Mark, here are some photos where the black plastic intake is a 1995 993 and the alum intake is about an 89 or 90 964. In the closer pictures of the 964 you can see the flap butterfly. Unfortunately I tossed this 964 part when I built a simplified replacement which is essentially a couple tubes, throttlebody, and your good to go. If you integrate a "flap" it was nothing other than a butterfly. Though this 964 part was magnesium it weighed quite a bit. Anyhow it's probably not difficult to find a used one.
Here is a description of the flap operation as written by Steve Wong:
Post from Steve Wong 5-25-2007
Checking Resonance Flap for Operation:
When the ignition is switched on, the resonance flap is closed by vacuum action. When the ignition is switched off, the resonance flap is opened (rest position). The opening and closing noise is audible in the engine compartment.
4. Resonance Flap
The DME control unit activates a vacuum-controlled diaphram valve which either opens or closes the resonance flap.
The resonance flap is closed between 3,000 prm and 5,000 rpm and at throttle opening angle of > 60 degrees.
Due to the ignition sequence, the intake system is alternatively supplied by both tanks. Due to the firing order, air is drawn in an alternating manner from both intake system tanks. If resonances occur, the intake frequency of one row of cylinders matches the natural frequency of the pressure vibrations in the respective tank. The natural frequency is determined by the geometry of the intake pipes, the resonance pipe and the tanks. A crucial factor, however, is the total length of the pipe from the actual intake cylinder to the next cylinder being supplied, the distribution in intake and the resonance pipe lengths as well as the depth of the tank in the direction of flow. In the no-current state, the resonance flap is open. As soon as the ignition is switched on, however, it is triggered and closed. If the DME control unit detects that the engine is being started, the resonance flap is opened again.
Pin 1 from the DME unit is used to control the resonance valve.
