What causes ignition breakup at higher revolutions per minute under boost in a Mazda RX-7 FD?

High-RPM ignition breakup under boost in the FD RX-7 is a common issue typically caused by the ignition system struggling against high cylinder pressures, degraded electrical components, or overly rich air/fuel mixtures.

Safety Warning: Be highly cautious when experiencing hard breakup under boost. Severe misfires and detonation can physically dent the rotor faces and cause catastrophic engine failure ⁵.

Here are the most common causes for this issue on the FD:

1. Ignition Coils and Wiring Harness (Difficulty: Moderate)

• Degraded Coil Harness: The factory coil harness sits in a very hot location and becomes brittle over time. This can cause the primary coil wires to physically detach from the connector or arc directly to the chassis instead of firing the coil ¹. Leading spark plug wires have also been known to fry completely off the ignition coil ².
• Failing Stock Coils: The stock coils degrade from prolonged heat exposure and can physically break down or turn to "goo" ¹.
• Ignition System Limits: At higher boost levels (over 1.2 bar / 17.4 psi), the stock coils often lack the energy to push spark through dense cylinder pressures, even when paired with a standalone amplifier like the HKS Twin Power. Upgrading to IGN-1A smart coils or a CDI setup is often required for high-boost stability ⁴.

2. Spark Plugs (Difficulty: Easy)

• Improper Heat Range: Standard factory heat range "9" plugs (like the BUR9EQ) retain too much heat at the spark plug boss on modified, boosted rotaries. Upgrading to cooler, race-style plugs such as the NGK R6725-10, R6725-105, or R7420 series resolves high-RPM breakup for many setups ⁵.
• Spark Plug Gap: High boost pressures can "blow out" the spark. Reducing the spark plug gap to roughly 0.024" (0.6mm) focuses the spark energy and makes it easier for the coil to fire ⁴.

3. Fuel System and Tuning (Difficulty: Moderate to Hard)

• Overly Rich Mixtures: Running excessively rich (e.g., Air/Fuel Ratios in the 10.4 range or low 11s) can drown out the spark at high RPM ⁵. Leaning the mixture slightly (closer to 11.0 AFR) often relieves the burden on the ignition system ⁴.
• Failing Fuel Pressure Regulator (FPR): A faulty FPR can restrict or improperly ramp fuel pressure under load, leading to hard stuttering and power loss ⁵.
• ECU Boost Cut Threshold: If the ECU's boost cut setting is mapped too conservatively, reaching higher boost loads in 3rd or 4th gear will trigger a fuel/ignition cut that perfectly mimics heavy misfiring ⁵.
• Low Octane Fuel: Running local pump gas on an imported ECU mapped for higher-octane fuel (like Japanese 100+ octane) can cause dangerous detonation and intermittent high-RPM misfires ².

4. Auxiliary Injection (If applicable)

• Excessive Water Injection: If you are running water/methanol injection, spraying large volumes of water (400cc/min or more) can completely overwhelm a stock ignition system at higher boost. Reducing the water volume or heavily upgrading the coils is necessary to prevent misfires under these conditions ⁴.