One mistake most people make is install a Y pipe Why???. Ram is choking the 5.7 and more the 6.4L Hemi's, I went to different exhaust places trying to either install an X pipe where the Y pipe was and dual 3 inch exhaust out the back. My 6.4L has increased fuel mileage by 5 mpg I have checked and rechecked I get more than 5mpg at times but average 5 mpg. depending on your muffler choice is what noise will be in cab along with increased mpg the extra power is amazing. I was considering selling and buying something I could pass a Prius with to leaving them in the dust, with exhaust opened up has increased power towing and raw power, increased mpg, fun to drive and sounds awesome in and out.
Don't continue to drive a vehicle that eats gas, pulls good but when accelerating can't pass anything and sounds like a 4 popper to one that is fun to drive get great fuel mileage, sounds great, when stepping on accelerator it goes and increase pulling.
It sounds like you're thinking all Y pipes are created equal and that is absolutely not the case, and any high end Y pipe made for these will not hinder anything at all instead they will help everything performance wise, unless you are some awesome big hp forced air build but thats a whole different deal.
The factory Y pipes yes you are correct they are terrible because they are crushed Y's with way to small piping but it's to please all the regulations. Pictured is a example of my current Y pipe, made by Kooks with there high flow cats next to the factory Y and cats, you can see there is a huge difference. I have Kooks 1 3/4" long tube headers and I've played with all kinds of setups and tunes for each setup, including running the open header which sounded awesome haha but was a **** on power because way to open. Also pictured is my DEQ pipe setup that I made so I can get through emissions without having to remove the long tubes, no Y just open pipes with factory cats and dumped thats it, also a **** but not as bad as open header. Slap my Kooks Y on and bam power is amazing. As mentioned I've had custome tunes made for all these setups to see if I could get them to run better but just no comparison to having a good Y pipe. These 5.7's have no chance of sucking in more air than they can push out of a quality made Y unless they have some pretty decent sized forced air.
Also I wanted to point out that the muffler alone isnt just what decides loudness and in cab volume as you stated, I could make a very loud exhaust setup with many different mufflers and still have it pretty darn quite in cab, changing location of exhaust exit/exits, tip size and shape, where the muffler is located, resonators, matting, piping size, header/manifolds used etc etc all play into decibel levels, tones and in cab sound levels.
Also OP stated they live in Cali and as most people know they have pretty much the most strict emissions testing of any state so theres a lot they cant do or have to consider.
Just sharing some quality info [emoji847]
Picture 1" my DEQ pipe setup.
#2: KOOKS Y with there HF cats vs factory.
#3: you can really see how bad the crush is on the factory Y in this pic.
#4: just for fun threw in a KOOKS long tube comparison vs factory manifolds lol
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