HUSTLE AND FLOW
Mark Stielow’s LT4-powered Gunner Camaro is the prototype for all-new mid-length and long-tube headers
Mark Stielow’s LT4-powered Gunner Camaro is the prototype for all-new mid-length and long-tube headers
It has been a while since we’ve visited the construction of Mark Stielow’s latest Camaro project. We’ve covered the build over the past year and you undoubtedly saw images of its debut at the 2017 SEMA Show in Holley’s booth, and on the cover of Chevy High Performance’s April 2018 issue.
Holley was a big contributor to the car, from the supercharged LT4 crate engine’s mounts to its exhaust system. In fact, it’s the exhaust system we’re addressing with this installment. Holley’s Hooker Headers division used the firstgen Camaro to develop LT swap headers and we were there to document the process, following how a collection of 304 stainless steel tubing was carefully cut, shaped, and welded by the company’s exhaust engineer, Doug Marino, to create a custom set of headers that ultimately served as the models for Hooker’s production headers.
Hooker prototyped mid-length and long-tube headers for the car, designed specifically for its Detroit Speed front subframe. The company has also designed LT swap headers for first-gen Camaros retaining the factory-original subframe.
Additionally, Marino fabricated the rest of the car’s exhaust system while on location at Detroit-area Sled Alley, where the rest of the car’s fabrication was handled.
“The LT engine swap is getting more popular every day and it was important to develop headers for not only the stock chassis but the Detroit Speed subframe that so many builders are using for track-capable Pro Touring machines such as Mark’s Camaro,” says Marino. “In fact, evaluating the intended use for the vehicle, from basic street performance
to dual-purpose street and racing, is the first step in our development process for every new header system.”
While we all know the purpose and function of exhaust headers, it’s worth revisiting the importance of their design for optimal engine performance. Exhaust flow increases with higher rpm and higher horsepower levels, increasing the importance of quickly and efficiently drawing exhaust gases out of the cylinders. And because combustion byproducts don’t burn a second time, any residual exhaust gases left in the cylinder during the next combustion cycle reduces cylinder filling and can contaminate the incoming air/fuel charge. The bottom line is it reduces performance and efficiency.
In the old days, it was a given that swapping the factory’s heavy, restrictive, cast-iron exhaust manifolds for a pair of high-flow headers was a sure way to uncork a few horsepower—and more when additional engine modifications were made. These days, the OEMs have gotten much better at designing
high-flow, low-restriction exhaust manifolds.
Even with a high-flow factory exhaust manifold, there are compromises, mostly to accommodate underhood packaging and noise level targets.
In a Pro Touring project such as Stielow’s Camaro, all those factory compromises go right out the window, allowing the exhaust system to be optimized for ultimate performance. That generally boils down to two factors: the diameter and length of the primary tubes.
Generally, a longer primary tube will promote increased torque lower in the rpm range by maintaining exhaust velocity and promoting low-speed scavenging. The shorter, larger primaries will enhance high-rpm scavenging. Mid-range and topend power will benefit from greater volumetric efficiency and better inertial scavenging at higher engine speeds, when the exhaust volume is increasing rapidly. That will shift the torque curve higher in the rpm range and low-speed torque will suffer due to poor inertial scavenging at low engine speeds.
Mid-length headers are a packaging solution, particularly for vehicles employing a dry-sump oiling system, which require more room for the oil lines. Compared to naturally aspirated combinations, supercharged engines do not seem to suffer as much from the lack of primary length, which makes for a good trade-off.
Scavenging—the practice of using the negative pressure generated near the exhaust valve to draw exhaust gas away from the cylinder—is instrumental in an efficient header’s design and has everything to do with
primary tube length and size.
“Ideally, equal-length headers, where each primary tube is the same length, offer the best scavenging,” says Marino. “When the tubes are the ‘correct’ and equal length, or very close to, the timing of the exhaust pulses between the tubes is optimized to create greater, more efficient spillover signal at the collector. Wave tuning is more accurate and the engine becomes more balanced.”
The economics of marketing affordable headers, however, typically means compromise. In other words, most headers out there aren’t equal length, including the LT swap headers developed on Stielow’s car.
“The realities of developing and manufacturing truly equallength headers makes it a pricey proposition,” says Marino. “Our experience and testing has shown that when the tubes are very close to being equal in length, as they are with these new LT swap headers, the performance difference between them and true equal-length headers isn’t significant for most applications.”
