Even though the MSD Blaster coil is a huge improvement, I was not satisfied with the loss of ignition energy as RPMs increase. So, after a bit of research, I picked-up another MSD coil to test. This is the most powerful, inductive coil that MDS offers; the HVC-II:
This coil is obviously not the same physical dimensions of the factory coil. However, it utilizes newer coil design, and is not difficult to retrofit for use in the 944 / 951. Testing this coil:
Immediately, we see that this coil charges extremely fast! Even with a little bit of hold-time at the end of the charge cycle, this coil hits peak current in a short 3.3mS! This is an improvement of 1.5mS over the MSD Blaster coil, and nearly half the time of the factory Bosch unit! Furthermore, this coil will not run short on charge time until 7000RPM. This means that there is no ignition energy loss for the entire rev-range of the 944 / 951!
This is an awesome coil!
But, like the problem mentioned earlier with the MSD Blaster coil, this coil charges very fast, and if the ignition dwell map is not adjusted to account for the fast charging, the DME transistor, wires, and coil will suffer. Further, this coil is much more likely to damage the DME transistor or wires than the MSD Blaster coil - the dwell map MUST be changed to account for this coil.
Let me say this again:
DO NOT RUN THIS COIL WITHOUT CHANGING THE IGNITION DWELL MAP TO ACCOUNT FOR THE FAST CHARGE TIME.
Seriously - don't do it, you will kill the DME transistor (just a matter of time).
Now let us finish-up by revisiting the energy equation earlier:
1/2 * Inductance * Current^2
We now know current, but what about the inductance? Like current, measuring coil inductance requires a special tool - lucky that I have such a tool.
Inductance is, essentially, the energy storage potential of the coil. More inductance does mean more potential energy. And though it would be tempting to make a coil with as much inductance as possible, inductance also adds to the overall coil impedance; higher impedance means slower charging. So, it is a bit of a juggling act of coil inductance vs charging rate.
That said, the stock Bosch coil has an inductance of 5.85 mH (milli-Henries). Now we can complete the calculation for peak energy (at peak current):
1/2 * 0.00585 * 9^2 = 237mJ (milli Joules)
This is actually quite a bit of energy! But remember that by 6500RPM, the Bosch coil's current has been significantly reduced:
1/2 * 0.00585 * 5.25^2 = 81mJ
Wow! That is a huge reduction in coil energy from the peak potential...
The MSD Blaster coil has an inductance of 4.36 mH, which is less than the Bosch unit, but since the MSD Blaster charges quite a bit faster, will there be an overall improvement?
First the peak energy (at peak current):
1/2 * 0.00436 * 9^2 = 177mJ
Interesting that the MSD coil actually has less energy at peak current, but lets see it as RPMs increase; here at 6500rpm:
1/2 * 0.00436 * 7.5^2 = 123mJ
Even though this coil doesn't have quite as much peak current energy as stock, it maintains a much higher level of energy as RPMs increase. By 6500rpm, the MSD Blaster coil is 50% more powerful than the stock Bosch unit.
Finally, the MSD HVC-II coil. This coil has an inductance of 6.23, which for how fast it charges, is a lot of potential! So, the coil at peak energy (peak current):
1/2 * 0.00623 * 9^2 = 252mJ*
WOW!!! This is by far the most powerful coil tested. AND remember that this coil does not lose any energy for the entire rev-range. So by comparison, this coil at 6500rpm has twice as much energy as the MSD Blaster coil, and three times as much energy as the factory coil !!!
*The coil was starting to saturate, so it will most likely have a little less energy than the linear calculation predicts*