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Just replace the muffler also. The flap is super close to the muffler. If you're set on doing just that the flapper, I think after muffler the 6.2 exhaust goes to around 3" and the 5.3 is around 2.25-2.5".
Full agree with hunter on doing the muffler and he is right on the sizes too.
The piping is weird in that area. I just got out my dial calipers here at the shop an my 6.2 catback that's on my truck is 3.5" entering the muffler and the muffler has a 3" exit. My flapper bypass pipe is 3" lightly spread on the front to go over the muffler outlet and it's not spread where it connects to the piping at where the rear side of the flapper would be, that's actually inside the bypass pipe. That pipe is super weird because it steps right back up to 3.5" on it's way out to the tailpipe.
My 5.3 catback with the flapper by pass in it is at home (where I'm not and the dial calipers are not at lol) so I can't measure it. But it does something very similar. If I recall it has a piece of 2.5" in it but it isn't spread and the oe pipe is inside it? I'd have to go pull it down off a 6 foot shelf and look at it. But man I'd just let the muffler shop take care of it. I got mine done for around $20 and several other people have too. It's really not worth fooling with it's so cheap to just have it done.
For what it's worth I wedged my flapper open and noticed no difference at all. This is all when I had the 5.3 cat back and stock manifolds and still had cats. A few days later I had it cut out and a magnaflow muffler installed at the same time. I picked up a couple of mpg and picked up 2-3 mph at my test spot. The stock muffler is pretty restrictive. The truck was hardly any louder at all with the magnaflow but had a little bit of sound. Enough to where I could finally hear it in the house when I fired it up with the remote start. But it was not loud in any way just a very very nice and very deep tone. A very noticeable low tone, but quiet overall and could not hear it at all out on the road except this super deep note if you turned off the radio and listened for it. With the radio even at low volume you couldn't hear it.
Then I got the headers with off road y pipe and put a 6.2 catback on it and picked up another 2-3 mph over the 5.3 catback with magnaflow. I'm running the stock 6.2 muffler in the 6.2 catback because I've heard no matter what muffler I do it's gonna be pretty loud since I now have headers and no cats. And larger exhaust.
Last edited by 00pooterSS; Oct 24, 2019 at 11:55 AM.
Here’s a couple of pics. Hope you can read the writing I can’t figure out how to use markup on the iPhone and write clearly.
This is after the Y pipe and going into the muffler on the 6.2
This is the back side of the muffler where the flapper was. I'll do a muffler on this 6.2 exhaust some day but with the headers I'm worried about it being too loud.
Its in the ball park, I am charging $40/weld these days. Including the section of pipe needed and the time to R&R I'd prob ask for $100 to do this is someone came to me
Want it done cheaper, spend $1500 on your own tools and do it lol
Oct 24, 2019 | 11:37 AM
