One of the easiest horsepower upgrades you can do to your car is to take off the stock exhaust and open it up with aftermarket headers and a full exhaust system. On our pet Fox Body Mustang, more commonly known as “Project 666”, we did just that. Because of the fact that Foxes are possibly the most popular performance cars ever built, there’s an almost limitless variety of exhaust parts and complete systems available straight off the shelf for any combination of chassis, transmission, and engine. Even if you’ve gone insane and built a small-block Chevy powered convertible Fox with a Powerglide, chances are that somebody out there makes headers and pipes that will work. By comparison, our Windsor-powered notch is positively mainstream, so our priorities were quality, performance, and ease of installation.

To find those attributes, we looked no further than Kooks Custom Headers and Flowmaster exhaust. Kooks is best known for their amazing stainless headers, with some of the best quality and fit in the industry. Flowmaster needs no introduction, being synonymous with performance mufflers – they hooked us up with their Delta Force System that sounds as good as it flows. Both of these kits came with everything we needed to easily mount them to our Fox Body Mustang.

“Project 666” has a 550hp 408ci engine that runs on pump gas and is fully street legal. The carbureted 351W-based powerplant sports Edelbrock’s Victor Jr. heads, an assembled short block from Ford Performance Solutions, and Harland Sharp rockers.

The specific Kooks headers we selected were their 2” Custom Headers. These headers came with an adapter plate to fit onto our Edelbrock Victor Jr. heads. Because these long-tube headers put their collector flanges farther back under the car than the stock manifolds, we knew that we’d have to do some minor modifications to mate them up to the Flowmaster X-pipe in the kit we’d selected, but a few minutes with a tape measure showed that by leaving the downtubes off the front of the X-pipe and splicing in a pair of 3-bolt flanges, we’d be all set.


Though we have the luxury of a 4-post lift in our shop, thousands of gearheads have done swaps just like this in their driveways using nothing more than a good set of jackstands.

Kooks Stainless Steel Headers for Victor Jr. Heads #5012

When designing the headers, there are a lot of factors to consider. Besides the obvious stuff like primary tube length and layout or allowing for spark plug access and steering shaft clearance, the head and block combination has to be accounted for as well. “When the deck-to-block height fluctuates, so does the position of the headers,” George Rumore from Kooks Custom Headers explains. Also, the cylinder head plays a major role because the exhaust ports on the cylinder head need to match those of the headers, and tubing size needs to be appropriate for the flow capabilities of the heads. “We offer tube sizes from 1 3/4″ to 2″ primaries for the Victor Jr. heads,” added Rumore. Kooks Custom Headers design their headers around these factors, and they manufacture them from 304 stainless steel, chosen for its durability and strength.

Kooks Headers feature:

• Stainless 304 Construction
• Wide range of applications for different engine combinations
• Designed specifically for the 9.5 deck with Victor Jr. heads
• Available in 1.75″ and 2” primaries for this application
• 3” stainless merge collector

Flowmaster Delta Flow Header-Back Exhaust #17389

The exhaust we used on our Fox Body Mustang upgrade was the Flowmaster Delta Force System. This kit is intended as an ‘off-road only’ race system designed to mate directly to stock manifolds or “shorty” headers with ball flanges in the same position as the stock exhaust outlets. As we mentioned before, we had to modify the system a bit to mate with our Kooks long tube headers, but because the Flowmaster exhaust is designed to use the stock hanger mounting points, everything else fit perfectly, just as intended. The kit comes with all necessary hardware, eliminating extra trips to the auto parts store. The Flowmaster system includes over-axle plumbing and polished dual rear-exit tips, but since we knew our Moser 9-inch rearend housing would make clearances tight, we decided to forgo running the exhaust all the way to the rear of the car and went with turndowns right after the mufflers. In addition to solving the clearance problem, the dumped exhaust is also a few precious pounds lighter than full tailpipes.


There’s plenty of clearance under our Fox for the big 3-inch diameter Flowmaster mufflers, which will quiet down our 408′s bark without choking flow.

Instead of a standard H-pipe or X-pipe, this kit comes with the Scavenger X-pipe. This patented design uses a parallel tubing section with a “D” port between sides, giving improved flow and a deeper sound than the standard H-pipe. “When paired up with different parts, like the ones offered in the Delta Force Kit, you definitely will make more power over the H-pipe,” Dale Dotson from Flowmaster explains.


Our Flowmaster exhaust system uses dual Super 40 Series 2-chamber mufflers, with 3″ inlets and outlets.

The mufflers that are used in this application are the Super 40 series race mufflers. These mufflers are the best of both worlds. They provide the deep, powerful sound of the standard original 40 series but with the performance and low interior resonance offered by Flowmaster’s Delta Flow Technology. “The upside to the Super 40 over the Standard 40 series is the bigger muffler that handles more power and produces a deeper sound,” Dotson explains. The Super 40 mufflers were developed from the company’s race muffler lineup and feature a black oversized case that keeps the resonance down on the inside of the cab without restricting flow.

Flowmaster Delta Force Kit Benefits

• Deep aggressive tone
• Bolt-n-go system for stock manifolds or shorty headers
• Super 40 Series mufflers
• Scavenger X-pipe

Installation of the Kooks Long Tubes

Installing these headers on “Project 666” was a breeze. Other than clocking the starter to make clearance for the headers, they went on very easily. The supplied adapter plates bolt to the head first, using countersunk hex-head bolts, then the headers bolt to the plates. One tip when installing any kind of stainless or ceramic-coated headers – after they’re bolted up, wipe them down well with a grease-cutting solvent and let them dry before firing the car up for the first time, or your dirty fingerprints will permanently discolor the finish.


The Kooks headers come with adapter plates to convert from the stock in-line bolt pattern to a diagonal 2 3/4″ pattern.


To install the adapter plate, first insert the first and last hex head bolts before inserting and tightening down the middle bolts.


After the adapter plate is on, carefully start mounting the headers by bolting them to the adapter plate.

Installing the Flowmaster Header-Back

To mate the Flowmaster Scavenger X-pipe to our long-tube headers, all we had to do was mount three-bolt flanges to the front instead of the supplied downpipes to match up with the collector flanges.


After the Kooks Custom Headers were mounted, we attached the collector on the end with the supplied bolts. After that was done it was time to attach our Flowmaster exhaust.


To span the gap between the collectors and the Scavenger X-pipe, we spliced in a pair of 3-bolt flanges.


Once the flanges were bolted together, we positioned the X-pipe using the supplied hangers and the factory mounting points. Flowmaster provides clamps to secure the pieces of their exhaust system together, but recommends welding the joints if possible, once everything is installed and adjusted properly.


Next, it was time to hang the Super 40 mufflers. We installed the ring clamps to the end of the Scavenger X-pipe and then slipped on the mufflers. Because we wanted to be able to adjust the mufflers to fine tune their position, we chose not to tighten the clamps all the way down yet.


After we installed the hangers on the rear of the mufflers, we were able to get the exhaust positioned, then tightened all the hangers, clamps, and bolts. All there was left to do was install the turndown tips.

In Order to Form a More Perfect Union…

We couldn’t have chosen a better pairing for Project 666 – The Kooks long-tube headers and Flowmaster exhaust are perfectly suited to the deep-breathing needs of our 408, and fit like a custom-built system though they’re both off-the-shelf part numbers. Installation really couldn’t have been much easier, and the sound is awesome, without being so loud that we’ll run afoul of our local drag strip’s noise police.