986 lower front flaps
 

I've just replaced the lower front flaps on my 2002 Boxster S. Both parts (left and right) supplied by Porsche cost about 23 pounds for both parts. I've seen posts that refer to them as mud guards, but in 'The Ultimate Owner's Guide' by Mark Bennet, they're described as 'helping to extract hot air from the radiators' and can effect the stability and perceived handling of the car.
My ones had become cracked and the near side one was completely gone. They tend to get damaged on hitting kerbs, bumps etc.
A ten minute job to remove and replace. Three philips screws holding them on.

p/n P996.504.503.00 and P996.504.504.00

Not sure I'll notice improved handling, but I suppose Porsche wouldn't have fitted them if they were not needed.

at first i thought you meant that they WEIGHTED 23 pounds for both parts. i looked them up with the intention of pulling them off the car to save weight! :)

those are actually to help cool the brakes.

Yes sorry for that. I have an HP laptop with an American keyboard which I picked up when I was working in New York, so no 'pound' sign.

Here's the diagram from Porsche showing the front lower flaps (attached)