Project FD RX7 Restomod: Part 34 – MSD PRO 600 CDI Ignition

The MSD PRO 600 Capacitive Discharge Ignition (CDI) system was developed to meet the extreme demands of top-tier “Pro Street” cars that compete in “Drag-and-Drive” events such as Hot Rod Drag Week and Sick Week.  Many of these V8-powered cars produce well over 4,000 horsepower yet must still be street-driven for hundreds of miles, requiring reduced spark energy at low load to prevent the spark plugs from melting. To accommodate these unique requirements, the PRO 600 CDI features switchable low and high-energy modes, with adjustable output ranges of 250–400 mJ in low-power mode and 350–680 mJ in high-power mode.

Unlike many CDI ignition systems that rely on fixed low and high-power output settings, the PRO 600 CDI offers a wide range of adjustable spark energy levels. This flexibility allows us to accurately determine the actual spark energy required for any given power level and RPM range in a rotary engine.

Unfortunately, the original designer of the PRO 600 CDI no longer works at MSD, and there is very little available information on the system’s internal architecture or definitive evidence that it could reliably fire two coils with the extremely short ignition split required in a rotary application. We chose to take the risk with the PRO 600 due to its wide spark-energy adjustability and its significantly smaller, more packaging-friendly design compared to running dual MSD DIS-4 Plus High Output CDI boxes, which has traditionally been Abel’s preferred ignition solution for his race engines.  (Spoiler: the MSD PRO 600 does indeed work in rotary applications).

SPARK ENERGY

Spark energy is a bit misleading because different brands quote or market either primary or secondary spark energy.  The highest output of the MSD PRO 600 CDI is 680mJ of primary spark energy, while IGN-1As are quoted at 103mJ of secondary spark energy.

Primary spark energy is the low(er)-voltage energy stored in the capacitor inside the CDI box that is discharged into the primary winding of the ignition coils.  This is how much magnetic energy is available to be converted into high-voltage energy inside the coil.

Secondary spark energy is what really matters because it is the high-voltage energy that is transferred into and discharged from the secondary winding of the ignition coil that jumps the gap in the spark plug to ignite the air and fuel in the combustion chamber.  In a CDI system, the secondary spark energy will vary depending on the coil packs used.  We converted and standardized various ignition system options for the rotary engine into secondary spark energy outputs:

IGN-1A – 103mJ

M&W Pro-14R – 115mJ

M&W ProDrag4R (low) – 160mJ

M&W ProDrag4R (high) – 250mJ

MSD DIS-4 High-Output – 165-175mJ

MSD PRO 600 with 8232 coils – low power (low) 250mJ = 81mJ

MSD PRO 600 with 8232 coils – low power (default) 325mJ = 106mJ

MSD PRO 600 with 8232 coils – low power (high) 400mJ = 130mJ

MSD PRO 600 with 8232 coils – high power (low) – 350mJ = 114mJ

MSD PRO 600 with 8232 coils – high power (med – 600mJ = 195mJ

MSD PRO 600 with 8232 coils – high power (high) – 680mJ = 221mJ

FT Spark-4 Gen II with 8232 coils – nominal – 400mJ = 130mJ

FT Spark-4 Gen II with 8232 coils – high – 750mJ = 243mJ

FT Spark-4 Gen II with 8232 coils – max – 850mJ = 276mJ

The clearest takeaway from our discussions with MSD about the PRO 600’s power settings is that the “nominal” (low-power) mode offers limited real-world usefulness, as it does not provide sufficient voltage for consistent combustion ignition. Even at the maximum 400 mJ setting, low-power mode is reportedly weaker than the 350 mJ high-power setting.

As a result, we will operate the car in the default normal-power mode at 325 mJ, which theoretically corresponds to approximately 106 mJ of secondary spark energy, though its ability to consistently support that output is questionable. From there, we can switch to three higher-energy settings—350, 600, and 680 mJ – delivering approximately 114 mJ, 195 mJ, and 221 mJ of secondary spark energy, respectively.

We paired the MSD PRO 600 CDI with the MSD 8232 CDI Ignition Coil due to having a similar form factor to the IGN-1A coils, and for having the longest spark duration of 380 microseconds, with the lowest secondary current of 1.3A, and 52KV (52,000 volts) of maximum secondary voltage with 50pF load.