I had purchased a Motion Motorsports underpanel a year or so ago, intending to install it on the E36 race car.  After evaluating how it installs and what I wanted to do with the race car, I decided it would be better suited for a road car.  My reasons: it’s unnecessary if running a front spoiler and it makes removal of the front bumper difficult.

That said, it does offer a number of benefits.  The stock plastic under-panel is a bit flimsy and only serves as a duct to the radiator, providing no real aero benefit or protection for the front of the engine.  The Motion Motorsports panel is made of a fairly thick sheet of aluminum and comes with some beefy hardware.  The install is relatively straight forward, just a few things to look out for and no special tools required.

I also ordered the radiator duct piece from Motion, which helps deflect and funnel air through the radiator, rather than around it.

Here you can see the stock plastic piece, which is held in with four screws.

And here it is removed, with the bottom of the radiator exposed.

Now for the hardware.  The kit includes four plastic nut inserts, which are inserted into existing holes on the front frame horns.  This is what the aluminum standoff plates are mounted to (sorry I forgot to grab a photo of that.)  The leading edge of the panel is attached to the urethane bumper using some T-nuts, small carriage bolts, and nuts.

There aren’t any existing holes on the bottom of the OEM bumper, so you have to temporarily install the panel on the standoffs and use it as a template to drill the holes.  Pay close attention to your placement, as the panel is an exact fit.  If you are shifted slightly to one side, it will not seat properly.  With the holes drilled, the T-nuts and carriage bolts are inserted from the top-side.  Then another nut is used to secure the bolt, which essentially sucks up the soft urethane plastic around the T-nut and creates a mounting stud.  See the photo at the right for a better idea.

While I had access to the back of the bumper, I took the opportunity to fix a loose center grill and also secure the OEM brake ducts which had a few broken clips.  Some HVAC aluminum tape worked great for both fixes, which are obviously hidden behind the bumper.

Here you can see the panel and radiator duct together prior to installation.  The radiator duct has a punch out for an electrical connector on the AC condenser, and the whole assembly can be shifted forward or rearward as needed.

The panel also replaces the flimsy pork chop panels that attach to the wheel well liners.  Here it uses some OEM style nut plates and the same bolts as used elsewhere.  Overall the whole system is very stout and adds some rigidity to the entire front bumper and liners.  Not sure if there is any appreciable amount of aero advantage, probably very very little, but it has to smooth out the air flow under the front a bit.  It also adds some protection to the front of the engine for the PS pump, AC compressor, and associated belts.