Deciding between a 4 inch vs 5 inch exhaust 6.7 cummins setup is one of those classic truck owner debates that usually ends with someone saying "bigger is better" while someone else argues about backpressure and drone. If you've spent any time on the forums or scrolling through diesel groups, you know the struggle. Both sizes have their place on a Ram truck, but the right choice really depends on what you're actually doing with the pickup every day. It's not just about which one looks cooler sticking out from under the quarter panel; it's about how it sounds when you're pulling a grade and how much clearance you're willing to sacrifice.
The Sound Difference: Tone and Volume
Let's get the most obvious thing out of the way first: the sound. There is a very distinct tonal difference between these two pipe diameters. A 4-inch system on a 6.7 Cummins is going to give you a very crisp, traditional diesel sound. It's definitely louder than stock, but it retains a certain "tightness" to the exhaust note. You get that signature inline-six whistle and a bit of bark when you step on it, but it's generally more manageable for a daily driver.
On the other hand, the 5-inch system is all about that deep, low-frequency rumble. It sounds more like a freight train or a big rig. If you want your truck to sound "mean" or "throaty," the 5-inch is the way to go. However, that depth comes with a trade-off: drone. Because the 5-inch pipe is essentially a massive echo chamber, it tends to resonate more at highway speeds. If you're cruising at 70 mph and your engine is sitting right in that 1,800 to 2,100 RPM sweet spot, a 5-inch straight pipe can get pretty loud inside the cab. It's the kind of sound that might be cool for the first twenty minutes but can start to wear on you during an eight-hour road trip.
Performance and EGTs
There's a common misconception that putting a massive exhaust on a truck will magically add 50 horsepower. In reality, unless you're pushing serious power—we're talking 600+ horsepower—you probably won't notice a massive difference in "seat of the pants" power between a 4 inch vs 5 inch exhaust 6.7 cummins.
Where the size actually matters is in your Exhaust Gas Temperatures (EGTs). The 6.7 Cummins is a big engine that moves a lot of air, and it generates a lot of heat, especially if it's tuned or if you're towing heavy. A 4-inch pipe is a huge upgrade over the restrictive stock plumbing and is usually plenty for most people. It allows the turbo to breathe much better and will definitely help drop those EGTs when you're working the truck hard.
The 5-inch pipe technically offers less restriction, which sounds great on paper. It can help shed heat even faster, which is a plus for guys running larger aftermarket turbos or aggressive tunes. But for a relatively stock truck, or one with just a light "tow tune," the 5-inch pipe can actually be a bit overkill. You might even notice a tiny bit of lost low-end spool-up time because you've reduced the exhaust velocity, though most people would be hard-pressed to feel that in the real world.
Installation Headaches and Clearance
This is the part most people forget to think about until they're lying on their back in the driveway with a pile of stainless steel pipes and a floor jack. Installing a 4-inch exhaust is usually a breeze. There's plenty of room around the frame rails, the transmission crossmember, and, most importantly, the spare tire and rear shock.
The 5-inch system? It's a tight fit. Because that pipe is so much wider, getting it over the rear axle can be a real pain. You have to be very precise with how you clock the pipes and where you place the hangers. If you're not careful, a 5-inch tailpipe will rattle against the spare tire heat shield or rub against the shock body. I've seen plenty of guys end up having to remove their spare tire or go with an "axle dump" style exit just to avoid the clearance issues that come with a full 5-inch kit. If you aren't a fan of wrestling with heavy metal pipes in tight spaces, the 4-inch is much more "bolt-on" friendly.
Appearance and Style
Let's be honest—part of why we do this is for the look. A 5-inch tip looks massive and fills out the space under the bumper nicely. It gives the truck a wide, aggressive stance from the back. A 4-inch pipe can look a little "small" on these newer, bigger Ram trucks, which is why most people who buy a 4-inch kit immediately slap a 5-inch or 6-inch tip on the end of it anyway.
If you go with the 5-inch system, you're usually looking at a 6-inch or even a 7-inch tip to finish it off. It's a bold look. If you like that "big pipe" aesthetic, there's no substitute for starting with a 5-inch base. Just remember that the bigger the tip, the more it acts like a megaphone, further amplifying that deep rumble and potential drone.
Towing Considerations
If you use your 6.7 Cummins for what it was built for—towing heavy trailers—this choice becomes even more important. When you're lugging a 15,000-pound fifth wheel up a mountain, your engine is under constant load. That's when drone is at its worst.
A 4-inch exhaust with a high-quality muffler is arguably the best setup for a dedicated tow rig. It keeps the noise levels down so you can actually hear your passengers or the radio, but it still flows enough to keep the EGTs in the safe zone. If you go 5-inch on a tow rig, I strongly suggest making sure the kit includes a muffler. A straight-piped 5-inch exhaust under load can be absolutely deafening for everyone inside the cab, and your neighbors at the campground probably won't be your biggest fans when you're idling out at 6:00 AM.
Price and Availability
Generally speaking, you're going to pay a bit more for the 5-inch system. It's more material, it's heavier to ship, and it's often marketed as a "premium" upgrade. The 4-inch kits are the industry standard, so they're usually cheaper and easier to find in stock.
When you're looking at a 4 inch vs 5 inch exhaust 6.7 cummins, you also have to think about the material. Whether you go 4 or 5 inch, you have to decide between aluminized steel or stainless steel. If you live in a place where they salt the roads in the winter, get the stainless. It doesn't matter if the pipe is 4 inches or 5 inches if it rusts through in three years.
Which One Should You Choose?
So, how do you actually pull the trigger? It really comes down to your goals for the truck.
Go with the 4-inch exhaust if: * Your truck is mostly stock or has a mild tune. * You do a lot of heavy towing and want to avoid annoying drone. * You want an easy installation process without hitting your spare tire. * You want a cleaner, more refined diesel sound.
Go with the 5-inch exhaust if: * You want the deepest, most aggressive rumble possible. * You're planning on building a high-horsepower truck (600hp+). * You love the look of a massive exhaust pipe and tip. * You don't mind a bit of extra cab noise or the extra work during installation.
At the end of the day, both options are a massive improvement over the stock exhaust. You're going to get better throttle response, lower EGTs, and a truck that finally sounds like a diesel should. Just be realistic about how you use the truck. Most guys who go with the 5-inch because "it's bigger" end up adding a muffler later to kill the drone, while the guys who go with the 4-inch usually just wish they'd bought a bigger tip for the look. Neither is a "wrong" choice, but knowing what to expect can save you from a lot of headache down the road.