In our last installment on Scarlett, our '72 coupe project, we shoehorned in a fire-breathing LS3 stroker motor that should rocket the former small-block car to far quicker times while reducing weight and adding a little bump in fuel economy, thanks to the EFI. With the 600-plus-horsepower 416 bolted in place, it's now time to let it breathe by installing the exhaust system. Even if I'd wanted to keep the headers, pipes, and mufflers that were on the old 350, the switch to an LS required a different header-primary configuration. And besides, the extra airflow from the bigger, hotter motor was going to need more than a pair of 2.25-inch pipes to breathe through.
Considering both the power and the general audacity of the shark body style, I decided early on that the car would need sidepipes—and not the shielded factory ones, either. While there's absolutely nothing wrong with that look, Scarlett's inspiration comes from the early road-race C3s, which frequently eschewed both factory sidepipes and undercar exhaust for OK Kustom side-mount headers, the design of which lives on in Hooker's sidepipes (which unfortunately are only available for conventional small- and big-block applications). Featuring four primary tubes that exit the underside of the car just behind the front wheel and dump into a single, exposed collector, the bundle-of-snakes look is both classic and unmistakably aggressive.
"In for a penny, in for a pound" is a pretty good way to describe building a car, not least because the further you get, the more things tend to cost. The pipes were no exception. As of this writing, several companies are beginning to make a side-mount-style header for LS conversions, but at the time we got this project underway, the only ones in regular production were from Stainless Headers Mfg. Made in 304 stainless steel, beautifully bent and welded, they're absolutely gorgeous, especially in the polished finish we ordered. They look the business on the side of a chrome-bumper shark—and they're priced accordingly.
The good news is that a fair amount of R&D goes into the Stainless Headers sidepipes. For example, the narrowing where the primaries enter the collector isn't there for looks, but actually increases power. The headers themselves were designed with input from Street Shop's Tray Walden so that they will fit both a factory frame (such as Scarlett's) as well as Street Shop's aftermarket chassis. They can also be had with bungs welded in for an O2 sensor, which you'll need for an LS conversion.
Since we're performing our conversion at Street Shop (and since they're a dealer for Stainless Headers), we ordered the pipes through them. The system came well-packaged, with both the headers and the separate sidepipes. The pipes allegedly came with a "muffler insert" in place, but I can say confidently that what's tack-welded in the pipe may be an insert, but it ain't a muffler. The part of me that loves the churning snarl of a big-inch engine through open sidepipes will have to hear it before we change it, but I see something quieter getting stuffed into those pipes before all is said and done.
Since I've learned to work under the assumption that everything has to be done at least three times, we started the installation by wrapping the headers and pipes carefully to protect them, and then installing the headers, without the sidepipes, while the car was up on the lift. They have to go in from the bottom, and while the job isn't terribly hard, there's a little moving around required to get them oriented correctly. Once they were bolted in place, we lowered the car to the ground and slipped the pipes onto the rear of the collectors, using a rubber mallet to tap them forward into place.
Having the pipes fitted also gave us a chance to assess how best to bolt them on. Each one has a pair of tabs that pass beneath the rocker moulding and bolt to the car, and each tab has a hole that should more or less align with one the few holes in the frame. After examining the frame holes, we chose to weld in a threaded insert that would let us run a bolt through the hole in the tab and directly into the insert. This obviated the need to fish a bolt through and hold it in place while attempting to tighten a nut on it.
Walden first double checked the hole locations, and then I hit the area around the holes with a sander to prepare it to be welded. A shower of sparks and a little bright-blue light later, and we had inserts welded in. Once they had cooled, I ground the welds back close to flush with the frame and applied some black spray paint to protect against corrosion.
One of the other things I learned from our initial header mock-up is that the primary tubes hit the rocker moulding where they passed under it on their way to the outside of the car. Accordingly, we were going to need to trim both the moulding and the fiberglass beneath it to make room. The keen-eyed reader will have noticed that since the sidepipes run the length of the car and bolt into place beneath the moulding, they have to be installed before the moulding can go on. As you might imagine, it can be tricky to ease this piece into place behind the pipes without scratching the paint.
For the initial install, though, we left the moulding in place and used a Sharpie to mark where the primary tubes contacted it. (Neither the sidepipes nor their mounting tabs interfered with the moulding.) With that done, I unbolted the headers and unscrewed the rocker moulding, noting how much of the fiberglass underneath it would need to be trimmed. Since that part of the fiberglass is concealed behind the moulding, I simply cut it straight across using an air-powered cutoff wheel and a reciprocating saw (aka wheelie cutter). Lest this seem indiscriminate, remember also that these pipes get hot, and having a little extra clearance around them reduces the risk of fire.
Again using a wheelie cutter, I then cut the moulding, removing the portion of the aluminum that contacted the tubes and leaving about a 0.25-inch clearance around them. I finished by cleaning up the cuts with a grinding wheel so the resultant cutout loosely followed the contour of the pipes. After checking everything, Walden and I bolted the headers back in place using a multilayer steel gasket and stainless-steel header bolts, smearing a little anti-seize on the bolts prior to assembly to protect the threads in the aluminum heads. While most of them are easy to access, note that the one farthest back, by the number-seven cylinder, is really tight.
