How much power loss can bad spark plugs cause on a Mazda RX-7 FD?

Failing or fouled plugs will cause substantial performance drops and severe drivability issues on the FD RX-7.

Depending on the condition of the plugs, you can expect the following symptoms of power loss:

• Perceived Loss of Boost: Because the rotary engine is exceptionally smooth, intermittent misfires from fouled plugs or high-resistance plug wires are not always noticeable as a traditional engine shudder. Instead, the engine may bog down under load—often around 3,000 to 4,000 RPM—and simply feel like a loss of turbo boost ¹¹.
• Inability to Rev or Produce Power: Heavily carbon-fouled plugs can cause the engine to sound extremely rough, similar to a lawnmower. In this state, the vehicle may completely refuse to rev freely, maintain a smooth idle, or develop any usable power ¹².

Important Note on Heat Ranges: If your engine is modified and you install plugs with the incorrect heat range, you might not feel a noticeable drop in power right away. However, running the wrong heat range on a highly tuned rotary can quickly lead to catastrophic engine failure—such as blowing an apex seal and damaging the turbo—without any prior warning in power reduction ².

Task Difficulty: Checking and replacing spark plugs is generally an Easy to Moderate task, though accessing the lower plugs can be slightly tight depending on the factory emissions equipment and intake layout still present on your FD. If you are experiencing hesitation or power loss under load, inspecting the plugs and testing the resistance of your plug wires is an excellent starting point.