When it comes to adding a performance sound to a Fox body, it can be easy to get lost in the TONS of different kits out there. One name that has always come to mind is Flowmaster, and the famous sound and the 5-liter seem to go together like ice cream and chocolate. Now, Flowmaster has upped the ante, adding 409S Stainless Kits to their already impressive resume of products for the most popular muscle car in the world.

So when we had a Mustang GT in need of a cat back, we knew right where to turn. Enjoy this tech article and video.. and check out the sound and performance. We sure did, and we didn’t get arrested either!

We have been more than familiar with Flowmaster’s popular American Thunder Series Cat-Back Systems. These systems have been around for a while and have proven themselves as one of the top level kits on the market.

The upgraded 409S systems feature the same design that we have come to love, but in full stainless steel. “The new stainless steel kit for the Fox Body mustang delivers the same high quality sound and improved performance as the standard tired-and-true aluminized kit but with the added protection of the 10-year warranty,” says Cam Benty, VP of Marketing for Flowmaster, “We know a lot of these owners are hanging on to their cars for a number of years and the added warranty is a plus for them.”

The heart of the Flowmaster Fox cat-back we installed was the Flowmaster Super 44 Mufflers, part number 817116. “We decided to use the Super 44 mufflers because they are one of our most popular, high-end mufflers,” explained Benty. “They are very desirable and we found that people were leaving them on their daily drivers due to their sound,” explained Benty. For those who don’t know the 44 is the little bother to the 40 series muffler. While the 40 is a really nice deep sounding muffler, the 44 provides a great sound that Flowmaster claims is their most aggressive sounding street muffler.


Best of all, the Super 44 uses Flowmaster’s new Gen II Delta Flow technology. This design inside the muffler works to give your muscle car a deep rich tone on the outside, while working to minimize drone on the inside of the car.

Now before you think that this stainless steel kit is going to leave you exhaust looking link the kitchen sink, pay attention to this. Flowmaster uses a 409S grade of stainless steel. This grade shares the durability of the 304 style of steel that you might see elsewhere, but without the mirror like finish. “We decided to do a 409 vs. a 304 stainless due to the cost and need,” Benty explained, “The 409 is actually more durable than the 304.”

The Car – 1989 Mustang GT


The Fox needs no introduction, but what it does need is a face lift on it’s exhaust. The years spent out in the elements of the road beat the exhaust up to the point of failure. Aside from the large amount of rust covering the entire system, there were holes both before and after the mufflers. This gave the car that nice exhaust leak sound that screamed lame more than performance. Upon starting the GT, the chocked down 5 liter struggled to push through the restrictive OE mufflers. We knew this car deserved a better system, and Flowmaster came to it’s rescue just in time.

Install: 409S Stainless Fox Kit

The Fox Body Exhaust is simple to install, outside of rusted together tubing. We started by removing the bolts that secured the the exhaust to the to the X-Pipe. Because our exhaust was already junk, we decided to make things a little easy on ourselves and just cut the stock exhaust out with a sazall. Not a requirement, but it did makes things a little easier.


With all the old tubing and mufflers tossed into the dumpster, we unboxed all the new Flowmaster tubing. Right away we noticed a few major things that separate the new American Thunder system and the stock exhaust:

  • First, the material and construction. The stainless steel that the Flowmaster kit is constructed from looks great and the mandrel bends are much better versus the compound bends of the stock exhaust. And because the kit is bent from 409 stainless steel, it will be able to withstand the elements. Even Flowmaster thinks so, as they back this kit up with a 10 year warranty to prove it.
  • Our Flowmaster system upgrades the tubing size from the 2.25 inch diameter stock size, to a 2.5 inch. This increase in size really opens up the exhaust of these cars that were emissions choked from the factory. The relieving of back pressure also is the main reason for the increase of horsepower.


We started by hanging the two Flowmaster Super 44 mufflers in the stock hangers and supporting them by attaching them to the rear of the X-pipe. We left the bolts loose for now, as it makes fitting the rest of the system much easier.

On the rear of the system, there are only two pipes to hang. We started by snaking the first Flowmaster 409S pipe up and over the rear end. We were doing the install on our 4-post lift, and we found that if you use a jack and lift the body up as much as possible it makes the install much easier. This lengthens the distance between the rear end and the body, letting you have plenty of room to fit the rear pipes.


We adjusted the fit of the Flowmaster system until we were happy with the position of the tips under the rear bumper. Our system was a great fit. The Super 44 mufflers were where they were suppose to be, the tubing didn’t interfere with anything else on the car and the tips kept their distance from the bumper.

Driving & Sound Impressions

We pulled the Fox Body out of the shop, and out on the street with the new American Thunder Series cat-back. Even as we started the 5.0, we were pleased with the sound difference. Gone was the muffled, choked down sound that plagued this stocker. In it’s place now is a powerful sounding exhaust that just begs for more RPM.

Even with the car stopped and the T5 in neutral, when we brought the RPMs up the car sounded great. The nice deep sound from the Super 44 Mufflers echoed nicely throughout the stainless steel tubing. The real question would be once the car was moving, would we have the same thoughts?

We tested the sound of this system in a number of different scenarios to try and create as many different real world environments. First up was the standard 45 MPH drive. Inside the car, the exhaust tone was noticeably improved. Then, we punched it to 70 MPH. The Fox Body came back with a deep aggressive note. But when we backed down to cruising speed, the tone was still tame enough that we could carry on a conversation inside the car without yelling at each other.

Not bad. It’s the Flowmaster magic in an updated package, with Stainless to Boot. Sounds like a perfect Christmas gift to us.