With the headers in place, the next order of business was to install the O2 sensor bung. Since our pipes came without bungs (fortunately we had a couple on hand), that meant going through the wiring harness and instructions for our FAST XFI computer and verifying that we would only be using one sensor, then slipping the sidepipes back into place to locate the hole for the bung that the sensor would screw into.
Since the purpose of the sensor is to provide feedback to the computer running the fuel injection, it needs to be located where it will get the best mix of all four cylinders on the bank feeding into the collector. Instead of placing it in the collector itself—where we'd run the risk of only reading data from one of the four cylinders—we situated it farther back in the pipe.
After deciding to use the passenger side for the sensor (it'll be closest to the computer there), Walden carefully marked the hole location, removed the pipe, drilled the hole, and welded the bung in place. After it all cooled, we screwed in the O2 sensor and slipped the pipes back onto the collectors. Since the pipes tend to run a little "downhill" as they head aft, we then used a floor jack to carefully raise them at the rear until the tabs lined up with their holes and the pipes could be bolted into place.
After carefully snaking the moulding in behind the pipes and installing the trim screws, what was once a relatively demure-looking bumper car was starting to look like something else entirely. Now it's time to get the 416 wired and plumbed, so we can hear that beast howl at the moon.
01 We ordered our Stainless Headers Mfg. LS-conversion pipes through Street Shop. The polished 304 stainless pipes are beautifully made, and the purge TIG welding on the flange is a sight to behold.
02 The attention to detail isn’t limited to the things that you can see. Note the smoothness of the inside of the collector, where there’s nothing to disrupt the flow of the hot exhaust gases as they leave the motor.
03 The sidepipes came with “muffler inserts” tack-welded in place. I’ll have to hear them before I make a final decision, but I’ll bet the local noise ordinance will necessitate the installation of something a little more restrictive before it’s all over with.
04 The sidepipes are mounted to the car using a pair of brackets that bolt to the frame beneath the rocker moulding. The nut is actually a threaded insert that will be welded into a hole in the frame to ease assembly.
05 Although we mocked up the first header install without them, for the final installation we used a multi-layer steel gasket. We also put antiseize on the header bolts to protect the threads in the aluminum heads.
06 The rearmost trim screw on each rocker moulding mounts through this fiberglass tab that hangs beneath the body. Don’t be surprised if one or both of them are hanging on by the proverbial thread when you get the moulding off. Ours will be glassed back into place.
07 For the first test fit, we wrapped the headers with protective foam, then slipped them up underneath the car and bolted them into place.
08 It didn’t take long to figure out that the primary tubes were hitting the aluminum rocker moulding and the fiberglass beneath it. With both the headers and moulding in place, we used a Sharpie to mark the moulding to show where we would need to cut it for clearance.
09 Also during our test fit, we used a rubber mallet to tap the sidepipes onto the rear of the headers to verify fit. This allowed us to check the alignment of their mounting tabs and corresponding frame holes as well.
10 With that done, we dropped the headers to cut away the fiberglass they were contacting. This is the driver’s side, where we removed pretty much all the black portion, some of which is backed with metal.
11 Here’s the passenger side, showing where we trimmed the ’glass using an air-powered cutoff wheel and reciprocating saw. Since this area won’t be visible with the rocker moulding in place, we cut it straight across.
12 We used a Sharpie to mark the fore-and-aft locations where the pipes were contacting the rocker moulding, then used a measuring tape and straightedge to lay out the vertical dimensions of where the moulding needed to be cut.
13 After marking the moulding, I used a wheelie cutter to rough in the clearance cuts. I then dressed the cut with a handheld grinding wheel so its radius loosely matched the curve of the pipes.
14 Once Tray Walden had checked the locations of the frame holes (to make sure they lined up with the mounting tabs on the sidepipes), I hit the holes with a disc sander to remove the paint around them and prepare them for welding.
15 Walden welds in the threaded inserts, giving the pipes’ mounting bolts an easy attachment point.
16 Once the inserts were welded into place, I ground the welds down close to the frame to give the mounting tabs a flat surface against which to lie. I then sprayed on some black paint to protect against corrosion.
17 Since the pipes tend to hang “downhill” once they’ve been slipped onto the collector, we used a jack at the rear to slowly raise them into the correct position. This allowed the mounting tabs to be bolted into place in the now-threaded frame holes.
18 Here’s the mounting tab bolted into place on the frame—note how the body interferes with the bolt head. Since a socket can’t fit on it in this position, this is open-ended-wrench territory. Remember, all of this will be covered by the rocker moulding once it’s reinstalled.
19 Once we knew where the pipes would sit when bolted in place, Walden welded the hole for the O2-sensor bung into the passenger-side pipe. While not usually needed for carbureted applications, this sensor is a must for an EFI motor that runs on a closed-loop system.
20 The O2-sensor bung is shown after welding. Once installed, the sensor should sit roughly horizontal. Note that it’s located behind the bulged end that slips over the collector. Any farther forward, and the collector would need to be cut as well.
21 While different computers require different setups, the FAST XFI system we’re using only needs one sensor (shown here screwed into place). It was supplied along with the computer and wiring harness.
22A Most side-mount headers are patterned after the OK Kustom pipes found on historic racers such as this one.
22B The aggressive, bundle-of-snakes look imparted by the four primary pipes is perfectly in keeping with the theme of our project.