Tuning your truck or 4x4 with the right exhaust mods is a tricky thing. If everything is set up correctly, then horsepower, torque, and mileage go up because the engine is working more easily and is therefore more efficient. It's a simple recipe with a lot of science behind it: You want straight-flowing, unrestricted exhaust without too big or too small of pipes. If the pipes are too small, exhaust flow will be restricted. If the pipes are too big, the exhaust will just tumble around inside the tubes and you won't get a proper exhaust flow.
Here are some things to look at when building your next custom exhaust system.
Headers
Cast-iron exhaust manifolds are big, heavy, and restrictive. Ditching them for a set of headers like these Brute Force long tube headers from AEM will free up power and will reduce engine heat under the hood since the headers hold the heat in the exhaust better than the radiating stock exhaust manifolds. This is the first part of your exhaust system, so if it's restrictive, what's the point of having any other exhaust work done? When adding a set of headers, you can run into some issues. This Dynomax header (A) is being bolted onto a 440ci engine, but it wouldn't fit in between the framerails without first lifting the engine a few inches off of the engine mounts. We also had to buy a new mini starter as well, since our new headers would not clear the original. Think about things like this when rushing hastily to your nearest mail-order catalog-you could be buying twice as much gear, and spending double the time to get what you want.
Catalytic Converters
You can get high-flowing cats for your trucks and 4x4s nowadays. You don't have to keep pulling them off every vehicle you buy. They do a lot for emissions reduction, and you're literally hurting yourself when you pull these off since you're the one most often around your own truck, breathing its exhaust. There's even high-flow catalytic converters for diesel trucks, like the one pictured here. The power loss from having a cat is so minimal that we'd be surprised if it registered more than even 1 hp. So if you can, keep your cat.
X-Pipes
If you are running true dual exhaust, then it's important to join it with an X-pipe or an H-pipe kit. If each side of the engine is separated, you essentially have two inline four-cylinder exhaust systems, and you won't get the proper scavenging effect that should be taking place in your exhaust system. You'll have a smoother powerband and a better sound if you keep the exhaust joined together properly.
Complete Exhaust Systems
Adding a whole new exhaust system from the header back is really the way to go. Otherwise, all you're really doing is moving the most restrictive area to another part of your exhaust system, if you're just adding a part here and there. This entire package is from Dynomax: headers, mufflers, tubing, an X-pipe, and tips. Not only will this give us the most optimum exhaust system, but it also came together as a package, which kept the overall cost down compared to piecing this system together separately.
Mufflers
Adding a less restrictive muffler is a no-brainer. Up until recently, the auto industry had been using exhaust tubing and mufflers that make us just want to cry. Getting rid of a huge muffler and replacing it with a shorter, baffled, chambered, or glasspack-style muffler like this Magnaflow will let you unleash a little more power. On newer vehicles, the OEMs got smarter and started to put on some pretty impressive exhaust systems. It's hard to make more than a couple horsepower just replacing the muffler on a newer ride, but it will definitely get you a throatier sound.
Larger Diameter Tubing
Having some big headers and a large-diameter muffler isn't going to mean anything if you have pea-shooter pipes. Have you ever tried to breathe out of a straw for a while? If not, try it; that's how your engine feels too. You can get complete tubing kits like this Flowmaster package with multiple straight and curved pieces to save time in bending and cutting tubing. Anyone with a 110AC welder could build himself an exhaust system with a kit like this.