There are two ways to go to simply disable the flapper valve.
1) Disconnect the vacuum hose to the flapper actuator, either at the flapper actuator itself, or where the vacuum line originates which is on a hose fitting located on the front throttle body.
Be sure if any vacuum line is disconnected, to plug that line or hose fitting.
2) Simply disconnect the flapper actuator solenoid connector attached to the left radiator on SP2.
The engine merely supplies the vacuum through a one-way check valve and held in a vacuum storage tank. The solenoid is activated by the ECU, which opens allowing the flapper actuator to be actuated.