The automotive ignition system is commonly one of the least understood parts of the powertrain. Unlike mechanical operation, it can be difficult to visualize exactly what is required to successfully fire the spark-plugs.
To make things more difficult, doing any quantitative testing of the ignition system requires specialized tools that very few people posses.
That said, I have been doing a lot of ignition system, and ignition coil testing lately, and (as always) I would like to share what has been learned.
To get started, let's go over the ignition system.
The ignition coil:
This is the Bosch "black coil", which is the OEM unit for both the 944 and 951. The purpose of the ignition coil is two-fold. Firstly, the coil has a primary and secondary windings. By having many more turns on the secondary winding than the primary, the coil acts as a "step-up" transformer. This steps-up the voltage to many thousands of volts - enough to fire the spark-plug. The secondary purpose of the coil is to store the energy needed to actually fire the spark-plug, and to keep the spark existing for a short time (typically less than 1/1000th of a second).
In the 944 & 951 (and nearly any electronically-controlled ignition), the coil is supplied with a constant +12 volts whenever the ignition is turned on. The DME controls the "ground-path", by a large transistor (the silver colored can-style transistor in the DME). To operate the ignition coil, it first must be charged, which the DME does by turning on the transistor, providing a ground for the coil - completing the circuit. Then, once the coil is charged, the DME turns-off the transistor, which opens the circuit causing the coil to fire. (yes there are more details, but they are not necessary for this thread - feel free to search/learn more about coils/inductors)
This digital (the transistor) control gives the DME very precise control of when the actual ignition event (spark) happens.
When the DME turns on the transistor controlling the ignition coil, current (amps) starts moving through the coil. Due to the property of coils, they resist changes in current flow (amps). So, initially when the transistor is first turned on, there is very little current flow, but as time increases, the current flow rises. Generally as long as the coil is not saturated, this current flow rise is a linear relationship with time (and voltage). Measuring the current rise is not an easy thing to do (especially without altering the charging circuit). However, it can be done, and here is a graph of the stock ignition coil being charged:
The cyan/blue trace (CH2) is the digital logic which turns on the transistor for the ignition coil. The important data is how long the logic pulse is "low", which is the length of time that the DME is charging the ignition coil for. Here each horizontal division (block) is 1mS (milli-seconds), the DME is charging the coil for 9.78mS. The yellow trace (CH1) is the coil current. Each vertical division is roughly 1.5amps worth of current.
So, as you can see, when the DME first turns on the transistor to charge the coil, there is very little current flow. But, as time increases, the current through the ignition coil starts to rise. After a few milli-seconds, the current reaches a peak and will not rise anymore - about 9 amps in this case. This is actually due to the internal current-limiting circuitry inside the DME. (more on this in a bit)
As one might imagine, all things remaining equal, more current means more ignition energy. So, it is ideal that the coil receives as much current as possible, to result in as powerful of a spark as possible. However, lots of current through a transistor, wires and coil can heat-up and generally be hard on the components. Look again at the graph, peak current is reached well in advance of the total charging-time. In-fact, peak current is reached nearly 3mS before the end of the charge-cycle. During these final 3mS, the coil is not gaining any energy, rather it is needlessly conducting current, which can heat-up the coil, wires, and DME transistor... So, what should we do? This:
Now the coil is still receiving the same peak current flow (9amps), but it spends no time holding at this peak current. If you inspect the blue trace data we see that the charge time is now 6.46mS (down 3.1mS from the previous graph). This is an ideal charging setup - we hit the maximum energy possible, and minimize the impact to the coil, wires, and DME transistor.
Now that we understand basic coil charging, and understand that it takes roughly 6.5mS to fully charge the factory ignition coil, the next thing to think about is how much time is actually available to charge the ignition coil. In the 944, one ignition coil must fire all 4-cylinders, which means there needs to be two ignition events per revolution. Therefore, as RPMs go up, the available time to actually charge the coils becomes shorter.
Here is a graph of the possible coil charge time, with actual ignition event time account for:
What should be peculiar is the fact that the possible charge time after roughly 4000rpm is less than the time needed in the earlier graph to reach peak current. Well, because there is less time available to charge the coil, the DME must reduce the amount of time it charges the coil for. By 6500rpm there is only ~3.6mS of time available to charge the coil:
A quick count of the divisions shows that at this shorter charge time, the coil is only reaching ~5.25 amps of current, which is nearly 4 amps less than the possible peak! Furthermore, coils store energy according to the calculation:
1/2 * Inductance * Current^2
Yes, that is current-squared. So, the amount of current is very important for the final coil energy value. Given our current data, we can see that at 6500rpm, our coil is down to ~34% of its peak possible value! Is it any wonder why we must run tight spark-plug gaps in order to prevent mis-fires.
So, what is the solution? Lets try a different ignition coil. The next coil I tested is the MSD Blaster coil:
This coil has nearly identical physical dimensions to the factory Bosch coil, which makes it an easy swap. Testing the blaster coil, we see this graph:
For this coil, we see that it takes 4.8mS to reach a peak current of ~9amps. This is a significantly shorter amount of time than the stock Bosch coils time of 6.5mS. So by using this coil it has time to fully charge until ~5200rpm - which is a significant improvement over the Bosch coils 4000RPM. Even though this coil charges faster than the Bosch unit, by 6500rpm it is also down on potential energy:
Since this coil charges faster, even at the short charge-time of 6500rpm, it has more current flow than the Bosch coil. We see it has ~7.5amps of current, which means this coil is only down to ~70% of its peak possible energy - a HUGE improvement over the factory Bosch coil (which was down to ~34% energy at the same RPM).
There must be a catch, right? Well, yes there is. Because the MSD coil charges faster, that means it needs less charge time to hit peak current. Remember the first graph, where the coil hit peak charge, and then maintained it for a significant amount of time? Well, that is exactly what will happen if we install the MSD coil without changing the charge time (known as dwell). The DME transistor, wires, and coil are fairly robust. So one could install the MSD coil without any other changes, and benefit from improved ignition energy, but this is needlessly hard on these components. What needs to be done is to change the charge-time data in the DME software (easy for us to do, and is user-available in the DME Tuner software).
(continued...)