell period, then an "open" connection (open points, ignition transistor "off", etc) to fire the ignition. The new universal load resistor device has 4 wires as opposed to the earlier device which had 2 wires.
So, there are two separate issues here really. The first is making the on-board computer happy and getting the ignition system working. The second is how to get your tach operating. The tach output of the MSD is a 12 volt pulse. This signal will run most aftermarket tachs, but will not trigger a tach that is looking for the 50 volt pulse from the coil. So, you can use an MSD "tach adaptor" to solve both problems, or give yourself a break and use a Jacobs kit. For almost any Volvo I recommend the new Mileage Master System available from Jeg's. (This is a multi-spark inductive unit.) If you have a really hot racing engine that runs up higher than 6000 rpm, go for the Pro-Street system. (The Pro Street is a capacitative discharge multi-spark unit.) One final important note: nearly any performance ignition will REQUIRE that you install a set of plug wires that will handle the upgraded ignition output: I recommend Magnecor 8.5mm helical core wires available from IPD.
Note on spark plug wires: Claims of low d.c. resistance are meaningless to a high voltage spark pulse. The Magnecor people explain some of these myths on their informative web site,
http://www.magnecor.com
I have been running a Jacobs "Pro Street" system in my 1994 940 for the last 5 years. The advantages are obvious right away. The car runs smoother, starts in a flash, and perks up in accelleration. I'm also getting about 10% better mileage all around, but that was not my first priority.
Keep in mind that most performance ignitions, while they provide a hotter spark, will require new plugs every few months. No sweat, because they can be the inexpensive copper electrode kind. For the B230F engine with 9.8/1 compression I've found that the plugs can be gapped to about .050". Lower compression engines that run on regular gas will experience even better mileage gains and you can open the plugs even more - say .055".
A note about plug gaps and ignition power: As a general rule, combustion becomes more difficult as compression goes up. Ignition also becomes more difficult as the mixture becomes leaner - that is, closer to the chemically correct 14.7:1 mixture. Thus the general rule of thumb is that with high compression and/or boost, you must decrease spark gaps. This is true if you are working with a limited ignition. Opening the plug gap will in almost every case result in improved chances of ignition as well as more complete combustion. However to run wide gaps under high compression and lean mixture conditions, you will need a higher voltage ignition (i.e. capacitative discharge) and most likely a multi-spark ignition to insure that the burning continues. Note also that you will need to select the right coil for your intended RPM range and ignition type (CD or inductive). Info on coils is tough to find, and even more difficult to decipher.
The Bendix variant system (actually 2 separate computers, one for fuel and one for spark), does not require the coil (-) pulse to run the injectors and the tachometer as the Bosch system does. But, that still leaves the requirement of a trigger signal for the aftermarket ignition. The Bendix has a crank trigger - but the output from this sensor must run through the car's Bendix REX computer to keep everything running. So, I have designed a (relatively) simple electronic circuit to turn the stock Bendix computer output of 3 volts into the full 12 volt trigger signal required by MSD and other aftermarket ignitions; this new circuit will get a "smoke" test as soon as the weather warms up a little here in Seattle. Anyone interested in electronic circuit building can email me for details. The circuit uses a single IC - ST Microdevices TD310 - is ideal to trigger the aftermarket Ignitions and eliminate the stock amplifer module and stock coil altogether.
In conclusion, you'll save yourself some wiring and interface work by using a Jacobs universal trigger system unless you have a vintage car that uses points or magnetic distributor pickup. Points or magnetic pickups will connect directly to MSD, Crane, or Jacobs. If the wiring task does not bother you, add the interface. Either way, an improved ignition IS worth the effort! Good Luck!
-Lazarus
