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The Kooks Custom Headers team is well-versed when it comes to fitting exhaust headers to swap the small-block Chevrolet—and other engines—into the Fox-body Mustang. The process is not as simple as taking an existing Fox-body header for a Ford engine and retrofitting it with SBC flanges; there are many other issues which need to be taken into account, such as vehicle body dimensions, other engine accessories, engine deck height, and more.

When we needed to install the supercharged 427 cubic-inch small-block Chevy engine into our Evil 8.5 project car, there were no headers on the market to fit the car that could be considered off-the-shelf products, so we entered into a discussion with Chris Clark, Kooks’ Vice President of Sales and Marketing, about swap headers and what’s involved for the company to develop these products.

Some of the components in our Kooks Race Header Builder Kit.

Some of the components in our Kooks Race Header Builder Kit. The kit includes a variety of J-bends, short bends, and collectors set to the specification required.

“As a manufacturer we look for the most popular swaps, as we are trying to reach the largest target audience,” says Clark.

“So we investigate which of these pockets of consumers are doing what. What K-members are they using, what motor mount combinations, control arm set-ups, starter location, whether the cars are using air conditioning, which cross member, and of course which engine with which heads.”

The parts awaiting fabricator Greg Holman's deft touch.

The parts awaiting fabricator Greg Holman’s deft touch.

The Drawing Board

The company has invested in a variety of different ways to develop its products, from the most advanced three-dimensional simulation programs and digitizing machinery to the simplest—building it on the car, with the selected components in place.

“Since we have a Fox-body house mock-up car and about every engine imaginable, we can easily swap engines and K-members into and out of this vehicle. In this case, it takes us much less time to swap an engine as it does to digitize said engine and combo each time. If we’re planning on doing a very large production run, we then will do the swap and scan it afterwards before creating fixturing,” says Clark.

The company is able to mock up a three-dimensional view of the header in the engine bay of the vehicle before the first tube is ever bent. This ability greatly lessens the expense and development time required to finalize design.

The company is able to mock up a three-dimensional view of the header in the engine bay of the vehicle before the first tube is ever bent. This ability greatly lessens the expense and development time required to finalize design.

The scan is created by a Kooks technician, using a device called a Baces 3D Coordinate Measuring Machine Arm, which measures and digitizes the dimensions for a particular combination with respect to the engine bay, the engine itself, the K-member, and any other components which may be in the way of the exhaust.

With respect to the small-block Chevrolet/Fox Mustang combination, the guesswork was done a long time ago as racers have been mixing the two since the chassis was recognized as a player on the dragstrip. We chose local fabricator Greg Holman of REF Unlimited to create headers for our specific application with the engine installed in the car.

Once the measurements have been made, the Kooks product designed can tweak and adjust as necessary to create the required product.

Once the measurements have been made, the Kooks product designer can tweak and adjust as necessary to create the required product.

Everything that can possibly be an issue, or challenge, is taken into account during the design process. As the Kooks team has over five decades of header and exhaust fabrication experience, they have developed a technique to ensure that each possible header configuration will work with the largest variety of tubing sizes.

Header flanges are from Schoenfeld and are CNC-machined 5/16-inch thick.

It’s a tight fit down in there. Building the headers in the car, as in the Evil 8.5 application, ensured us that we could sweep the collector out by the front tires.

“We take everything into account. We know going into the job if we are trying to make a big-block Chevrolet-swapped Fox-body header for the standard bore, that a 5-inch bore-spaced engine will require a completely different header. In general, we normally create each system as its own beast, and try to take nothing for granted,” says Clark.

“When we are developing the initial prototype we will always use the largest size tube that we think we are ever going to offer. So if we are planning on doing an LS swap, let’s say with a 1 3/4-inch or 1 7/8-inch primary offering, we would most likely use 2-inch tubing for our template. With using the bigger tubes we have those answers in our design process initially.”

Clark says that there are always tradeoffs when it comes to header design, especially in a full-bodied car, because parts like strut towers, A-arms, and other body and engine components can create fitment challenges.

Even though Evil 8.5 is not a full-bodied street car, there are still plenty of challenges to getting the exhaust system to clear all obstacles.

Even though Evil 8.5 is not a full-bodied street car, there are still plenty of challenges to getting the exhaust system to clear all obstacles.

The Spike

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Some years back, it was discovered that a spike welded to the junction of the tubes inside the collector made a large difference with respect to performance and scavenging. By smoothing the air as it comes together in the collector and preventing the exhaust from one cylinder from “running into” the airflow from another, it helps to reduce turbulence, improve the scavenging effect and boost horsepower in the process.

“The spike is what ties the header all together. The spike is built with a beveled rotation that forces the exhaust gas to mix as it exits the cylinder heads creating a greater vortex of air. The length of the spike is really going to be due to the length of the collector. You will need a wider spike if the diameters of the tubes are larger, so you don’t have any exhaust gas leaking,” says Clark.

“When we are doing production, we listen to our customers up front. If the majority of them are running eighth- or quarter-mile drag racing on N/A LS applications, for example, we will build a header that has a longer primary to help with keeping the car in the torque band, while conversely, someone who has a big block with a blower just needs zoomies to expel the spent exhaust gases,” he says.

Tubing dimensions—including length and diameter—are the most important variable which can be controlled for a specific application. Many feel that it’s the length of the tubing, but for an odd-fire V8 engine as in this application, tubing diameter is far more critical than length. Specify the tubing diameter correctly and the engine will make optimal power, but under- or over-size it improperly, and power will suffer, in some cases greatly. Primary tubes which are too small for an application—especially on a boosted engine—will hinder flow, while tubes that are too large will allow the exhaust gas velocity to slow down and hurt performance in the process.

