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This will be some testing info I've tested personally over multiple trucks for those looking for that perfect sound. Really has zero effect on power but if you want that great cummins grumble we all know and love then think about this one>>>> so your in the market for an exhaust? There is tons of options and brands....so brand is pretty much irrelevant... only changes really are the outlet angle. MBRP comes back on a 45* outlet, Diamond eye 90* outlet and some others do 60*. Really it doesn't make that big of a difference other then the amount of sound you hear in the cab. Windows down 45* is quietest, 90* is loudest. You'll hear it crack and echo off everything. Windows up 90* is quiet at speed USUALLY, 45* can start to drone at speed due to the low pressure area behind the exhaust pipe/tip. It's not much but it exists. Larger the tip the more drone at high speed. So brand isn't a big deal. Now 4" VS 5", so there isn't a difference in power, we run 4" down pipes on second gens and the flow rate of 4" OD pipe is (3.8" roughly ID) is such a high CFM that it doesn't really matter on a 5.9 liter. Your Peterbuilt 13l CAT engine would like 5". So that said 5" is deeper, tighter fitting and tends to drone more in the cab. Yes it sounds better and I use 5" whenever possible. I love the deep grunt and burp sound, it just screams cummins! Cheap 4" systems will gain you the same power but it's a little more raspy. Now the MOST IMPORTANT part! The material!!!! This makes a huge difference in sound, no performance gains but the sound change is amazing. So alluminized tubing that 80% of us run is honestly crap...in salt conditions it corrodes in a way that turns it a black color over the years. The wall diameter is usually a hair thinner depending on manufacture. The pipe itself TYPICALLY isn't truely round, when it cools the seem in the tubing warps a little So assembly sometimes can be a bear. My personal pet-peeve...drone. Alluminized vibrates, bad...you hear it under the truck, you hear a sound reverberation in the exhaust note and worst of all it vibrates and resonates into the cab from the pipe vibration under the truck. I absolutely hate drone, it's annoying. That said, stainless steel is the best! Even with a thin wall tuning the vibration/reverberation and sound dampening of the stainless is second to none. Yes the tubing is vibrating still since it's connected to the engine and of course everything vibrates but, it flexes less and holds up better to conditions. No corrosion. Some surface rust may show since it's probably from china but it'll last much longer. From personal experience a stainless system also has a much more crisp and clean sound. A to B the drone is near gone in the cab even with 5" straight pipe and a large s300 or 400 turbo that's loud beyond loud. One reason stacked trucks sound the way they do is because the stack is stainless which cleans up the exhaust note. A 4" stainlesss exhaust vs 5" alluminized, the 4" sounds better and cleaner. Much more of the deep burp noise. My personal fav is the 5" stainless though, if you want to make people think you have a small pecker in your wanna be Peterbuilt this is what you use. Deep, crisp sound with little to no drone. Each truck is different, the amount of hangars and style hangar make a big difference as well. Stacks attached to the bed, vibrate the bed and cause droning well same with under carriage exhaust. Make sure your insolators are soft and properly sized. It'll help tremendously. ive personally had 7" stack systems that you couldn't hear in the cab with the windows up, a floating stack system looks a little weird when driving, it moves independent of the body floating on the insolators attached to the frame. But it's very quiet when you want it to be. This also reduces the stress on the pipe, turbo and manifold as the engine twists under load. from personal experience: I've got 2 2nd gens, one with 4" stainless straight pipe and one with 5" alluminized with a diamond eye stainless muffler (came with the truck brand new but I'm going to switch!) if you listen to both trucks in person the 4" stainless sounds much cleaner, crisper and more angry when your on the throttle. The 5" sounds like the echo in a tunnel. Like multiple vehicles are racing due to vibration and reverberation. Now for some word to the wise info...please for the love of god don't use the stupid crush clamps... they are crap. They bend the alloy pipe so bad it hardly ever comes apart without a torch. If you want to swap turbos or mufflers or whatever don't use crush clamps. I've had so many that are so bad I just cut pipe, butt it together and either band clamp it or weld it. It really sucks to do guys... if your gonna run stainless, good stainless pipe doesn't like to bend, so the crush clamps will actually bow and not compress the pipe but rather itself. it tends not to hold and a few miles later you got yourself an axle dump you didn't want it even an open down pipe and the muffler is in the grill of a Prius....don't use them. SO that said, the best, easiest and most adjustable/usable way is the overlap-band clamp. Slide your pipe seconds together, overlap the band clamp and get it really nice and tight. Some will have a pin hole leak right around the bolt location, if that's a problem you can use some paste on the pipes swelled overlap to help seal it for inspections and such. Now you can switch down pipes, mufflers, axle dumps etc without getting the hammer out. Just unbolt and slide apart! Stainless once again is the easiest for this and less opertunitly to leak. DO NOT use alloy band clamps. They are so weak you'll just break them! Regardless of what pipe you have to use get the stainless clamps. They hold up much better and longer. They keep there shape a little better also and seal/grab way better. If you don't have a swell overlap there is Butt-band clamps for 2 identical pipe sections to mate. Again stainless clamps are best. Little flexing means a tougher exhaust and better connection. Now that your up to date....spend the dang money and get a good exhaust, the $260 special online will work yes but sound isn't great, it doesn't last as long and though stainless is much heavier it's only about 20lbs. Unless your doing a full race build it's not gonna matter... you'll be happy you went for stainless in the end. Exhausts usually go on and stay, make sure your gonna be able to live with it for 5-20 years of ownership when the dodge body is gone and your riding the frame and engine cause it's all that left lol