Q:  Why is my FC bucking under boost?  What is overboost fuel-cut?  What is a Fuel-Cut Defensor (FCD)?

The FC3S stock ECU is programmed with an overboost fuel-cut that cuts fuel only to the rear rotor.  The overboost fuel-cut will trigger approximately 1psi over the stock boost levels.  The Zenki FC3S turbo stock boost is about 5.5psi; the Kouki FC3S turbo stock boost is about 7.5psi.  The stock overboost fuel-cut is will trigger approximately at 6.5psi and 8.5psi respectively.

An FCD is a popular option to combat this overboost fuel-cut.  There are two types of FCD's, but the most common one is the voltage clamp type.  The voltage clamp type is characterized by a voltage setting.  Any voltage input that is under this setting is allowed to pass through to the output.  If the voltage input is over this setting, the circuit "clamps" the extra voltage and only outputs a voltage signal equal to the maximum voltage setting.  This allows the FC3S boost sensor to work normally under the fuel-cut boost level but prevents the overboost fuel-cut from triggering if you are running more boost than stock.

Now, before you start to worry about inadequate fuel delivery by the limited boost sensor signal, rest assured that the boost sensors does not affect fuel delivery when the engine goes into positive boost.  That's right - most boost sensed by the boost sensor does not cause the stock ECU to add more fuel.  The airflow meter is the primary engine load sensor for the FC3S.  Fuel delivery is mostly dependent on the airflow meter signal.  What the boost sensor does do when the engine sees positive boost is retard the ignition timing.

Now, let's get back to engine damage from the overboost fuel-cut...  Many will argue that if all fuel is totally cut from the engine, then there should be no engine damage.  It does sound logically, but...  We've killed engines banging against the overboost fuel-cut.  What happens is a very fast execution of a damaging cycle that causes a lean condition.  When fuel-cut occurs, the boost drops.  Once the boost drops, the fuel-cut is disabled and fuel turns back on.  Boost starts to climb again, hitting the overboost fuel-cut again.  This cycling induces a lean condition which will kill the engine.  An FCD is highly recommended if you're going to be running higher boost levels.

Questions?  Comments?  Send mail to:  reted@fc3spro.com