Although a smaller tube will flow less volume than a larger tube, the velocity will be high, and until an engine reaches the RPM level where the smaller tube simply can’t evacuate the engine, the smaller header will be more efficient.

“We’re always playing with tube lengths and this is a question we get all the time. What it really comes down to, is what application is this being used for. Engine size, weight, type of racing or driving, and RPM band all play into selecting the tube size. If you have a max-effort, all-out engine, it makes sense to consult with us, so we can spec the ideal header for your application,” says Clark.

Here Holman tacks tubing sections together.

Here Holman tacks tubing sections together.

Evil 8.5 And Header Construction

Crossbreeding

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Dragzine’s Evil 8.5 Ford Mustang project—despite being a fine example of what the Fox-body Mustang can be as a drag car—does not rely on Ford power for motivation. Instead, the performance is based around a Brodix BD2300-headed 427 cubic-inch small-block Chevrolet engine designed, assembled, and built by Steve Morris Engines. It’s pumped up by one of Vortech‘s Xi-Billet centrifugal superchargers.

At the end of last year, Evil 8.5’s engine hammered the needle of the Steve Morris Engines dynamometer by producing an outstanding 1,834.4 horsepower at 7,500 rpm and 1,383.4 lb-ft of torque at 6,500 rpm with the Vortech Xi supercharger onboard.

“This blower is extremely aggressive in the lower parts of the RPM range. On this particular motor that Steve Morris built for Evil 8.5, it will make a lot of boost in less than a second. That’s what really accelerates the car, and that’s where you get those great 60- and 330-foot incrementals,” says Vortech’s Lance Keck.

With this in mind, getting the air out of the engine and shoved into the atmosphere becomes a critical challenge, one which Kooks was up to the task of solving after a bit of research. We discussed the concept of using straight-diameter header tubes, stepped header tubes, and various collector diameters to achieve the goal most efficiently. Ultimately, we settled on a 2-1/8-inch straight primary dimension to go with a 4-inch collector, and since Kooks did not offer a shelf-stock header for our application, we chose to order the Race Header Builder kit [Kooks PN WK2880 with flanges, $899.99].

“For this application, we wanted to make sure there was enough diameter to expel the gases, while fitting in the tight envelope that is the Fox-body chassis. For the blower cars, a step does not always mean you’re going to pick up power. Again, it’s all application-specific,” says Clark.

Here's a typical set of small-block Chevy/Fox Mustang swap headers, with 2-inch primaries and a 3.5-inch collector.

Here’s a typical set of small-block Chevy/Fox Mustang swap headers for use with 18-degree heads; they feature 2-inch primaries and a 3.5-inch collector.

“If you have a max-effort all-out engine, it makes sense to consult with us, so we can spec the ideal header for your application,” – Chris Clark, Kooks Custom Exhaust

One rule of thumb is to take the primary tubing diameter and multiply it by 1.8 to determine collector diameter—in our case 2.125-inch—and then select the nearest upsized tubing diameter for the collector dimension. Our calculation becomes 3.825, so we selected the 4.000-inch collector.

With respect to collector diameter, Clark says that it generally depends on the particular application, and the company sizes the collector appropriately given usage requirements and performance demands from the customer.

“Headers are not miracle workers; they are hard parts which are designed to best accommodate and optimize whichever application they are being installed on. In general, you have to think that a collector has four tubes going into it at once, so you have to make sure you have a large enough collector to accommodate the diameter of those tubes. Typically, each size tube will have 2-3 collectors that will fit the application. For 2-inch pipes, you can use a 3-inch, 3.5-inch, or 4-inch collector depending upon the engine’s requirements. Any smaller then that won’t fit, and any larger will not seal,” he explains.

"Have tools, will travel..." REF Unlimited is based in Kingman, Arizona, but Greg Holman has established himself as the premier go-to guy when it comes to on-car header fabrication on the West Coast.

“Have tools, will travel…” REF Unlimited is based in Kingman, Arizona, but Greg Holman has established himself as the premier go-to guy when it comes to on-car header fabrication on the West Coast.

As Kooks has been in the header manufacturing business much longer than the Fox Mustang has been used in drag applications, it’s only fair to assume that the company has a monstrous catalog of header choices for the platform. In fact, they offer 80 distinct header designs for the Fox Mustang—too many part numbers to list here.

There are header designs for the small- and big-block Ford, Ford Coyote, older 4.6-liter Modular engines, LS-based engines, the small-block Chevrolet as we’re using in Evil 8.5, and big-block Chevrolet, all offered with a variety of flange styles, tubing diameters and collector sizes to satisfy a wide range of customers. In addition, the legendary Kooks fitment is offered with a choice of 18-gauge mild steel or 304 stainless steel to appeal to a wide range of budgets.

Measure three times, cut once.

Measure three times, cut once.

In Conclusion

The science of header design and manufacturing can be explained in two words: it depends. The powerplant combination, its airflow requirements, and the specifics of how each particular engine fits into the chassis where it is to be installed all must be taken into account during the design process.

The Power Automedia TIG welder got a workout during the construction of the headers for Evil 8.5; the end result was well worth the effort.

The Power Automedia TIG welder got a workout during the construction of the headers for Evil 8.5; the end result was well worth the effort.

With their modern facility, equipment, and decades of manufacturing and design expertise, the Kooks team is well-positioned to create the ideal product for just about any application. Evil 8.5 hit the track for the first time earlier this year in the NMCA West Outlaw 8.5; in its first weekend of competition, the car turned in a best pass of 5.21 at 144 mph—a solid showing for the first time, out, and an encouraging performance to build upon for the future.