Flowmaster’s New GM 6.6L HD Direct-Fit Muffler Adds Power And Rumble

The 6.6-liter gas V8 in the new General Motors (GM) HD trucks is a powerhouse, but you’d hardly know it from the sound of the stock exhaust. For owners who want to hear that V8 without waking up the entire neighborhood, Flowmaster just dropped a new FlowFX direct-fit muffler. It’s a simple, bolt-in replacement for the massive factory muffler (P/N 718155) that adds a healthy rumble and a bit of real-world power to 2020–2025 GM 2500HD and 3500HD trucks.

A Deeper, Drone-Free Sound

The FlowFX series is Flowmaster’s answer for people who want a great sound without being obnoxious. It’s deeper and more aggressive than stock, but it’s not as wild as their Outlaw or American Thunder mufflers. The most important part for a truck you actually drive is that the “straight-through” design was specifically engineered to kill any annoying drone at highway speeds. So you get the noise when you want it, and quiet when you’re just cruising down the road.

The Flowmaster FlowFX Direct-Fit Muffler Advantage

This muffler isn’t just for noise; it actually makes a bit more power. Flowmaster’s dyno testing showed it’s good for an extra seven horsepower and five lb-ft of torque at the wheels. Flowmaster builds the whole thing from tough 409 stainless steel and backs it with a Lifetime Limited Warranty, so it can handle whatever you and the weather can throw at it.

Bolt-On Simplicity for the Driveway Mechanic

The best part about this direct-fit muffler is that it’s a true driveway-friendly job. Flowmaster designed it to be a simple bolt-off, bolt-on swap that uses all the original hangers and connections from your factory exhaust system. There’s no cutting or welding involved, and it comes with all the hardware and instructions you need. You just unbolt the huge stock muffler, bolt the new Flowmaster in its place, and you’re done.

Finding Your Truck’s Voice

Not everyone is looking for the loudest possible exhaust for their truck. Sometimes you just want to adjust the tone to something that sounds like a proper V8. Flowmaster’s different muffler series lets you pick your volume, and this new FlowFX is the perfect option to give your GM HD a deep, powerful sound without making your ears ring on the highway. It’s all about finding that perfect, drone-free voice for your truck.