Welcome to the
!!! OEM COLD AIR INTAKE !!!
Below is the best possible routing and design for a cold air I take on a VW diesel unless modified some ungodly HP!
Reality is that having a vacuum within the air box for air to expand cools it down. This is accomplished by utilizing a sealed box that you control the dynamics and operation.
Below is the tube that wraps around the battery, it's sized larger than the lower boost pancake tube. (which is a far better place to consider an upgrade to the OEM system with an OEM designed part using the upgrade metal lower boost pancake tube.)
This tube is specific flow rates being considered and on a stock to slightly modified engine is more than adequate.
The tube on the upgraded plenum box (in one of the last pictures below) that points towards the engine is the OEM upgrade for vehicles with greater modifications that might occasionally need to draw a bit more air than the tube around the battery.
Where it comes out in front of the battery and behind the headlight
This is the area beside the battery and to the right is the opening for air to enter.
The same picture as above with the rear cover below installed creating the completed cold air intake.
The inside of the rear cover
This is the bottom of the 1.8T air box to the left you have the added volume air valve tube that points towards the engine above trans.
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This is the CAI installed
The picture below show's the bottom of the original ALH CAI filter plenum which only has one inlet for air.