What is the recommended water/methanol injection mix for a Mazda RX-7 FC?

The recommended water/methanol mix for your FC RX-7 depends entirely on whether you plan to retune the ECU for it or if you are just bolting the system on as a safety net against detonation.

Here is how you should approach the mixture:

1. Untuned (Safety Net): 100% Distilled Water If you want to keep your current fuel maps exactly as they are, run 100% distilled water. It provides excellent knock suppression and intake charge cooling without altering your air-fuel ratio ⁵. If you add methanol to an untuned setup, the methanol acts as a fuel and will richen your mixture, which can actually cause you to lose horsepower ⁵.

2. Tuned (Power & Cooling): 50/50 Water/Methanol A 50/50 mix of distilled water and pure methanol is the standard setup if you want a balance of intake cooling and the octane-boosting properties of methanol ³. Because methanol is a fuel, you must tune the ECU to pull base fuel out of your maps to compensate ². Some owners run 100% methanol for massive power gains, but a 50/50 mix is generally preferred for its combined cooling and knock-deterrent benefits ⁴.

3. Running E85? If your FC is fueled by E85, skip the water entirely. The consensus is that water injection combined with E85 usually causes a loss in power unless you are pushing extreme boost levels (over 35 psi) ⁴.

Safety and Setup Notes:

• Fluid Purity: Always use pure methanol and distilled water. Avoid off-the-shelf windshield washer fluid, as the additives and dyes can clog your injection nozzles over time ⁵.
• Failsafes: If you tune your engine for a 50/50 mix or straight methanol, your rotary is now dependent on that system. If the pump fails or a line clogs under heavy boost, the engine will instantly run lean and detonate. You need to use quality components and ensure you have a reliable failsafe wired in to cut boost or pull timing if fluid flow drops ³.