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Developing Headers for an LT4-Powered 1969 Camaro

Hustle And Flow: Mark Stielow’s LT4-powered Gunner Camaro is the prototype for all-new mid-length and long-tube headers

Barry Kluczyk Feb 21, 2018
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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
Part Number Description
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
71222017-HKR Transmission crossmember
71221024-HKR Engine mount bracket kit

002 LT4 Header Exhaust Prototype Install 2/24

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 solution. That’s where Holley’s Hooker Headers division stepped in, using the project to prototype LT swap headers for first-gen Camaros.

003 LT4 Header Exhaust Prototype Install 3/24

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 should, while giving a target point to aim the primary tubes.

004 LT4 Header Exhaust Prototype Install 4/24

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.

005 LT4 Header Exhaust Prototype Install 5/24

Hooker Headers’ Doug Marino holds up a curved length of 1 7/8-inch-diameter 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 Detroit Speed subframe.

006 LT4 Header Exhaust Prototype Install 6/24

Another length of tubing is cut to add length to the first primary tube. It is angled slightly to ensure clearance within the chassis.

007 LT4 Header Exhaust Prototype Install 7/24

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.

008 LT4 Header Exhaust Prototype Install 8/24

The first tack-welded 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 sufficient clearance around the spark plug wire.

009 LT4 Header Exhaust Prototype Install 9/24

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.

010 LT4 Header Exhaust Prototype Install 10/24

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 cylinder No. 4 to the far side makes them closer to the same length, which is ideal for optimal performance.

011 LT4 Header Exhaust Prototype Install 11/24

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 primary tube away from it, but doing so with enough clearance to prevent a heat soak problem—while also enabling the starter to be serviced (removed).

012 LT4 Header Exhaust Prototype Install 12/24

When the header’s design is mostly determined, a position for the oxygen sensor bung must also be accommodated.

013 LT4 Header Exhaust Prototype Install 13/24

Here, Marino confirms the final fitment and positioning of the headers. Mid-length and full-length headers were prototyped on the car, and this is one of the full-length headers, which will maximize horsepower output at higher rpm.

014 LT4 Header Exhaust Prototype Install 14/24

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.

015 LT4 Header Exhaust Prototype Install 15/24

For Stielow’s car, some careful finish-welding and finish-work with some steel wool on the tubes produces a gorgeous, premium brushed appearance.

016 LT4 Header Exhaust Prototype Install 16/24

Back at Hooker Headers’ production facility, the digitally scanned dimensions of the prototype headers are fed into a computer-controlled tubing bender, which produces perfectly accurate primary tubes by the score.

017 LT4 Header Exhaust Prototype Install 17/24

Fixtures hold the flange, primary tubes, and collector in place so they can be welded by Hooker’s craftsmen.

018 LT4 Header Exhaust Prototype Install 18/24

Finally, racks of new headers are prepped for shipping.

019 LT4 Header Exhaust Prototype Install 19/24

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 fit to the long-tube or mid-length headers and is offered with or without resonators—and with or without a brushed finish.

020 LT4 Header Exhaust Prototype Install 20/24

The flow tubes for the X-pipe wrap tightly around the contours of the new Hooker transmission crossmember designed to accommodate the T-56/T-56 Magnum transmission with the Detroit Speed subframe.

021 LT4 Header Exhaust Prototype Install 21/24

A pair of Hooker’s VR304 stainless steel mufflers complements the headers. They feature a straight-through flow design for minimal restriction. In fact, they can be mounted in either direction, which makes custom fitment easier. They each measure 14 inches long by 9 inches wide and 4 inches tall.

022 LT4 Header Exhaust Prototype Install 22/24

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 ground clearance.

023 LT4 Header Exhaust Prototype Install 23/24

The exhaust outlets are routed over the axle and as far away as possible from the fuel system’s components.

024 LT4 Header Exhaust Prototype Install 24/24

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 exhaust system is complete.

Photos by Barry Kluczyk

Sources

Scoggin-Dickey Parts Center
Lubbock, TX 79424
877-604-6057
http://www.sdparts.com
Holley Performance Products
Bowling Green, KY 42101
866-464-6553
http://www.holley.com
Detroit Speed
Mooresville, NC 28115
704-662-3272
www.detroitspeed.com
Sled Alley
586-630-0171
www.sledalley.com

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