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Installing Flowmaster's American Thunder Dual Exhaust System For 1964-1972 GM A-Bodies

By: Jeff Smith | 04/15/2021 < Back to Motor Life Home
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Exhaust systems are often lower on the wish list of items for a performance car. The exhaust is hidden under the car and many enthusiasts don’t realize how much power may be hiding under all that sheet metal. Sure, headers help, but a well-planned and executed exhaust system can also offer performance advantages well past the sound you desire.


Flowmaster has recently upgraded its American Thunder series of exhaust systems with some slightly redesigned pipe layouts and improved exhaust hangers that warranted our attention. We decided our 1966 Chevrolet Chevelle needed a new system so we ordered one of the American Thunder kits complete with the Super 40 mufflers. This same kit can be ordered without the mufflers if you already have the mufflers you need. Our buddy Scott Gillman helped us with this installation and his fabrication skills were greatly appreciated.

One major reason this is such a nice system is that all the bends are made with a mandrel that prevents kinking the pipe. This produces a much smoother flowing system. Kinks in the pipe reduce the overall inside diameter and affect flow. We wanted to use the 70-series mufflers but with the body not yet in place on our chassis, the longer mufflers might be a tight fit so we used the Super 40s. It’s possible we could make these larger mufflers work but this will require modifications to the H-pipe assembly.


The American Thunder kit comes with all the hangers and clamps you’ll need but we also wanted to add a couple of other items necessary to complete the installation. For headers, we chose a set of ceramic coated Hooker Super Competition 1¾-inch primary length headers. We discovered that we needed to clearance the headers in order to clear a set of ARP head bolts, and we also had to tweak a couple of tubes in order to clear a brake line on the passenger side.

We later tested fit a set of 2.0-inch Hooker Competition headers (PN 2213HKR) that seemed to fit a little better that appeared to require much less clearancing. We originally chose the Super Competition headers because of their smaller primary tube diameter to work with our smaller displacement 396ci big-block.

There are a few important installation details that are worth mentioning for first-time installers. The first point of interest is Flowmaster’s use of band style clamps instead of the more common crimp clamps. The band clamps offer the potential to more easily disassemble the system at a later date. Crimp clamps make this nearly impossible without destroying one or both of the pipes/mufflers.

One item that we thought could be better on the Hooker header collector flanges is the collector flange is fully welded rather than a floating flange. When welded, the third bolt is installed on the bottom. This part of the flange may likely be the lowest part of the car and if so, will tend to drag on speed bumps and steep driveways. We would have preferred to have the third bolt facing upward, closer to the floor to allow more road clearance.


There are a few important installation details that are worth mentioning for first-time installers. The first point of interest is Flowmaster’s use of band style clamps instead of the more common crimp clamps. The band clamps offer the potential to more easily disassemble the system at a later date. Crimp clamps make this nearly impossible without destroying one or both of the pipes/mufflers.

Certain sections such as the leading edges of the H-pipe are cut to allow these band clamps to squeeze the pipe to create a positive seal. In other cases, where a pipe is expanded to allow another pipe to fit inside, these will require a crimp style clamp to prevent leaks. Another optional band clamp is Flowmaster’s “TORCA” style band clamp that allows the two tubes to butt rather than overlap.


Another option you might consider is Flowmaster’s ball flange conversion kits. These kits are intended to replace the three-bolt gasket style header collector flange with a spherical ball flange that requires no gasket. Each kit comes with two pairs of fittings to convert two collectors. This will require fabrication and professional welding to install.

Once we read through the instructions, the installation only required about two hours of effort, including a trip to the muffler shop to have our header collectors expanded for a slip fit. Our installation is not actually complete yet until the body is fitted to the chassis, and the fuel tank is installed. Once that is accomplished, we can make the final adjustments to ensure the system fits as tightly to the body as possible to enhance the Chevelle’s appearance.


Chevelle exhaust pipe cut

Gillman cut this pipe long to allow us to creep up on the length. Cutting too short is something you should try to avoid. We used a portable band saw to make a clean, quick cut and then deburred the sharp edges with a die grinder and 60-grit sanding roll.

Chevelle exhaust tube measurement

With the rear length set to align the front, we marked the position of the tube to allow sufficient length to engage the collector. We’ve removed the collector and slipped the pipe inside the header to determine the first rough length to cut

Chevelle exhaust length check

Here’s the left or driver side checked for length to the reducer.


Chevelle exhaust reducers

We took our reducers to a local muffler shop to have the collectors expanded to allow the lead-down pipes to fit inside the collectors. The shop charged us $10. This connection will likely be welded.


Chevelle exhaust locking rings

Flowmaster also supplies these 7/16-inch locking rings that slide over the muffler hanger round pins to keep the hangers in place.


Chevelle exhaust Ball Flanges

Flowmaster makes these slick ball flanges for header collectors as an option. The ball end is welded to the collector while the female side is welded to the exhaust pipe. This eliminates the need for a collector gasket and offers a more leak-free seal.


Chevelle exhaust TORCA clamps

Hooker’s TORCA-style band clamp allows the installer to butt adjoining tubes together and clamp them in place.


We assembled the entire system at first only loosely clamped so we could judge for clearance around the mufflers. Obviously the body is not in place so we will have to wait to do the final adjustments once the body is installed.


Parts List

ComponentsPart Number
Hooker Ceramic 1¾-inch Headers2118-1HKR
Hooker 3-inch to 2¾-inch Pipe11040HKR
American Thunder Exhaust Kit, 2½-inch, No Mufflers17107
American Thunder Exhaust Kit, 2½-inch, Super 40 Mufflers17119
American Thunder Exhaust Kit, 3-inch, Super 40 Mufflers17202
American Thunder Exhaust Kit, 2½-inch, Stainless817412
Flowmaster 70-series Mufflers, 2½-inch, Full Offset52573
Flowmaster Flanges, 3-inch Collector, 2½-inch Exhaust15925
Flowmaster Flanges, 3-inch Collector, 3-inch Exhaust15930FM
Flowmaster Flanges, 3½-inch Collector, 3-inch Exhaust15923
Hooker TORCA 2½-inch Band Clamp, Stainless41172HKR
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