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Flowmaster Original 40 Series Offset/Offset Oval Muffler - 2.5 in. (79-04 All, Excluding 99-04 Cobra)

Market Price $98.99 You Save 13% ($13.13)
Our Price

$85.86 (each)

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  • Item 18401
  • MPN# 42543

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Features & Details

  • Signature Flowmaster Sound
  • Original Two Chamber Technology
  • Offset Inlet and Outlet Design
  • 2.5" Inlet/Outlet Diameter
  • Aluminized Steel Construction
  • Sold Individually
That Flowmaster Sound. Improve your Mustangs exhaust flow for an increase in horsepower and a deeper throatier exhaust note with a set of Flowmaster 40 Series Mufflers for a truly classic muscle car sound.

Superior Design. The Flowmaster 40 Series Mufflers are MIG welded from durable 16 gauge aluminized steel for durability and value. The 40 series mufflers use Flowmasters original two chamber design to increase power and sound. And each muffler is embossed with the "Flowmaster" name so you know it is the real article.

Application. The 40 Series Muffler's outer case measures 4" x 9.75" x 13", and features a 2.5" diameter inlet and outlet in an offset configuration. This is the perfect muffler for the 1979 to 2004 Mustangs. Excludes 1999-2004 SVT Cobra.

Fitment: 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Details

What's in the Box
  • (1) Muffler
Installation Info
Tech Specs
40 Series Delta Flow versus 40 Series Original versus Super Flow 44 Series
  • Minimizes interior resonance
  • Aggressive exterior tone
  • Two Chambered Muffler
  • Made of Aluminized Steel
  • No minimization of interior resonance
  • Aggressive tone inside and out
  • Two Chambered Muffler
  • Made of Aluminized Steel
  • Delta Flow technology minimizes interior resonance
  • Aggressive deep powerful tone
  • Two Chambered Muffler
  • Made of Black Aluminized Steel
Will it fit my Mustang?
  • Bullitt - 01
  • Foxbody - 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93
  • Mach 1 - 03, 04
  • V6 - 94, 95, 96, 97, 98, 99, 00, 01, 02, 03, 04
  • Cobra - 94, 95, 96, 97, 98
  • GT - 94, 95, 96, 97, 98, 99, 00, 01, 02, 03, 04

People Who Bought This Item Also Bought

Most Helpful Reviews

Takes some time

I had 44's on but nowhere near the same sound as the original flow 40 series...going through tunnell's is heaven and shifting at 2500 rpms is heaven sent lol they were a tad quiet at first but after 100 ...

Read More
Helpful (38)

Classic 40s

I had these welded in today they sound incredible I have the stock h pipe with cats and it is still nice n loud, it has a great deep tone and it was a great 1st mod for my car. 2200 RPM is the sweet s ...

Read More
Helpful (24)

Overall Rating 4.8 out of 5 stars
Read All 41 Reviews

Customer Photos (7) Submit Your Photo

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12 Questions / 6 Answers

Ask a Question

Which muffler is louder original 40 or super 44s?


  • Honestly, they are very very close in overall sound, but the super 44's are slightly louder, the original 40's are a little deeper.


Helpful (1)

I already have an X pipe on my 03 GT and cold air intake but would this make my car a lot louder or just a little? could you hear me coming down the street with these?


  • The Flowmaster Original 40 Series Offset Muffler will increase the sound of your mustang and give it that classic muscle car sound.

    AM MikeW

Helpful (0)

could i take off my stock muffler on my 04 mustang 3.9L v6. and put this on without any problems and would it sound better?


  • Yes, you would be able to install this on a 2004 3.9l V6 mustang. The sound being better would be personal preference, it would increase the sound of the car with this muffler.


Helpful (0)

Do they still offer a cat back with original 40's?


  • Hey Murphy, The 40 series mufflers are utilized in the Flowmaster American Thunder Catbacks that we carry.


Helpful (0)

I have a 2002 mustang gt with stock exhaust. Do I need a 2.25 or 2.5 flowmaster muffler. And do they need to be welded or just clamped


  • The 2.5 inch one is what will line up with your stock exhaust. However, this 40 Series Muffler's outer case measures 4" x 9.75" x 13", so not only will you have to pick the correct diameter but you need to choose something as close to your current size in order for the least amount of modification to make it work.


Helpful (0)

I have a 2012 Mustang GT. I took off the stock mufflers and ran straight pipe, axle back. I hate it, its WAY too loud. I want to put FLowmaster40s on, does anyone know how these sound on the 2012?


