[WD40]

I experimented with this idea and it failed miserably, to the point it was bringing exhaust fumes into the car.
The biggest reason for the experiment is that the civic hatchback has very inadequate ventilation to begin with.
My plan, use the same vent idea that cars already use, the flapper that opens when you shut your car door or trunk.
So I cut a suitable sized hole in the tailgate of my civic hatchback behind the license plate.
I then installed a flapper from an old volvo approx 5" x 7".
The plan was to vent any excess pressure from inside the car and hopefully fill the low pressure air vacuum behind it.
Well for some reason it drew the exhaust into the car.
Then I had the bright idea to install a 120mm fan to force the air out, end result was the vacuum was to hard to over power.
So now I have gorilla tape blocking the vent inside and out.

I think the idea to fill the low pressure air vacuum behind the car would be better implemented via duct work from under the front of the car and exited in the void area, sort of like and exhaust system.
If I were to experiment with that plan I think I would start the duct work from the grill as its a great catchment area.
If it was done right the duct work could double as part of a belly pan.
Seems to me someone already implemented something similar to that on a VX, but I would exit it up higher in the void.