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Torque Box Reinforcements

        The upper and lower torque boxes have routinely been the subject of criticism among Mustang enthusiasts who enjoy pushing their cars at the track or even on the street. We regularly hear about some of the more experienced racers reinforcing their boxes before they install aftermarket control arms, sticky tires, or just plan to put the car thru the rigors of spirited driving. So lets start with the basics... What are torque boxes, what do they do, and where are they located?

        To put it simply, the torque boxes are thin sheet-metal boxes that serve as the link between your rear control arms and the chassis/body of your automobile. All 79-04 Mustangs have two upper and two lower torque boxes that are factory spot welded to the chassis in just a few locations. If you have a mostly stock car and never intend to step up to a tire with more bite such as slicks or drag radials, chances are that your boxes will never require any special attention. If, however, you are using an aftermarket set of control arms with poly or solid bushings, plan to run stickier tires, or just enjoy abusing your car, it may be wise to address the structural integrity of your torque boxes. Remember, repairing torque boxes is almost guaranteed to be far more costly and time consuming than reinforcing them the first time and being worry free.     

        The following are among the three most common methods used to reinforce the boxes (always make sure to disconnect the battery when doing any welding and be cautious of any fuel lines within the welding area):

1.) Weld up the seams: This involves following every single torque box seam that contacts the chassis or floorboard and welding it for added strength. Note that the paint should be stripped with a wire wheel prior to welding to ensure a strong weld. This adds significant strength to the weak factory welds.

2.) Upper and Lower "Battle Boxes": Although a shop called "Wild Rides" coined the name "battle boxes" years ago, a number of companies have since copied the simple but effective design. These upper and lower reinforcements are unique in that they spread the forces over a larger area and prevent smaller problematic areas from failing. They come with undercarriage plates as well as interior cabin plates that are used to "sandwich" the floor of the car. Although these can be just bolted in, it is wise to weld them in order to prevent the plates from shifting under extreme load. These are available from UMI Performance, UPR Products, and Wild Rides.

 3.) Lower "Boxing" Plate: These 1/8inch reinforcements are responsible for physically "boxing" in the lower torque boxes. It maintains the straightness of your existing thin boxes and dramatically increases their strength. These are available from Competition Engineering and Wolfe Racecraft.

        Because I decided long ago that I plan to keep my 93 coupe for a long time and would like to be able to occasionally race it at the strip without worrying about its structural integrity, I knew that torque box reinforcements were in order. I decided  that I wanted to do this once and not cut any corners so I planned out how I was going to bullet-proof my boxes by welding up all the seams, installing battle boxes, and the boxing plates. I purchased the battle boxes from UMI for around $135 shipped and the Wolfe Racecraft boxing plates for around $35 shipped.

        When I received the UMI reinforcements I decided to mockup the lower L-brackets to see how the fit was. I quickly noticed that the control arm hole in the L bracket did not line up with my existing control arm holes. I called UMI and they said that these cars have a great deal of tolerances from car to car in the torque box area. They asked me to go ahead and mark where I needed the holes drilled and send the L-brackets back to them. I did just that and UMI re-welded the holes and re-drilled according to my markings. They fit perfectly. I highly recommend UMI for standing behind their products and working with me (not to mention they have the cheapest upper/lower reinforcements that I could find).

        Once I had all my parts I was ready to begin. I put the back of the car as high as possible, disconnected the battery and pulled the carpet back. Next I removed the wheels, disconnected the rear brake lines, removed the driveshaft, and dropped the rearend. Although full removal of the rearend is not necessary to install the boxes, it makes things significantly easier and since I knew I was going to fully weld the seams, I did not want the rearend in the way. This gave me an opportunity to replace all the bearings and seals in my rearend, strip it, and paint it. Here is how it turned out:

        Now that the rearend was out, I could easily proceed. I first wiggled the L bracket into place. The front of the lower control arm should be in place already and the control arm bolt should now be tightened down. I can't stress how important this step is because it ensures that you drill the holes in the correct place. I have known people that just mocked up the L bracket and drilled the holes without the control arm tightened in place. Once they bolted everything together they noticed that the holes were off by 1/4inch or so once the torque boxes flexed a bit due to the torque. Tightening the front of the lower control arm will simulate how the L bracket will sit when everything is completed.

With the lower control arm and the L bracket bolted together, you can go ahead and drill four 3/8" holes straight up thru the floor using the upper L bracket holes as templates. Inside the cabin you should clearly see where you drilled the 4 holes thru the floor. Take your square interior reinforcement plate, and match the holes you just drilled to that plate. Mark the perimeter of the plate on the sound insulation and with a utility knife cut the perimeter of the sound insulation leaving an extra 1/2" along each side for welding purposes. Then use a gasket scraper to remove the sound insulation in those areas. I made sure to retain as much sound deadening as possible to prevent excessive noise in the cabin. Also remove the paint in those areas using a wire wheel on a drill or a grinder. Once the sound insulation and paint has been removed, go ahead and lay down the square plate and bolt it to the L bracket using the supplied grade 8 nuts and bolts. Tighten firmly. Repeat on the other side.

        The uppers are a bit more tricky. Make sure the upper control arms are removed and out of the way. Since there is a sequence to installing the bolts in the uppers, I have numbered the holes going from top to bottom from 1-4. Here is an illustration:

Begin by drilling a 1/2" hole straight up thru the factory supplied 1/2" hole (using #1 from the illustration as a template). Now take the large curved bracket and match it up to the hole you just drilled from inside the cabin. It is a good time to make your markings once again, and remove the sound insulation in that area. Have a friend help you install a bolt thru the undercarriage bracket, thru the floorboard, and finally thru the curved reinforcement plate in the cabin. Tighten loosely. Make sure to use to the supplied angled spacer. Now you can go ahead and drill the 2nd hole from inside the cabin using the curved bracket as a template. Drill straight down. Once again install the bolt and tighten down loosely. Holes #1 and #2 from the illustration above should now have bolts running thru them. Now you could simply drill holes #3 and #4 from under the car using the above brackets as templates. These two will require a little force since you not only have to go thru the floor-board, but you actually have to drill thru the 1/8inch steel reinforcement as well. Once the holes have been drilled and the insulation/paint removed, tighten all the bolts firmly and repeat on the other side.

        The next step is to strip all the paint around the torque box seams. I used a combination of a drill with a wire wheel, 4 1/2inch grinder, and a dremel to get the hard to reach places. Using a mig welder capable of welding at least 1/8inch steel, weld all around every single seam on the upper and lower reinforcements. Because I did not have a welder, I payed a friend to do the job (Thanks a lot "JohnB" from www.cafords.com). He did a great job welding all the seams and welding the battle boxes in place to keep them from shifting. After he completed the job I painted all the areas that were stripped and reassembled everything.

Here are the upper and lower torque boxes just after dropping the rearend:

Here are pics of the reinforcements installed prior to welding:

Below are pics of all the seams after welding:

These are a shots of the interior reinforcements:

         After some debate we decided not to use the Wolfe Racecraft boxing reinforcements. My car happens to already have extremely long home-made subframe connectors. These connectors completely interfered with a significant portion of the area that the Wolfe plates were supposed to contact. Although we thought about notching the plates or cutting the subframe connector welds and slipping the plate underneath, we agreed that it would be best to leave the lower plates out. Since the subframe connectors are so long, they already somewhat add to the structural integrity of the torque boxes. That combined with the battle boxes and welded seams should be plenty strong.

        If you have any questions, please feel free to email me at info@mustangmotorsport.com

        Luke87GT (Luke Idzkowski)