Underside plastics.

Underside plastics.
"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 Skidplate. 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 here. 

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 vertically at par with the bumper. The M3 V8 instead sits lower, making for 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 

This was my second damaged center undertray section on the E9x M3.  

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. They were often 3x the 200$ cost of the original undertray.

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.

Inspiration from the V8.

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. 

The Performance Aluminum Skidplate incorporates the large center cutout located behind oil cooler as found on the M3 V8. On the S65, it has been proven effective at funnelling air behind the oil cooler to enhance heat extraction away from the cooler and radiator. 

Below is the underside of the undertray with the center opening, matching the oil cooler's width.

The internal design has two walls, with the higher section situated closer to the engine and the lower section 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. 

The original BMW plastic sleeve is a useful feature for the oil cooler's heat extraction by allowing air to pass under the mechanical engine fan and the radiator before being extracted. 

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

The undertray is laser cut from 5251 aluminum and weighs +/- 3.6 kg (7.9 lbs). It's adding a few pounds to the front end. 

It is powdercoated in satin black for subtle aesthetics. 

It is fitted with threaded inserts and the matching hardware to install the undertray. 

The undertray is a critical component to holding your front bumper in proper position. The rear of the undertray has slotted cutouts to provide some adjustability when bolting to the engine's belly pan. 

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. 

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.


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