Bellypan and Ventilation Idea
 

What about constructing a duct to direct all the warm air out under the rear bumper?

You have a bellypan that extends back and it is all sealed up except for a 4' by 3" metal duct that extends the length of the entire car with the muffler cat etc all contained inside. The fan could be moved to the rear of the engine compartment to blow into the duct. The cat and engine would warm up quick and the underside of the car would be smoooooth and quiet.

That oughta work but heat build up would be a worry, esp. in summer. Heat wrap around the exhaust plumbing (as commonly sold in speed shops) would help insulate and quieten the exhaust. Keeping the plumbing hot would also prolong its life, since less water would condense inside, leading to less corrosion.

Actually, something as simple as aluminum rain gutter material, which comes on a roll and is shaped to a given length, may work well for this application.

Thermocouples could monitor internal heat, for cheap.

You may even get a tiny bit of thrust out of such system.

I'd want to arrange constant airflow if I was boxing in the exhaust. It is somewhat theoretically possible to design it to produce thrust, retroengineering it into the vehicle would be something of a challenge.... but if you're gonna do it from the ground up, wrap a 10" concrete form with carbon fibre for your backbone chassis....

Exactly, it traps much more moisture on the outside than a regular pipe.

A friend once gave me a piece of advice for exhaust heat wrap. First, coat the exhaust pipe with a thick coat of high temp exhaust paint (sold in speed shops as well) While that is still wet, wrap the exhaust. Once the paint is dry, give another coat of exhaust paint. Now your insulation is sealed in and won't soak up moisture to hold against the pipe.

My preference would be to duct the air from the radiator out the hood and seal off the rest of the engine bay. Most cars have a low pressure area right behind the leading edge of the hood anyways. Google pics of the ford GT-40 for a good example of this ducting.

I wouldn't be boxing in the exhaust. The exhaust pipe would run out the back of the car as before. It would be inside the duct though.

After looking again under my CRX I see that it is dished out in the middle so it would not have to go down 3 inches. I think a 1" sliding door type alluminum D shaped channel right inside where the jacking points all the way forward to the front wheels would work and only lower the lowest point of the frame about 1/2". Then a about 3 foot wide aluminum sheet could be slid in from the rear. The Skinnier section between the front wheels would side in on top of the front of the rear sheet. It would be slid in from the front into the same D channel material but only about 2 1/2 feet wide

I would not want to wrap the exhaust. The whole point is that the air gets heated more and more as it heads to the rear of the car to create a convection type air flow like opening the front door on a wood burning stove. That way I would be creating a little passive boost to the air flow on the way through hopefully negating the drag of the radiator

Exhaust wrap insulates the plumbing and keeps the heat in. Racers use this because hotter exhaust retains more energy and reportedly scavenges the cylinders better, for more power per gallon of fuel. It also reduces underhood temps, keeping intake manifold cooler for cooler and therefore denser intake air, i.e., more power there, too. A cooler engine bay is less taxing on the engine cooling system, so the water pump works less, with subsequently less fuel wasted pushing coolant through the radiator.

Fortunately, my cars have stainless steel exhaust plumbing, so corrosion from exhaust wrap retaining moisture would not be a problem. Besides, not that much external moisture would be in contact with the exhaust system if an undertray were installed on the car.

And, since most exhaust system corrosion is from internal corrosion from condensed moisture and acids, and since exhaust wrap keeps it hotter and therefore less prone to condensation on internal walls, I suspect the exhaust-wrap-causes-corrosion argument is suspect or bogus.

Finally, the exhaust pipes may be bell-like and vibrate, causing unwanted noise. Exhaust wrap would dampen such vibrations, attenuating noise.

I've heard plenty of real-world stories about "wrap rotted my pipes" that it's a valid arguement. unpainted mild steel pipe with wrap on it will rot faster and from the outside due to the wrap retaining moisture. solve this as mentioned: paint well to protect it and the bellypan will keep most moisture away altogether.

I was kicking around the idea of putting a vent pipe around my exhaust. The ones with the split down the side should simple to get on. But I wanted the scavange the exhaust heat for the WAI.

The water pump is engine driven and is turning all of the time no matter what. If anything, a hotter engine would result in less water pump drag because the thermostat would be open, resulting in less resistance to flow and less power being drawn by the pump.

Cooler underhood temps will help anything rubber or plastic live longer though.