On my first E34, a 1994 530 v8, I bought a Dinan "tube" that fit into the stock airbox and was like a stovepipe to the air intake behind the right front headlight. Removed the silencer and installed and even if it did not make any more HP, it sounded great and retained the complete stock airbox and filter.

I reused the Dinan tube on my last E34, 1989 535i by screwing it into the flange that comes out of the front of the airbox when the silencer is removed. Same result. Sold that car with the Dinan mod.

Now with my new to me 1992 535i (5-speed, Granitsilber, BAV Auto Chip), I wanted to get rid of the silencer and I needed a device to attach to the box and get close to the large round hole for the factory cold air intake. A trip the the plumbing department at Lowes filled the bill with a black rubber 3" to 4" plumbing pipe adaptor. They conveniently come with two stainless hose clamps. I removed the 4" clamp as it is not needed. The 3" side just fits the flange from the stock airbox and the approximate 6" length of the adaptor is right in the hole for the cold air behind the headlight. So now I have a $6.82 ram air for my air box. Quite a savings over the $75 I paid for that Dinan tube.