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 first-gen 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 top-end 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 equal-length 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
|Hooker Headers 1967-’69 Camaro LT Swap Components For Detroit Speed Subframe And Suspension|
|70101356-RHKR||1 7/8-inch primary long-tube headers|
|70101358-RHKR||1 7/8-inch primary long-tube headers – brushed finish|
|70101359-RHKR||2-inch primary long-tube headers|
|70101360-RHKR||2-inch primary long-tube headers – brushed finish|
|70201318-RHKR||1 7/8-inch primary mid-length headers|
|70201319-RHKR||1 7/8-inch primary mid-length headers – brushed finish|
|705013156-RHKR||Exhaust for DSE subframe and suspension|
|705013157-RHKR||Exhaust with resonators for DSE subframe and suspension|
|705013161-RHKR||Exhaust for DSE subframe and suspension – brushed finish|
|705013162-RHKR||Exhaust with resonators for DSE subframe and suspension – brushed finish|
|71221024-HKR||Engine mount bracket kit|
Photos by Barry Kluczyk