Saturday, July 18, 2020

DIY - Under Panel Drain (Speed) Holes

I read about Model 3s losing their bumpers due to filling up with water. When I look underneath at the rear diffuser and I see that there are some small holes where it meets the undertray for drainage, so I decided to make some enlarged holes along the slope. Drilling holes in cars always reminds me of this clip from the Simpsons.

"Speed holes"

I decided to perform this modification without having to remove the diffuser from the rear bumper as that was way too much work. This is where my "speed" holes are position and they do not interfere with the rear deep trunk section.

Approximately 6.5 inches back from the edge of the point where the undertray had a second bend on both sides. Look at the starting point of my tape measure. Going any further will be underneath the "under" trunk storage space and and closer will be too level.
Location of each drain hole.
I did not have to jack up the car, I simply used a 5/8" hole bit and hand twisted it using a ratchet so that it would not hit any parts with power tools and then used tiny blade to clean out any plastic burrs. The good thing about 5/8" is that it is easy to find plugs to seal the holes in case I made an error with my speed holes idea in the future.
Starting left side hole using a 5/8" bit

Made a parallel hole on the right side.
Drain hole on right side.
To test this, I poured a gallon or two of water in the rear windshield and saw water coming from my new speed hole. What I am unable to test is if the holes allow water to be sucked in while driving.


When I was installing new spring in the rear, I had to remove the rear diffuser, so I decided to study the pattern of the dirt that was collecting and staining the rear diffuser. The is the result of ~5000 miles of paved road driving. 

Dirt Collection from ~5000mi
So based on this I can really imagine someone collecting a few pounds of dirt from driving a couple of thousand miles on unpaved dirt roads.

Observing the rear diffuser top side reveals the stock drain holes in green. My earlier holes are in red and the dirt concentration I have shown with a yellow arrow.

Dirt concentration spots
So based on the observation it appears these V-shapes serve a structural purpose and possibly an aero purpose, but each segment must be able drain the liquid and dirt.

Dirt collects on edges.
I made some new "speed holes" using a smaller drill bit to match the side of the factory holes on both ends of the tray as shown in red. The other other red hole was my original one from before which I can plug up, but leaving there for now. This should allow for more water drainage from the diffuser.

New smaller drain holes added to each edge
One of the reasons I left the bigger holes in place is that if dirt collects, I can easily stick my finder of a small tool to and see if it comes out dirt to give me an indication of the amount of dirt and sand trapped in there to warrant an future diffuser removal.