"In this Special Series about the Mechanical Restoration, I document the repairs, the replacement parts and the mechanical upgrades to the E46 M3. 

In this entry, I'll be doing it once and I'll be doing it right by documenting and fitting the Performance Aluminum Undertray. It promises durability, and cooling benefits."
-Matt

What undertray?

The previous owner of my E46 M3 had supercharged its S54, resulting in many modifications to engine components along with the removal of ducting on the front carrier.

The previous owner simply didn't bother fitting the OE plastic undertray section: it wasn't even there. It's unclear whether the undertray was removed for clearance or suffered road damage.

Having had little experience with the E46 M3 back in 2021, I only took notice of the missing undertray during the Summer of 2022 as we begun the mechanical restoration. 

The center undertray section was missing. It would have been bolted to the original aluminum belly pan. The E46 M3's front fender liner sections also make up the corner sections of the undertray as seen below. 

E46s don't crack*. 

E46 M3 owners do not face cracking undertray problems to the extent M3 V8 owners do in part due to its design sitting at par with the bumper. The M3 V8 instead sits lower, making for a prime target for road debris. 

Back in 2020, I documented BMW's feeble M3 V8 undertray design in the entry "Ticktock, Ticktact". I cracked and replaced my E92 M3's undertray twice before calling it quits and fitting the Lightweight Aluminum Skidplate.  

Hunting for a viable design.

Consequently, there were few options available for the E46 M3. The available products were 1:1 replicas of the original plastic undertray although manufactured using aluminum with varying finishes from raw to wrinkled black. 

I felt finding a design that was cost effective and offered some performance improvement would be of value to other E46 M3 owners beyond those who face the rare scenario of a previous owner not fitting an undertray.

I previously documented the original E46 M3 oil cooler design and its plastic sleeve in the entry "A little more".

The solution. 

In January '23, I sourced the Performance Aluminum Skidplate for documentation. It promised to increase durability at an appropriate price point with the added benefit of increase oil cooling performance. 

To begin with, the undertray is laser cut from 5251 aluminum and weighs +/- 3.6 kg (7.9 lbs). While adding a few pounds to the front end, it also improves durability, ground clearance and aesthetics; it is powdercoated in satin black.

Design update 2025. 

As of 2025, the undertray is now a 3 piece design to facilitate shipping and installation. The installed pictures of my E46 M3 will showcase the former 1 piece design. 

Inspiration from the V8 and V10. 

Most importantly, the Performance Aluminum Skidplate incorporates the large center cutout located behind oil cooler as found on the M3 V8 and M5 V10. This design feature has been proven effective at funnelling air behind the oil cooler to enhance heat extraction away from the cooler and radiator.

The underside of the undertray displays the center opening - it matches the oil cooler's width. The topside has two walls; the higher wall is situated closer to the engine and the lower wall by the oil cooler. 

They act as wind deflectors, forcing air passing under the car to move through the engine bay, with the hotter air going through upper sections of the hood. They are fitted with rubber insulation to eliminate vibrations and seal the opening. 

 

Adjustability. 

The hardware kit includes 4x aluminum brackets to allow height adjustments. They are installed with the front swaybar brackets and replace the original aluminum brackets on the side components of the undertray. They are bolted through the 2x slotted cutouts on the picture below. 

To help adequately position the front bumper with the undertray, the rear / engine side has slotted cutouts to provide some adjustability when bolting to the engine's belly pan. To do so, your front bumper will need to be drilled in 3x locations as pictured below. 

What wasn't even on my radar to begin with ended up as another example of using contemporary knowledge to benefit our classics. 

Overall, I feel this should be on your radar even if your original undertray remains uncracked after 20 years. 

I can source this year long for your E46 M3 here.