I agree that an inflatable boattail would be safer in an accident, but I am not sure mylar is a good material. I think somethink like tent fabric would be better. When mylar rips/punctures the rip just spreads and would destroy the whole thing. In my mind it doesn't have to be completely air tight.
I see 2 options for inflation:
1) Air ducts from lead from the rear wheel wells to the boattail through a simple check-valve (ping-pong ball in a PVC pipe kind of thing).
2) A "Y" is put into the exhaust, one leads to the boattail through the same type of check-valve, and the other into the atmosphere.
The issue with #1 is that I doubt there will be enough pressure to keep it inflated continuously.
The issue with #2 is that you would have to have either a manual control to determine how much exhaust goes into the boattail vs. how much into the air. Or perhaps air could more directly lead into the boattail via the exhaust, and a smaller dia (1/2-3/4") pipe could be welded to the exhaust and make a 180degree bend. This would provide a straight-shot into the boattail, but still provide an automatic escape route for the exhaust when the pressure in the boattail is creating too much backpressure.
This is the kind of thing that I would LOVE to build, and would, except I dont have a car I could do it with. I have been trying to sell my truck for 6months now to buy a small efficient car, but apparently no one wants it.
