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  1. #1
    Join Date
    Jun 2012
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    Long Beach, CA
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    Reduce exhaust size from 3" to 2.75" or 2.5"?

    My engine build is in my signature. I initially had the car with 5.7's oem butterfly deleted catback adapted to fit the SRT8 manifold, mids and cat. After changing the to a Magnaflow clone which is unfortunately 3" as the magnaflow, there was a noticeable loss in power. Not a mind thing.

    Here's my dilemma, I LOVE the sound of my current exhaust but want to regain the lost power. I'm here in SoCal and the shop I got it tuned by can never be reached so tuning is out of the picture. It is also difficult to find a competent muffler shop. I want to know if I should find a shop to change the 3" pipes to either 2.5" or 2.7" as I am pushing out about the same as an oem srt8. Since the only OEM SRT8 catbacks I can find are from that of a Challenger, I am considering getting that and having a shop extend and fit that to my car, probably keeping the rear cans.

    I don't know and want opinions. If I could find a Charger, 300 or Magnum's OEM srt8 catback here, I'd most likely get that as it would just bolt on with a perfect fit. I wouldn't even mind it being quiet.
    Last edited by EsQueue; 08-11-2019 at 09:59 PM.
    2006 Dodge Magnum R/T

    Inertia Motorsports Big Valve Heads, Spartan Plus Cams, SRT8 Headers, Mids and Radiator, SRT8 Magnaflow clone catback.

    Honda S2000 retrofitted projectors in CCFL ringed housing, Denso Ballasts, Phillips X-TremeVision Gen2 D2S bulbs. all LED switchback Amber/White parking/signal in front, signal in rear, parking, running, Fogs, License, sidemarker, interior, hatch and HVAC cluster lights.

    376 rwhp 405 rwtq



  2. #2
    Join Date
    Sep 2018
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    Springfield, MO
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    Another thing you have to consider is exhaust flow choking anything

  3. #3
    Join Date
    Mar 2016
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    2,457
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    Leave the exhaust. Add more air to the intake!


    Sent from my iPhone using Tapatalk

  4. #4
    Join Date
    Jan 2006
    Location
    Kingsport, TN
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    Get 2.75...better TQ...3in is for really extreme mods..






  5. #5
    Join Date
    Sep 2006
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    Calgary, Alberta
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    A 3" CAT back is, as you have found out ridiculously too large for a 5.7 no matter how its built. And 2.75" - it as well is not justifiable. However, it would be likely easier to find an OEM SRT CAT back to at least get a constant ID system that you could modify with different mufflers. All over the place as take-offs.

    Having a shop bend pipe, it must be mandrel bent - otherwise the losses make it a waste of money.

    Not aimed at you - but others on here who soo want to believe the misnomer bigger is better. In this situation...it ain't!
    Last edited by Hemissary; 08-14-2019 at 12:23 AM.
    2005 Magnum RT---Viper Venom Red----440ci Aluminum block----Short Runner Valve Intake--410mm BAER 6S Monoblock Extreme--Eibach Multi-Pro 2

    Custom--Grille Work--Hood--Headlights--Side View Mirrors--Rear Spoiler--Rear Diffuser--SRV Control System--Turbine Wheels

    Likes StrokedRT liked this post

  6. #6
    Join Date
    Jul 2011
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    Jarrell, TX
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    I did 6.1 shorties & mids... But, I had Corsa Sport cat backs. So, took the mids and cat backs to the muffler shop and had them swage the cat back to fit OVER the mids.

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    2017 Granite Crystal Scat Pack
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  7. #7
    Join Date
    Aug 2016
    Location
    Idaho
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    Quote Originally Posted by Hemissary View Post
    A 3" CAT back is, as you have found out ridiculously too large for a 5.7 no matter how its built. And 2.75" - it as well is not justifiable. However, it would be likely easier to find an OEM SRT CAT back to at least get a constant ID system that you could modify with different mufflers. All over the place as take-offs.

    Having a shop bend pipe, it must be mandrel bent - otherwise the losses make it a waste of money.

    Not aimed at you -b ut others on here who soo want to believe the misnomer bigger is better. In this situation...it ain't!
    Do you think a 3" catback is overkill on a 5.7 making 575 to the wheels?