To put it another way, the performance delta between them ain’t nothing compared to the cost delta, so almost- equal length offers the best cost/performance balance.
Marino spent the better part of a week at Sled Alley crafting the headers and exhaust system on Stielow’s Camaro. After the prototype headers were completed, he returned with them to Holley’s headquarters in Bowling Green, Kentucky, where they were laser-scanned in preparation for manufacturing. They’ll be on the market when you read this.
The rest of the custom exhaust system’s construction is detailed in the accompanying photos—and we’ll spare you the pun of calling it an exhaustive look at header construction. You’re welcome. CHP
17 | Finally, racks of new headers are prepped for shipping.
15 | Back at Hooker Headers’ production facility, the digitally scanned dimensions of the prototype headers are fed into a computercontrolled tubing bender, which produces perfectly accurate primary tubes by the score.
13 | Here’s a look at the finished prototype headers, which served as the models for Hooker Headers’ new LT swap headers. Along with versions designed to fit the Detroit Speed subframe, they’re also available for the stock first-gen Camaro subframe.
16 | Fixtures hold the flange, primary tubes, and collector in place so they can be welded by Hooker’s craftsmen.
14 | For Stielow’s car, some careful finish-welding and finish-work with some steel wool on the tubes produces a gorgeous, premium brushed appearance.
09 | Here’s the primary tube for cylinder No. 4 mocked up. Note how it pushes farther away from the flange to position it at the far side of the collector. Routing the tube from cylinder No. 2 to the near-side position on the collector and the tube for...
07 | The first tackwelded primary tube is shown here mocked up on the engine and routed to the collector. Note how an additional length was added to the tube for cylinder No. 2 at the flange, pushing the primary tube out far enough to provide...
10 | The steering linkage and starter are the two biggest obstacles when it comes to header design. In some cases there’s no choice but to run the steering gear through the tubes. When it comes to the starter, the challenge is not simply keeping a...
08 | The LT swap headers feature a conventional four-into-one collector with a 3-inch outlet. Some mistakenly call this a merge collector, but a merge collector necks down after the tubes come together and widens at the rear to create a venturi effect.
11 | When the header’s design is mostly determined, a position for the oxygen sensor bung must also be accommodated.
01 | Mark Stielow’s Gunner 1969 Camaro was built with a Chevrolet Performance LT4 crate engine obtained from Scoggin-Dickey Parts Center and installed on a Detroit Speed front subframe—a great combination that didn’t have an off-the-shelf exhaust...
02 | Rather than starting at the exhaust ports on the cylinder heads, the headers’ design actually begins at the end point: the collectors. Hooker Headers works from the rear of the vehicle forward to ensure the rest of the exhaust system fits as it...
05 | Another length of tubing is cut to add length to the first primary tube. It is angled slightly to ensure clearance within the chassis.
04 | Hooker Headers’ Doug Marino holds up a curved length of 1 7/8-inchdiameter 304 stainless steel tubing to the exhaust port outlet on the flange for cylinder No. 2 as he begins to gauge the general routing requirements for the LT engine in the...
03 | Building the headers on the car starts with bolting on the same 3/8-inch, water jet-cut, stainless steel flange that will be used on the production headers.
06 | The additional length is tack-welded to the first length of tubing. At this point, it’s only necessary to hold the two pieces together to check the overall fit in the chassis, so no finish-welding here.
21 | Here’s a look at the exhaust system flowing to the rear of the car. Ideally, it would have been great if the driveshaft wasn’t trapped by the exhaust system, but it was the only way to keep the system tucked up in the chassis to maintain adequate...
18 | The rest of the 3-inch exhaust system on Stielow’s car consists of a mix of previous first-gen LS swap exhaust components developed on his Jackass Camaro, and the new X-pipe that will complete the LT swap exhaust system. The exhaust is a direct...
20 | A pair of Hooker’s VR304 stainless steel mufflers complements the headers. They feature a straightthrough flow design for minimal restriction. In fact, they can be mounted in either direction, which makes custom fitment easier. They each measure...
23 | Like the headers, the exhaust outlets are made of 304 stainless steel and after the tips were trimmed flush with the bottom of the body, a little more treatment with steel wool produced the finished, brushed appearance. From stem to stern, the...
22 | The exhaust outlets are routed over the axle and as far away as possible from the fuel system’s components.