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If I was to use a larger exhaust pipe than the muffler in ( ex. a 2.5" pipe from cat in a 2.25" muffler ), will that remove the advantage of the larger pipe ? Also from muffler to back, if the muffler has a 2.25" out, is there any advantage of using a 2.5" pipe from muffler to tip?
 

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An exhaust system is complicated. Total exhaust flow is always going to be limited by the smallest orifice, the 2.25 dia in this case, but we're also dealing with the length of pipe before and after the restriction, their diameters, number of bends and the radius of the bends, etc. Then there are the issues of desirable backpressure, tuned exhaust length, blah, blah, blah... not to mention the intake side of the engine.

While there is nothing stopping you from increasing the dia of the pipe before or after the muffler, it's not going to give you any gains at all. Even the best designed full exhaust will only add a little rwhp, unless you change other things as well.

If you're going to spend time and money, do a full cat back as a minimum. You'll notice that the exhaust mfg's typically use the same dia pipe before and after the muffler as the muffler inlet/outlet dia.
 

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Discussion Starter #3
I got a tuner and CAI also. But you answered my question, and from what I understand, there is no point in going larger than the muffler in / out when it comes to pipes, I mean no gain in that.
 

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your exhaust is only as free-flowing as the least restrictive part will allow. you will gain nothing if you use say a 3" pipe if your muffler is only 2.5" (just an example). the only thing that may help you is on the bends. a piece of bent tubing with a larger diameter will flow the same as a piece of straight tubing of smaller diameter. for example (and i'm not an engineer so don't quote me on this) a 3" diameter 90* bend might flow similar to a straight section of 2.5" piping. again, I'm not an engineer, but I'm just giving you the general idea.
 

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Discussion Starter #5 (Edited)
When we say a pipe of 2.25", 2.5" or 3", are we talking about exterior diameter or interior ? I measured my stock 2011 exhaust, and outside diameter is; from cat to muffler 2.0" and from muffler to tip 2 1/3" . Is that the correct measurement ?
 

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your exhaust is only as free-flowing as the least restrictive part will allow. you will gain nothing if you use say a 3" pipe if your muffler is only 2.5" (just an example).
What you're saying makes perfect sense. But then why would companies like Magnaflow for example market an exhaust system for the X like #15583 with 2.5" tubing all around, while I read somewhere that the diameter of the out from the cats is 2" on the X , and so is the diameter of the front pipes between cats & muffler? Is it for marketing purposes, or it does give slight edge ?
 

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It's all about pressure drop through the system (or, how much pressure it takes to make exhaust flow through the system: more pressure drop = more HP lost to the exhaust flow.)

So. Technically there is a slight edge in larger diameter cat backs due to a smaller pressure drop from the cat exhaust to the exhaust pipe end. However, it will be so small you will most likely not notice as the real restriction is the cat pipe diameter. What SHOULD make a noticeable difference is replacing the factory Y-pipe (where the exhaust pipes from each cylinder bank come together between the cats and the muffler), which places a fairly large restriction on flow. Look at it and you will see what I mean- it looks like one side is hammered flat. Any good "cat back" system should fix this. Most "cat back" systems will also drastically change the sound of your truck, which is something not everyone wants. However, they are cheaper to install than a full aftermarket exhaust, and should free up a couple of HP- nothing drastic.
 

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When we say a pipe of 2.25", 2.5" or 3", are we talking about exterior diameter or interior ? I measured my stock 2011 exhaust, and outside diameter is; from cat to muffler 2.0" and from muffler to tip 2 1/3" . Is that the correct measurement ?
Just about every pipe is measured in I.D. ( inside diameter), and tubing is O.D. ( outside diameter), .... So the factory secondary cats( B-pipes) is 2.0" i believe, the Y-pipe to the muffler is 2.25", but gets crushed down to 1"(each side) right before the muffler, now the muffler inlet is 2.25 inlet, and 2.5 outlet....
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What you're saying makes perfect sense. But then why would companies like Magnaflow for example market an exhaust system for the X like #15583 with 2.5" tubing all around, while I read somewhere that the diameter of the out from the cats is 2" on the X , and so is the diameter of the front pipes between cats & muffler? Is it for marketing purposes, or it does give slight edge ?
these guys already answered for me, but I will add to them.

right, if magnaflow makes an exhaust that has a bigger diamater just from the cats back, it will free up the exhaust a little bit, but like i said, it is only as free-flowing as the most restrictive part. when you pair their exhaust with say some headers and b-pipes, then you will REALLY feel a change in the exhaust. when you replace one part of the exhaust it doesnt do a whole lot, but when you replace the ENTIRE system and it isn't restrictive that is where you will feel the gains. also remember, what you're doing is allowing the engine to breathe better which is whats making the extra power. if you want the total package, add a high flow intake to the mix and you should notice a nice improvement over stock.
 

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Discussion Starter #10
It's all about pressure drop through the system (or, how much pressure it takes to make exhaust flow through the system: more pressure drop = more HP lost to the exhaust flow.)

So. Technically there is a slight edge in larger diameter cat backs due to a smaller pressure drop from the cat exhaust to the exhaust pipe end. However, it will be so small you will most likely not notice as the real restriction is the cat pipe diameter. What SHOULD make a noticeable difference is replacing the factory Y-pipe (where the exhaust pipes from each cylinder bank come together between the cats and the muffler), which places a fairly large restriction on flow. Look at it and you will see what I mean- it looks like one side is hammered flat. Any good "cat back" system should fix this. Most "cat back" systems will also drastically change the sound of your truck, which is something not everyone wants. However, they are cheaper to install than a full aftermarket exhaust, and should free up a couple of HP- nothing drastic.
My goal is to eliminate the Y pipe, and I do not want louder sound. Already have a K&N intake, and happy with the sound. So I'm getting a 2 in 1 Pathfinder V8 muffler, based on another user on this forum ( thanks for the suggestion ). That should take care of the Y pipe. muffler has same diameter in / out as stock pipes. So based on what you're saying, and only for my application ( no other exhaust system to be used ) I won't gain anything by replacing stock pipes with larger diameter ones . Correct ?
 
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