Smoothing a Firewall: Build a Custom Shaved Engine Bay
How to Make Your Engine Bay Look Good
The OE manufacturers didn't really care how things looked under the hood. The expectation was that it would be a greasy and dirty space seen only by the owner when they checked the oil once a month or by the local mechanic when something needed to be fixed. But as gearheads, we do care. In fact, some of us care more about the cleanliness and trick appearance of the engine and custom engine bay than we do about the rest of the car. There's nothing quite like that collective gasp from the crowd when you roll up at the local cruise spot and pop the hood, revealing some mechanical masterpiece. One quick way to get there is by creating a shaved engine bay and a smooth firewall.
How to Shave an Engine Bay
Since the OE's saw the engine bay as an industrial space, they didn't care how many holes they poked in things like the firewall and core support. The result is an ugly expanse of sheet metal that is riddled with holes for things like bundles of wire, climate control and hoses, and little plastic plugs in all the rest. It's cluttered and distracting. We want the real star of the show to be the engine, and there a million different ways to achieve a shaved engine bay that acts like a jewelry case for your hopped up engine.
We've all seen the really nice cars at the show that feature a ton of custom work under the hood. While we're not all Chip Foose, we can create some neat details that correct or cover up the factory ugliness. While most of this article is devoted to shaving the firewall, don't overlook other opportunities to do something unique. If you want to practice your sheet metal shaping chops, try making a cover for the area between the grille and radiator core support. Often, this can be a simple piece that really cleans up the engine bay. Take a look at other areas that are distracting and could be made better, Try hiding things like coolant overflows, bulkheads for AC hoses, and wiring under fender edges or under custom panels. Just be sure to put function ahead of form, keeping an eye on any places that hoses or wires could rub through on moving suspension or engine components.
How to Fill Holes in a Firewall
In the above video, we tackled a C10 shaved firewall on our '68 project truck. We opted to work with the existing metal and use one of our Shaved Door Handle and Smooth Firewall Kits. We can't say enough about these. Basically, it's a bag full of round patches of different sizes in a variety of gauges ranging from 11 gauge (perfect for frames and structural components) up to the 18 gauge kit that we used, perfect for firewalls. These are perfectly sized for the holes that you're likely to find in your firewall, and even have little "teeth" cut into the perimeter, allowing them to bite into the surrounding sheet metal and providing a perfect gap for your MIG weld. We used a bunch of these on our C10, and often they were sized so perfectly that we just tapped them in with the hammer and dolly and they held themselves in place while welding.
Our first step was to circle the holes that we wanted to fill. We plan to retain the stock brake booster, steering column, and heater plenum, so those holes were left alone. In most cases, the holes in our firewall perfectly matched those in the patch kit. In a couple cases, we drilled the holes out slightly to size, then installed the patch. Also note that not all holes are round. We faced an oval hole next to the steering column and the square hole where the old wiring bulkhead passed through. For both of these areas, we used a fresh sheet of 18-gauge steel and cut our own patch. Take your time and fit carefully. Be sure the metal is clean and free of paint, primer, and dirt.
As noted in the video, be sure to take your time here. Welding puts a ton of heat into thin sheet metal, and it's guaranteed to shrink and warp. The best strategy to keep this under control is to tack, then planish with a hammer and dolly to stretch the metal back out. Time, care, and practice will yield impressive results.
Custom Sheetmetal Firewall vs. Smooth Firewall Kits
Sometimes, the approach that we took of simply filling holes in the existing firewall is not the best option. Maybe the stock firewall is damaged. Maybe you want to really raise the bar and create something truly unique. Whatever the case, there are a few ways to approach this. One is to purchase a smooth firewall kit that's designed to fit your specific application. These are usually roughly cut to the same size as your existing firewall and often have some sort of recess built in to clear the back of the engine. Take care when selecting a kit for your car and be aware that one size does not always fit all. In other words, though it says it's designed for your '69 Camaro, expect to do some serious cutting and trimming to make it work. We have, however, had good luck with a few fill plate kits, like those from DSE. They're designed to fill the hole that the stock heater and AC plenum left behind, and they're high quality parts that don't replace the entire firewall, just the area where the large climate control holes were.
Another option here, and by far the most ambitious, is to fabricate your own custom firewall. If you're doing something really unique or accommodating a crazy engine swap or setback, this may be the best option for you. It's important to remember that you're dealing with a significant piece of your car's structure. That means that before you just blast the stock firewall out with the plasma cutter, you'll need to add some bracing to keep the body square. 1" x 1" mild steel tubing works well for this. Also keep in mind that the piece you fabricate to replace the stock firewall needs to be sufficiently stout. The stock firewall may have been made from thin material (more on this in a minute) but it was very stiff because of all the shape in it. That's why those ribs and recesses are there. Simply skinning your firewall with some thin 20 gauge sheet and calling it a day is not sufficient here.
Firewall Sheet Metal Gauge - Car Firewall Thickness
What gauge sheet metal should you be working with to create your custom smooth firewall? There's more than one right answer here. Typical cars from the '50s-70's used slightly thicker sheet metal on the floor and firewall than on the rest of the body. While it will vary from manufacturer to manufacturer and car to car, it's fair to assume that body sheet metal will be in the neighborhood of 20 gauge and floor and firewall sheet will be close to 18 gauge. The only way to be truly sure is to measure with a sheet metal thickness gauge. Once you've established what you're working with, you can select the appropriate thickness for your patches. On our C10, the 18 gauge patch kit was perfect.
As discussed above, if you're building something from scratch, be sure to use material that is thick enough to handle the forces applied in this area of the car. A large, flat expanse that is otherwise unsupported may need to be 16 gauge or even 14 to provide enough strength. The tradeoffs here, of course, are weight and workability. It's hard to roll a bead in 16 gauge, and darn near impossible in 14 gauge. It's also possible to work with thinner material if there's a tube structure built into the cowl to carry the load. Often, pro-built custom cars will use a structure like this to support the pedals and steering, stiffen the cowl, and allow for custom firewalls to be built from thinner, more shapeable sheet metal.
In the end, a little time spent cleaning up your firewall and engine bay can yield impressive results. On our C10 truck project, we're hoping to direct attention to the LS engine, not the holes and barbs in the stock firewall. The engine is the heart of the car, and a clean engine compartment is a great way to give it the home it deserves.
