Why is my Mazda RX-7 FD hitting 12 psi of boost when the Greddy Profec boost controller is set to off?

There are two primary reasons your RX-7 is hitting 12 psi when the Greddy Profec is "off," depending on whether the unit is entirely powered down or if you just have the duty cycle/limiter parameters zeroed out.

1. Running on Mechanical Wastegate Spring Pressure When an electronic boost controller like the Greddy Profec is completely turned off, the control solenoids do not intervene, and the turbo system defaults to running entirely on your mechanical wastegate spring pressure ¹. If your physical wastegate is equipped with a 12 psi spring (or a combination of springs yielding 12 psi), the car will naturally boost to that level and hold it, completely independent of the electronic controller.

You can confirm this by bypassing the boost controller solenoids entirely. Run a vacuum line from the compressor housing to a brass T-fitting, and route it straight to the bottom port of your wastegate(s) ⁸. If the car still holds 12 psi, that is your mechanical spring pressure.

2. Unit is Set to kPa Instead of PSI If the unit is powered on but you have the LIMITER set to 0% (which the manual indicates as "off"), the controller might actually be bouncing off a misconfigured warning limit.

A common mistake with the Profec series is having the display set to kilopascals (kPa) rather than PSI. If your WARNING parameter is set to "180" (with the intention of it being a safe 18.0 psi), the controller will read that as 180 kPa. 180 kPa translates to 12.4 psi. Once the boost reaches this ~12 psi threshold, the warning safety feature triggers, cuts your gain to the 0% limiter setting, and drops the boost back down to wastegate pressure ¹. This often causes a rapid boost flutter right around 12 psi.

Diagnostic Steps (Difficulty: Easy):

• Check the display units: Verify your Greddy Profec is configured to read in PSI and not kPa. If it is in kPa, you must recalculate and adjust your WARNING and SET GAIN values accordingly.
• Verify Spring Pressure: If the unit is entirely powered off (or bypassed) and you still hit 12 psi, you are simply hitting the mechanical limit of your wastegate spring. If 12 psi is higher than your desired baseline, you will need to physically swap the wastegate spring for one with a lower pressure rating.