    Sent from my SM-N960U using Tapatalk
    2007 R/T, long tubes, off road mids, borla atak exhaust. Inertia cnc big valve heads, inertia raging trojan cam, D1SC procharger 6.1 srt aluminum intake, mopar performance tcm, Trufiber Carbon Fiber SRT8 hood.

  8. #8
    Join Date
    Jul 2004
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    City of Trees
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    Quote Originally Posted by Hemissary View Post
    A 3" CAT back is, as you have found out ridiculously too large for a 5.7 no matter how its built. And 2.75" - it as well is not justifiable. However, it would be likely easier to find an OEM SRT CAT back to at least get a constant ID system that you could modify with different mufflers. All over the place as take-offs.

    Having a shop bend pipe, it must be mandrel bent - otherwise the losses make it a waste of money.

    Not aimed at you -b ut others on here who soo want to believe the misnomer bigger is better. In this situation...it ain't!
    I agree, 3 in. duals is way to big for the NA 5.7 displacement. A 2.5 in. stock type SRT setup should work fine though or, even go back to the stock 2.25 in. cat. back. IMO, you also havea little too much cam for the stock 5.7 displacement and compression ratio and, also, the heads/valves are also a little too big for the displacement. Your setup would be great for a larger displacement, higher compression ratio engine....something that'll move more air. Then it would run like a raped ape. The stock 5.7 displacement, at 100% Volumetric Efficiency (VE), is only capable of moving just under 700 cfm, no matter what you do to it.....except forced induction.

    I went through a bunch of this when I built my 5.7 Jeep and 5.7 Magnum....both modded almost exactly the same. Big valve, ported heads will give you only about 12 hp., if that, over the stock ones when using a fairly mild cam. Using your bigger cam probably negates any gains from the heads. This is according to a Hot Rod Network article on using big heads on an otherwise stock rotating assembly: https://www.hotrod.com/articles/hemi...mild-and-wild/ On both of mine I went with the old, long tested and proven FRI Sidewinder cams, advanced 6* on the Jeep and 10* on the Magnum. I was after a little more low end, which is what advancing the cam will do.....if you don't go too far. I had read the HRN article before I did the heads on both, so I stuck with stock valve sizes and opted to raise the compression ratio about a half point by having .030 in. milled off them. Don't want to go too far here either. The stock 5.7 NA rotating assembly just won't horsepower super well when many of the "upgrades" end up being "downgrades". A good solution, since you have all the cool stuff already, is to go with a fully forged, stroked 5.7 or 6.1 short block and all your goodies. I know if I ever blow either of mine, I'll be going with a forged 5.7 based 410 stroker, and keep the MDS too.....as I still have. Best of both worlds....more power and better fuel economy. FWIW, just my 2 cents.
    Last edited by magnuman; 08-13-2019 at 09:49 PM.
    06 300CSRT8, Comp cammed, 85mm TB, HP/DS Tuners, 06 Jeep GCO and 05 Magnum RT both modded almost the same, MDS "ON" Lt, Fumoto Drain Valves, BT Catch Cans, SRT8 CAIs, AFE Filters, Sonnax Trans Line Press Booster, SC, DS and HP Tuners, Transgo Shift Kit, ARH Long Tube Hdrs w/3 in Magnaflow/Xlerator AP Exh Sys, Bilsteins, SRT Springs/Sways, Cust Eng Cvrs, SRT8 frt/rear bumpers, FRI Sidewinder Cams advanced (Jeep 6*/Maggie 10*), 6.1 valvetrain, SLP 25% UD Pulley, BBK and Arrington 90mm T-Bodies, SOS Ign Coils,TEA ported/milled heads w/new valve seats, SRT Reps.

  9. #9
    Join Date
    Sep 2006
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    Quote Originally Posted by StrokedRT View Post
    Do you think a 3" catback is overkill on a 5.7 making 575 to the wheels?

    Sent from my SM-N960U using Tapatalk
    Yes; exhaust velocity is your friend especially as it cools rapidly before existing the tail pipe.
    Last edited by Hemissary; 08-14-2019 at 12:28 AM.

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