With the roll cage and suspension set up, the rest of the work in the trunk can be completed. There are a few technical aspects of the build that must be covered here, and it all depends on how the car is being built. The stock gas tank was in the way of the rear end, thus it was removed for a fuel cell. We also opted to mount the battery in the trunk. These two details require installing a rear firewall to separate the trunk from the rest of the interior. While we were chopping up sheet metal, we built our own wheel tubs and replaced some rusted trunk floor.
This is a street\strip car, so a tiny fuel cell just won’t cut it. We ordered a new 16-gallon polyurethane fuel cell (with a fuel level sender) from RCI. This allows us to pass a few gas stations on the way to the local cruise spot, not to mention a little extra weight in the trunk will help plant the tires. To provide some more weight transfer, we decided to mount our Optima Red-Top battery in the trunk as well. Yeah, this will require running a few more feet of heavy wire to the starter, but the master battery switch needs to be in the back anyway, so that wire would have been run either way.
Mounting the trunk in the battery does not automatically require a rear firewall, you could mount the battery in a sealed box with a vent to the outside, but a fuel cell in the trunk does require a rear firewall. According to the NHRA rule book, the rear firewall must cover the rear seat area including the rear deck, to seal off the interior of the car from the trunk. This is to prevent a fire from the fuel tank to spread rapidly into the rest of the interior. A steel firewall must have a minimum thickness of .024”, while and aluminum firewall needs a minimum thickness of .032”. We went with 22 gauge steel, which measures .0299” thick.
In the trunk, the stock wheel tubs were way too small to fit the 30×9 Toyo Proxes slicks. We used the plasma torch to remove most of the stock trunk sheet metal during the roll cage and ladder bar installations, so there was a fair amount of sheet metal work to do. We needed new wheel tubs and a large section of the trunk floor. We again went with 22 gauge steel for lightweight durability. Most of the pre-built wheel tubs are made from 24 gauge steel, which can be a little flimsy, but still cost $80 or more and require almost as much fabrication as the method we used here. Our wheel tubs are squared-off, not rounded. It is functional and still looks pretty dang good.
The results of this project can easily be replicated by just about any builder with a few tools you likely already own. One that you might not have is a sheet metal nibbler. We picked ours (an air-powered unit) up from Harbor Freight tools several years ago and it works great. You could use a cut-off wheel, but that takes more time and is just not worth the effort. We did have the steel supply shop cut our two 4×8 sheets of 22 gauge into specific sizes, and then custom trimmed them in our shop. If you took the time to make cardboard templates beforehand, you could have the steel supply cut all of your steel, but you run the risk of something not fitting right when you get it home. A plasma cutter is another invaluable tool for not only this project, but the entire build, not sure where we would be without it.
The entire process of fabricating and assembling the rear firewall, wheel tubs and floor took about a week, working 8-hour days. This is a huge task, but it is well worth the effort. We also buttoned up a few other odds and ends on the roll cage while we were at it. With this task completed the chassis is finished and we are moving on to the drivetrain. There is more work to be done on the body like the wiring and paint\body work, but we will get to that soon enough.
01. The stock subframe was spot welded to the trunk floor. Since a good portion of it was rusted and we needed better support for the rear roll cage bars (they are part of the suspension after all, supporting the bulk of the rear suspension), we used the plasma cutter to remove the rest of the thin sheet metal.
02. Then we took some 2×3 tubing, split it on the 3” side and cut sections to match the curve of the subframe, just like we did to mount the down bars. Then each section was welded to the subframe and each other. This is much stronger than the factory.
03. We boxed in the bars with another section roll bar pipe. This will provide more strength and eliminate potential flex in the down bars should the Scamp hang the front wheels in the air (which it probably will).
04. This is what two sheets of 22 gauge steel looks like after we had it sheared. The large piece in the center is a trapezoid shape that is a perfect fit for the rear firewall. This eliminated a bunch of extra work. 4×8 sheets of thin sheet metal are not only difficult to cut without a sheer, but dangerous as well.
05. Using the Esab Handi-Plasma 380, we cut out the driveline hump in the rear firewall.
06. Then we marked and trimmed out the area around the rear down bars.
07. We laid down some stitch welds every couple of inches. There is no need to fully weld it, you can even secure it with screws if you wanted to.
08. The rear deck offers a bit of a trick, since the back glass is curved inward. We pulled the trim (where the vinyl top was) and used it as a guide since it matches the curve. We ended up with a perfect fit.
09. The finished firewall will protect the driver from a nasty fireball in the event of a fuel rupture. We also welded the diagonal support beams to the ladder bar crossmember through the floor.
10. The wheel tubs began with a single sheet positioned for the rear of the tub. We trimmed it to fit the curve of the quarter panel. Then, using a magnetic angle finder and a straight edge, we extrapolated the angles and measurements of the wheel tub side piece.
11. We transferred these measurements to the pre-cut rectangle (we previously measured for the wheel tubs before we ordered steel) and trimmed it out with the plasma cutter. The frame was traced on to the panel to get the lower shape.
12. This is a critical cut. Since the firewall is part of the front of the wheel tub, the side panel needs to fit clean. We cut around the X-brace on the back of the seat so the side panel would slide up tight.
14. Since the coil-over shocks mount to this crossmember between the rear cage bars, we had to box in this area. We cut a strip of steel and welded it in place to the crossmember and existing trunk floor.
15. The fuel cell needs support, as does the trunk floor. We welded in two lateral braces, this one, just behind the rear down bars and another at the very back of the trunk. This will provide a solid structure to support the 16-gallon RCI fuel cell as well as the large section of trunk floor.
16. We cut the floor panel to fit and stitch welded it in place. With this in place, there is not as near as much room to work as there had been. It is starting to look like a race car.
17. We could have just welded up another piece of steel to cover the shock area, but access through the trunk is always nice, so we opted to add some 1\4-turn fasteners (from Summit Racing). We marked the panel and the tubes for the locations.
18. Using a welding magnet, we positioned the weld-on tabs for the fasteners. It is important that these are flush with the top of the bar and level, which is easier said than done.
19. The panel was place over the tabs and each location was marked for the fasteners.
20. The fasteners are riveted in place. The springs are also riveted to the welded tabs.
21. These things are awesome, they may be a pain to install at times, but once in place, they are so easy to use. A simple 1\4-turn and they tighten or release.
We mounted the fuel cell and the Optima battery to make sure everything fit and worked before finalizing the install. The fuel cell mounts before the access panel installs, the panel covers the front bolt for the fuel cell straps. We also had to cut a section of the new floor out so the rear sump on the fuel cell could drop through.
24. The rear firewall got the same sealer treatment. Once this cures and the body work is done, the entire trunk, interior and undercarriage will get sprayed with Al’s Liner, which is a tintable bedliner material.
25. The Optima battery is a sealed battery, so there are no worries about fumes. We built this tray using some angle iron, welded it together and then welded it to the floor. A basic battery hold-down will secure the battery. To be NHRA compliant, you can’t use J-hooks without welding the open J shut. The best method is a pair of 38” bolts that tie to the frame.
A life-long gearhead, Street Tech Magazine founder and editor Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 5 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced. You can follow Jefferson on Facebook (Jefferson Bryant), Twitter (71Buickfreak), and YouTube (RedDirtRodz).