The air vent in the Tiger's firewall was altered when the vehicle's engine was changed from the HL 210 to the HL 230 in May 1943 [3] on hull #250251. The diagram above shows the new design for the air vent.
The HL 230 had a small gearbox at the top rear, to drive its cooling fans. The fans could be decoupled from the engine; this was necessary in the Tiger for underwater travel. To decouple them, you only had to pull a handle at the front of the engine. This handle, indicated by an arrow in the photo [4] above, was connected by a long rod to a lever on the gearbox at the back.
Therefore the crew needed access to this new handle through a door in the firewall. But the location for the door was already occupied by the original air vent.
The designers eliminated two of the four holes in the vent, replacing them with a hinged door. These diagrams show the rear side of the vent mechanism.
The sliding panel was made smaller so that it would not collide with the new door.
This is a German photograph [4] of the new vent in a tank manufactured around June or July 1943. The door is designed to fit between the guide rails for the sliding panel. Notice that the door is held shut by two springs.
This detail from a German drawing [1] shows the "size of the open door" behind the firewall. We are looking to the right. The door is compared to the water pipes (both old and new) that connect to the front of the cylinder head block.
From an early stage it was planned to upgrade old Tigers with the new HL 230 engine. But in these vehicles, the firewall already had 4 holes cut for the old air vent. (There were also unused firewalls in the factory). Therefore, components were designed specifically to upgrade old vents. A surviving German drawing [1] tells us, in part, how the upgrade was done. These instructions are at the side of the drawing:
"c) The old air vent in the firewall is to be removed and replaced by a new vent that has a door. Before installing the new vent, 2 air holes in the firewall are to be combined by cutting out the crossbar. The guides for the vent remain unchanged."
And written on the drawing itself, beside the air vents, is:
"c) cut out the crossbar"
This detail from a German manual [3, see page 47] completes our picture of the upgrade. The old sliding panel was replaced by the new one. The direction of the handle was reversed. Two of the four holes were combined. A box was welded behind them, sealing the opening and forming a recess. A small door in the back of the box gave access to the "cooling gear handle" on the engine.
I have seen only one photo of this kind of vent [2, see page 23] . It shows inside of the box painted a dark colour. I have no images of the door in the box.
[2] Tiger in Action, Squadron Signal # 27
[3] DW to Tiger 1
[4] German report on experimental ventilation gear for Tiger 1, now at Chatellerault archive