Going With The Flow: Installing Flowmaster Muffler’s American Thunder Exhaust

What’s the first thing you notice on a car? A shiny red paint job? Or a set of trick wheels? For most of us, before we can even see a car, we pick up on the exhaust sound. We can usually tell if it’s a four-cylinder or a big-block just by the note the exhaust makes. The exhaust is what gets us to drop the wrench and look into the street to see what’s motoring on by. Or when you’re at a show and do an abrupt 180 degree turn to see the monster that’s tickling your inner ear. Nothing looks as good as hearing a motor test its limits going down the track!

Recently we were working on a 1965 Chevelle that’s equipped with a 500 horsepower LS2. The problem was when you were standing next to the car, it sounded like a 283 with a two-barrel carburetor. Sure, that made for a great sleeper, but this Chevelle wants to make a statement when it pulls into a parking lot, but also be great for cruising coast-to-coast. The cheap exhaust on the car wasn’t up to the task, so we called one of the leaders in exhaust, Flowmaster Mufflers. They make much more than just mufflers, and we were looking for a complete exhaust kit.

Making Calls

Catching up with Mark Emerson, we told him that the car was a 1965 Chevelle, with an LS2. We asked him what he would suggest. We knew we wanted something that would be heard, have a great exhaust note, but not be too loud inside the cabin. The last thing we want is to have our ears ringing after going down to the gas station.

“I would suggest our new American Thunder Kit, part number 817412. This kit is a refined kit based off an older version. We wanted the kit to be a little more specific to the 1964-1967 A-Bodies, and fit perfectly. They are made out of stainless steel and have a limited lifetime warranty on them. They come complete and include an updated muffler, our Super 40 Series; this muffler offers better bass tones and even less drone than the previous muffler thanks to it’s larger casing and Delta Flow internal design,” said Emerson.

We noticed that this kit comes as an H-pipe, not as an X-pipe or straight exhaust. Emerson filled us in on the details, “In reality the H-pipes and X-pipes offer about the same amount of performance on most street applications. Their purpose is to balance the pressure differences in the cylinder banks and to help cancel out drone. In some racing applications we have seen X-pipes out perform H-pipes, but for most applications the H-pipe is the way to go to maintain a deep V8 rumble. Plus it’s a much easier fit under the car.”

We were sold on that kit, but had to figure out how to attach the kit to our existing headers. “You could use a traditional three-bolt flange, but we have a new ball and socket style( part number 15925) that works much better. They offer more ground clearance, no gasket is needed and on top of that are more forgiving if the exhaust isn’t lined up perfectly,” continued Emerson. Since this A-body has been lowered a few inches, every bit of ground clearance we could get was important. The ball and socket has one side that has a radius slightly towards in side of the pipe, while the other half has a radius towards the outside. This allows the two pipes to push against each other, forming an air tight seal. This is the same principal used with an AN fitting and it’s angled fitment.

With our kit in the mail, we headed to the garage to start the install.

Removal and Installation

The first step we took was to get rid of the old exhaust. Removing a few bolts and with a sawzall, we had the old exhaust out of the way in less than an hour. We then took the Flowmaster kit out of the box, and laid everything out so we knew what goes where.

Installing the kit was a simple one-man job. Basic hand tools and some sort of power tool to cut the exhaust tubing, was all that’s needed, although we highly recommend a welder as well. Following the directions we started by mounting the brackets that hold the back half of the mufflers to the frame. This mounted to a hole already in the Chevelle frame, and came with the correct nuts and bolts for the job. With the mufflers hanging in their brackets,we installed the forward exhaust pipes that have the H-pipe already welded in. We used a small rack under our lift with some wood to hold this piece up and make sure it was at the right angle and tucked nicely under the car.

From here we installed the back half of the exhaust system. This is where we ran into a bit of an issue. This particular Chevelle had an aftermarket, over-sized Rick’s tank. When the pipes come down from going over the axle, they shoot out to under the rear quarter panel. However, the pipes were rubbing against the exhaust and the tire. We stopped and thought about it for a minute and realized that if we trimmed the length on that pipe, it moved it closer to the car and left plenty of clearance around the gas tank. The way the kit was designed and where it comes apart leaves a lot of room for making changes, like in this case.

The last pieces of pipe to install were the ball and socket connectors that connect to the H-pipe. Our headers were about a half-inch wider on each side than the Flowmaster kit, but we easily got it all together with no leaks thanks to the ball and socket connector.

With everything together we went around double checking clearances, and tightening all the clamps down from the front to rear. After that, we grabbed the ol’ buzzbox and fully welded each connection. This ensures we will never have any leaks, rattles, or problems in the future.

The absolute last step to installation was to trim up the exhaust tips. On the 1964-1967 A-Body cars, the exhaust exits between the rear tires and bumper, under the quarter panel. These pipes from Flowmaster are purposely left long, so you can cut to fit the right length. We’re always a fan of exhaust being heard not seen, so we trimmed the pipes flush with the quarter panel.

With the exhaust 100% finished, the only thing left to do was fire her up. The exhaust sounds amazing – there is plenty of rumble and not any annoying drone. We like that it allows the lumpy cam to come through, but not make us deaf after a few minutes. The whole process took about one days time, but could have gone quicker with another set of hands. The only tools needed are basic hand tools, a sawzall or band saw, and a welder. An easy days work for a huge improvement!

Hop on Flowmaster’s website to see their full line of products, which is a lot more than just mufflers! If you prefer, here’s a video from Flowmaster of the installation of the old version on a 1966 Chevelle. The installation is exactly the same as our kit: