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Discussion Starter #1
I've been doing a lot of research over the past couple of years on "stage-1" performance mods and such. The 5.7L automatics come with a 2.25in dia exhaust pipes and manuals come with 2.5in (at least in the older models, I have a 11 charger RT). I never really thought much about why until recently. I looked under my charger to see what kind of muffler mod was done to it before i bought it and it had a flowmaster american muscle "style" config. I stress "style" because what the prior owner did was a reso delete with dual flowmaster super 44 muff install using the original pipes. The reso delete is fine but they installed a muffler with 2.5in openings! So, there are welded reducers on each side of the muffler and the out pipe on the left muffler is touching and vibrating off the trunk base and they tried to mitigate the vibration by putting some type of high heat RTV between the trunk and pipe. Overall I discovered a pretty shabby job which I don't like.

So now I'm looking at doing a cat-back replacement myself but I also noticed that the cat piping is 2.5in and I'm pretty confident that its stock - I assumed that it would also be 2.25in though. But thats what got me thinking about the 2.25in dia on automatics. It doesn't make sense that the same engine would use two different size exhaust systems due to transmission factors. So now I'm thinking its because of the MDS system - less scavenging on 4 cylinders (thoughts?). I'm getting a tuner to turn off the MDS anyways so I'm going with 2.5 in pipes.
 

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The Charger only offered an automatic transmission (manual transmission only available on the Challenger). The 2.5" exhaust did come standard on R/T Chargers in 2012+ models. I'm not sure about 2011 and older models ... they may have been 2.25" exhaust. Had nothing to do with MDS.
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Discussion Starter #3
The Charger only offered an automatic transmission (manual transmission only available on the Challenger). The 2.5" exhaust did come standard on R/T Chargers in 2012+ models. I'm not sure about 2011 and older models ... they may have been 2.25" exhaust. Had nothing to do with MDS.
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You're absolutely correct! Chargers come in auto only. I had the challenger in my head because that's the car I was going to buy originally and most of my research was on it. I failed to clarify that. Its still the same engine and the question still stands for me on why they were different. So 2.5 is standard on 2012+ you say...interesting, lessens my concern on potential power loss.
 

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Good question on why the manual would have larger diameter exhaust than a automatic on the Dodge Challenger. I have not researched the Dodge Challenger so my guess is based on what has been done in the past on other cars.
It was not unusual for the manual models to have a few more HP than the automatic (same engine but rated HP was different by about 5 HP). The reason is the automatic with it's torque converter provides a smoother transition on take-off and shifting between gears. The manual is more abrupt. Drivers can not work a manual clutch as smooth as a torque converter. The abrupt application of the clutch requires a little more HP/torque to overcome these more abrupt transitions.
One way to increase the HP/torque is to have a larger diameter exhaust (within certain limits). The difference with MDS, in the manual vs automatic, is that manual shifting is unpredictable and there's no freewheeling with the manual (unless you push in the clutch). With the automatic transmission, in full automatic mode, the engine management system can control the shifting. This is why the Dodge Charger will disable MDS if you switch the transmission from Full Auto to AutoStick (manual mode).
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I can't speak for other model years, but my 2015 R/T from the factory had 2.5" exhaust after the cats that necked down to a 2.25" x-pipe and remained 2.25" out to the back, including the resonators. I replaced my x-pipe with a true 2.5" version and made all the remaining pipe to the rear the same size. It makes no sense to me why Dodge would do such a ridiculous thing, but they did.
 

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Discussion Starter #6 (Edited)
Found a vid called "No More Exhaust Myths" on the tube with a Maganflow engineer that provides a pretty decent yet basic explanation of how exhaust systems are designed and work based on fluid dynamics and flow rate. There's an algorithm factor of 2 to 2.1 that is used by Magnaflow. Basically, you multiple the engine HP by this factor to get total CFM. He used a 500HP engine as an example which yields 1000 total CFM. For a dual exhaust thats 500 CFM per side. He said at that flow rate, its near the boundary for 3 in pipes. I'm sure there's more that goes into the design but it does give us an educated figure to work with.
 
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