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.
“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 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 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.
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.
“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.
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.
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.
Evil 8.5 And Header Construction
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.
“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.
“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
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.
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.
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.
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.