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

Reliable spark energy is critical for rotary engines operating above 8,000 RPM, especially during sustained use in high-powered cars. We are using a programmable MSD PRO 600 CDI ignition system to evaluate how much spark energy a rotary engine actually needs across varying power levels and operating conditions.

Rather than relying on the well-established IGN-1A Inductive Smart Coils, which remain the most common, well-supported, and straightforward ignition solution for rotary engines; we chose a more challenging path by being the first to run a rotary engine using the insane MSD PRO 600 CDI that was originally designed to support over 4,000hp Pro Mod and Pro Street V8 platforms with over 680 millijoules of primary spark energy.

Originally, we were going to use the tried-and-true IGN-1A coil pack kit.  This easy plug and play solution receives the ignition timing inputs from the ECU through the factory spark ignition module (ignitor) connector while powering the coils directly from a dedicated power cable attached to the battery.

The factory ignitor is located near the firewall on the left, driver’s side of left-hand-drive cars.  For the IGN-1A installation, the ignitor is removed and the new coil pack harness plugs into the connector.

The IGN-1A coil kit is nicely packaged on a bracket that holds all 4 coils, 2 for the primary spark plugs and 2 for the secondary spark plugs.

This bracket mounts the coils cleanly next to the brake booster on LHD cars, replacing the factory ignitor.

IGN-1A coil packs are more than sufficient for nearly all rotary applications, and only a small percentage of engines ever approach their performance limits. However, our engine builder Abel Ibarra built us an exotic semi-peripheral and bridge-ported engine with a 9,000–10,000 rpm redline and future power outputs exceeding 600whp with a larger turbo. Based on his experience building numerous engines at this level, Abel recommended a capacitive discharge ignition (CDI) system, a suggestion that aligns with feedback from many rotary experts when pushing a 13B into these upper extremes of rpm and power.

IGN-1A INDUCTIVE SMART COIL LIMITATIONS:

IGN-1A coils are notoriously sensitive to heat and are often pushed close to their thermal limits in rotary engines simply from generating discharge energy, this is before even factoring in engine bay heat soak. For most rotary owners, this isn’t a real-world issue since they are rarely at full throttle for more than 10–15 seconds at a time, whether on the street, dyno, or drag strip.

Those limitations begin to surface when the engine is pushed beyond 8,000 rpm, and especially during sustained full-throttle operation. This includes standing-mile events with 30+ seconds of continuous load, or extended 20–30 minute road course sessions where the car may spend well over 60 seconds per lap at full throttle.

CDI ignition systems are far more tolerant of heat, sustained use, and short dwell times, maintaining consistent spark energy long after inductive smart coils begin to fall off. Unlike IGN-1As, which can lose 15–20% or more of their effective spark energy as heat builds and dwell time shrinks at higher RPM, a CDI system doesn’t rely on dwell in the same way

Rotary engines are especially demanding, firing the ignition system twice as often as a piston engine, effectively cutting available dwell time in half as RPM increases. While an IGN-1A may be rated at 103 mJ, real-world sustained output can be closer to 90 mJ or less, whereas a 108 mJ CDI system typically delivers that energy consistently across a wide range of conditions.

IGN-1As work extremely well in piston engines and big-power V8s, but once a rotary is pushed beyond ~600 whp and 8,000 rpm for more than short bursts, the shortened dwell window becomes a real limitation. That’s where CDI systems stop being optional and start becoming mandatory for high-RPM, sustained-load rotary applications.