Back when my daughter bought her (new) '07 S60 2.5T, and having read (in IPD's FWD catalog that these cars have a vulnerably-placed oil sump (compared to a higher underside 240 that I was used to), I thought that I would buy her a metal "skid plate" to protect the underside of the engine sump and transmission from things like hitting a cinder block or other road debris.

Wanting something better than IPD's aluminum shield, I found that Volvo actually sells a steel protective plate. It's p/n is 9451784-4.

But I also discovered that with the part comes a warning -- paraphrasing (it's been a few years since I looked into this, so I'm going by an almost retired person's memory :-), it cautioned that this plate should not be installed in areas where there are high temperatures, as this part increases underhood temperature.

It's seem obvious, then, that any shield under the car will interfere with air flow through the engine compartment. The 240s' "splash pan", extending from the radiator back to the crossmember under the front of the engine, does protect from road splash and is useful in this regard, but it's easy to see that a farther back plate would prevent air from freely flowing over the engine compartment and exiting under the car. Of course, the relatively simple engine compartment of 240s hardly compares to the astonishly cramped compartment of my daughter's S60, and virtually all other contemporary cars I've seen (a do-it-yourselfer's nightmare, but that's another story :-).