Motion Motorsport Underpanel
When I rebuilt the cooling system the first time, I noticed this plastic bit that BMW calls an "Air Duct Front Bottom" (51712496714), which clips onto the bottom of the radiator, had apparently come loose at some point prior to my owning the car and received some custom machining from the power steering pump pulley:
The OEM "air duct" is actually two parts (Air Duct 51712491034 and the above pictured Air Duct Front Bottom), but for some reason the two items are given a single number designation (2) on the BMW exploded diagram of the assembly:
After tinkering with the OEM underbody plastics, I liked the idea of installing a Motion Motorsport underpanel for the Z3 as an aluminum "air duct" replacement, so I ordered one; unboxing the MM part:
The MM underpanel is nicely finished, and comes with a a set of optional threaded inserts for mounting aero bits or other items to the panel; I'm of the opinion that the car bodywork is low enough already for street duty. There are clues that previous owner(s) often tested the front bumper height, in light of the extensive scuffing of the underside of the front facia. I'll deal with that at some point; the bumper cover in pretty good shape overall, but has plenty of paint chips, which is typical for low slug cars like the Z3.
I needed to slightly massage the MM underpanel and mounting brackets to get it to properly fit my car, which is no big surprise since the item is obviously a racer oriented part. I'm relatively certain that my car isn't tweaked since I have seen no evidence of prior front end collision damage.
The fender liners also needed to be modified; a 30mm X 100mm chunk needs to diced out of each liner to provide clearance for the MM underpanel mounting brackets. I added a 2mm thick piece of rubber strip, about 50mm X 300mm long, tucked between the underpanel and the bumper facia to isolate the two parts; after tracking down and dealing with an array of M Roadster rattles and squeaks I didn't want some sort of new noise generated by the aluminum underpanel. I also fabricated a small bracket that accommodates the OEM captive nut positions to tie the right fender liner to other under body plastic. One of the fender liner mounting points is the OEM air duct, and the MM underpanel installation left it hanging. Overall, I'm happy with the panel installation; it does seem to tighten up the underbody trim assembly, but it is somewhat more difficult to remove and reinstall than the OEM plastics.
I suffer from severe fastener OCD; Motion Motorsports supplies 1/4-20 hardware to mount the underpanel, which needed to become metric with some proper washers to prevent a splinter in my mind:
The OEM "Air Duct Front Bottom" fits into the spot where the oil cooler would reside on an S50 / S54 powered Z3M, preventing airflow from bypassing the radiator on S52 Z3M cars which were not equipped with an oil cooler; it appears that 51712496714 forces more airflow through the radiator, at the expense of airflow through the lower portion of the A/C condenser. Omitting the part when installing the MM underpanel results in the gap illustrated below:
Since my "Air Duct Front Bottom" was already damaged, I had nothing to lose by dicing it up in an attempt to fill the gap below the radiator; modifying the part as shown below allows it to work with the MM underpanel:
Modified "Air Duct Front Bottom" installed:
Power steering hose routing:
The Motion Motorsports underpanel was on the car for about a year; when I installed an S54 radiator and BMW oil cooler I went back to OEM plastics, replacing my front fender liners along with the OEM underbody air ducting (including the mysterious S54 oil cooler airfoil).
Here's a view of the relatively minor fender liner modification needed on each side to clear the Motion underpanel brackets: