There are a few modifications that everyone should do before adding tuners or modules that increase horsepower. Generally, everyone will tell you that gauges, a free flowing intake, and a free flowing exhaust are needed before you start to spend money on bigger HP modifications. There are other articles written about the importance of a free flowing intake and installing gauges, so those won't be covered here. Instead, I'm going to be going over the basics of upgrading your truck's exhaust system, why you should do it, and what to look for in choosing an exhaust. The main reason people want to upgrade the exhaust is the benefit of lower EGT's (Exhaust Gas Temperatures). It's very important to keep EGT's low to ensure the longevity of your diesel motor. Pulling heavy loads for extended periods of time, lugging down your motor, and hot-rodding can all lead to EGT's rising into the "danger" zone (generally thought to be above 1200 degrees Fahrenheit pre-turbo.) One way to combat high EGT's is to make the exhaust less restrictive, make the pipe larger, or a combination of both. Common upgrade's are 4 inch and 5 inch pipe. The larger pipe allows hot exhaust gasses to escape more quickly, thus lowering the EGT's in the motor. Other benefits of the upgraded exhaust may be a horsepower gain and/or a fuel mileage gain, because the engine does not have to work as hard to expel the exhaust.
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The exhaust system also might be suitable for the "Heavy Breathing" section because the exhaust system is what might be considered the engine's "exhale." It doesn't matter how much the engine can inhale, if it can't breathe out, it's going to limit the power output. More air and fuel means a larger system is needed to handle the increased exhaust flow. That's accomplished with larger-diameter pipes, free-flow mufflers, and mandrel-bent tubing. A restrictive exhaust will increase EGTs. On a stock engine, a low-restriction exhaust will drop a lot of EGT under a load and sometimes gains a boost for a small increase in power. When combined with other mods, the exhaust works in concert with the other stuff to increase the benefits or to make those benefits possible.