CRX wheel skirts and belly pan.
 

I want to begin doing some aero mods to my CRX. I really want to try some wheel skirts on the rear however I have read a few places that this can actually hurt mileage on the CRX. The possible explanation was that additional air gets trapped in the wheel wells and adds to drag. I am planning on building a rear belly pan that would streamline the huge parachute area that the rear bumper creates. I am also wondering if I could somehow enclose around the bottom of the wheel wells in addition to the skirts and prevent air from traveling up into that cavity. Anyone know of any potential drawbacks or problems I may encounter from attempting this?

Crx
 

I have the '84 CRX.I did full rear skirts and suffered no loss.
In order for the 'diffuser' section to work you may need to clean up the underside from the rear axle all the way to the nose.

Well I am not opposed to doing a large amount of paneling under there to smooth things out. However I don't see how the exhaust system area can be covered without creating a fire hazard and trapping a whole lot of heat. Covering the bottom of the engine compartment would also create some other problems.

i have covered the entire engine compartment of my car and haven't seen any indication of a heat problem,(even with a partial grill block) i did however use thin aluminum for the part that goes over the actual engine bits (oil sump and exhaust) at the front of the front of the tray i have some louvered vents... i'm not sure they do much, but they're there.
one this is that any oil leaks will accumulate on the tray, so some sort of drain is advisable, also you now have to pop the hood to look for leaks rather then spot them on the driveway
i have just have tank to tank results so no accurate data, but subjectively a front and rear undertray made a big difference.

my rear undertray covers mainly the right side as the exhoust sits on the left. as such i only extended the lower edge of the left wheel well (mainly to prevent dirt from getting on top of the undertray) when driving on the highway with this mod for the very first time a very small part of this extention came loose and this caused a loud occilating noise. after popping the little piece back in place the noise was gone on the way back.

also if you have a torsion bar rear suspention you can extend the coroplast tray from the back up to this bar. this could cause it to flex along with the suspention so this scared me at first but in practice it worked flawless. mine is held on to an anti roll bar with zipties and as amateuristic as it may seem it works perfect.

as for wheel skirts i can imagine even half skirts can have a signifficant effect, so if for any reason a full skirt doesn't work, perhaps a "mini skirt" is worth exploring.

VWs come with a molded plastic "sound deadener" panel under the engine from the factory. It stops just short of the catalytic converter. If you avoid the cat/con and don't have plastic touching the exhaust, concerns about heat are unwarranted IMHO. My coroplast belly pan goes from behind the cat/con to the back bumper and even touches the exhaust pipe on my diesel and there is no indication of heat damage whatsoever. Diesel exhaust is much cooler than gassers (petrol).

Coroplast is also available in a heat resistant panel if you're really worried about heat.