Found this info from someone on Audizine called "Old Guy":
"There is a fluid chamber in the mount with a plastic body separating the upper and lower section. There is a small port in the plastic separator. This allows fluid from the top chamber to pass through the plastic body and into the bottom chamber. The bottom chamber is rubber lined and can deform under pressure. There is also an airway vented to atmosphere with a rubber membrane separating the fluid from the atmosphere. The solenoid
valve opens or closes the airway to control vibrations from the motor. Below 1100 rpm the engine vibrations are not enough to force fluid from the top chamber to the bottom chamber so the airway (solenoid
valve) is open. The rubber membrane creates a soft cushion
which absorbs the high frequency vibrations. Above 1100 rpm the solenoid valve is closed which blocks the airway and consequently "hardens" the mount (the fluid has nowhere to go). thus the mount is effectively solid."
Judging by this it appears the b5 RS4 had solid mounts not offering electronic solenoids as they did not have a solenoid valve and will not be adaptive remaining stiff at all times... hmm can't tell if this is a good or bad thing, surely they could cause additional shaking in the cabin but would last the life of the car. If anyone has used them please share experiences (including Google44
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