[wagonman76]

Regardless of heat or poisons, I would think the restriction placed on the exhaust would more than kill any effeciency gained by the kammback. The bleed off would have to be a pressure higher than required to keep the kammback stiffly inflated.

But they make 12v high volume inflators for air mattresses and things like that. Just fill it when you need it before driving, maybe even incorporate a pressure switch that kicks the inflator on and off if needed.

Back when I was a kid, I remember this "as seen on tv" product that was for jacking up your car in a pinch. It was a big balloon that attached to your exhaust pipe. Didn't see it advertised for too long and never heard about it again, probably for obvious safety reasons.

If vehicle mods are basically banned, my first thought is to make it something that you could consider a piece of cargo, rather than something attached to the vehicle. Maybe make something that fits securely in your trunk/hatch with the lid bungeed down to an appropriate angle, that sticks out and forms an envelope that happens to be shaped like an appropriate boattail. If you could make it foldable or inflatable it would be even better since if you are deemed a safety hazard by some bad officer, you could just collapse it and take it with you.