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.
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