Your have the same engine as my '92 400E. On my car there is a clip (missing of course) on the intake manifold side, towards the front of the engine that holds down the front of the airboxes. Release the front clip and the rear part should slide out forward, if it's like mine. Could be they changed it for your car.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)