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I recently installed a 4" DP and rest of the system is stock 3.5" - the muffler.
So I am really thinking I might run a single stack up behind the cab. So question being from my 4" DP should I increase to 5" or 6" up through the bed. Also because of the shorter run will it be super loud or on par with strait 4" back. Im not looking to increase the exaust noise and as it sounds nowthe turbo sound is great and the exaust sound went down from using the 3.5" stock DP. Also any hints on what to do or not do when installing a stack would be great.
To be able to accurately answer your question, we'd need to know what other mods, if any, will be added to the truck in the future. Assuming you're going to stay relatively mild with it, a 4" exhaust is all you really need until you approach 450rwhp. Few ever get to that level. You could easily fab up a 4" system, and have it exit above/behind the cab in a normal stack configuration. Also remember that as the pipe diameter increases, the tone of the exhaust lowers. There would be a noticeable difference in the sound if you went to 5" or 6", but the expense of the exhaust pieces themselves is something to consider.
I've got a 5" exhaust system on my truck, not because I'm over 450rwhp, but because I prefer the deeper sound. I noticed that the components to put together my 5" system were considerably more expensive than the 4" pieces they replaced. I also considered 6" pieces, but gagged at the price.