  • This Flowmaster Original 40 Series Offset Muffler - 2.5 in. is for a 1979-2004 Mustang GT and will not work on a 2012. I would recommend checking out our axle-back exhausts for your 2012 Mustang GT. I will put the link to our selection below.


Helpful (0)

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Customer Reviews (41) Write a Review

Overall Rating 4.8

out of 5 stars

Installation Time:Full Day

Installation Cost:$100-$250

  • Loudness / Volume 3.95 4.0
  • Exhaust Note 4.45 4.5
  • Fit & Finish 4.65 4.7
  • Bang For Your Buck 4.65 4.7

  • 5/5

    July 10, 2010

    Takes some time

    I had 44's on but nowhere near the same sound as the original flow 40 series...going through tunnell's is heaven and shifting at 2500 rpms is heaven sent lol they were a tad quiet at first but after 100 miles there starting to wake up!! dont get any but the original flows!!


    Helpful (38)

  • 5/5

    April 05, 2010

    Classic 40s

    I had these welded in today they sound incredible I have the stock h pipe with cats and it is still nice n loud, it has a great deep tone and it was a great 1st mod for my car. 2200 RPM is the sweet spot.

    Dale P

    Helpful (24)

  • 5/5

    February 25, 2010

    great mufflers

    I put on a pair of Flowmasters and what a difference they made in sound and horsepower. Getting these mufflers along with a cold air intake and your mustang will improve performance 100%. I recommend these mufflers to other mustang owners.


    Helpful (22)

  • 5/5

    August 27, 2010

    Great sound

    Installed Original 40 Flowmasters and am very please with the sound. The only problem I had was I had to adapt the flanges to my orginal equipment pipes. Be sure you know what size pipes you have before ordering.


    Helpful (19)

  • 5/5

    October 06, 2009


    Very nice product, Adds the nice deep throaty exhaust note


    Helpful (10)

  • 5/5

    March 05, 2010

    Great sound

    I have Original 40 series on my '01 V6 model and to my ears it is a great sound. People ask me a lot if it is the GT model, and one GT owner told me, "That sounds amazing for a V6!" It doesn't get too raspy like some other kits might. Overall a great choice.

    Daniel C

    Year: 2001 Submodel: V6

    Helpful (7)

  • 5/5

    November 17, 2010

    Folwmaster 40

    sound great. one of the best mufflers ive had. Make my mustang sound like a gt. =]

    Michael Jaser

    Helpful (6)

  • 5/5

    August 17, 2010

    97 v6 mustang

    i have 2.25 pipes to 2.5mufflers to 2.25 tips. it has no h or x pipe and it sounds good at low rpms get a little raspy at high rpms. i was worried at first but the sound is great with a bbk throttle body and k and n cai. i would recommend them to other v6 owners. the were also a great price and the shipping is free and fast


    Year: 1997 Submodel: V6

    Helpful (6)

  • 5/5

    Installation Time: Shop Install

    October 10, 2011

    Sound Awesome

    Replaced old mufflers once they started to rust at the inlet pipe, these new mufflers sound amazing on my dual exhaust V6.


    Year: 2004 Submodel: V6 Lethbridge, AB, Canada

    Helpful (4)

  • 5/5

    July 15, 2011

    the best

    had to order new mufflers and went with the original 40s and man do they sound that original flowmaster sound!!!!


    Year: 2004 Submodel: Mach 1

    Helpful (3)

  • 5/5

    November 05, 2010

    flowmasters on my 02 mustang

    I wanted a nice rumble with the windows down...and a quite with the windows up..... Perfect sound, so i read they break in after 100 miles.. alittle louder would be good also. Not damn on my auto v6 shipping as good, the faster the better.... Cheers american muscle -kriss-

    kriss daigle

    Year: 2002 Submodel: V6

    Helpful (3)

  • 5/5

    January 21, 2010




    Year: 2001

    Helpful (3)

  • 5/5

    Installation Time: Less than 1 Hour

    January 01, 2014

    Can't beat the classic note of these 40 series mufflers

    I bought these to replace a set of awful sounding mufflers the original owner of my cobra installed. I'm very happy with the classic note of these mufflers. I am very happy to see the quality of flowmaster remains the same it was back in the 90's! I also commend American Muscle for their customer service. There was a small issue over shipping but customer service was quick to communicate and solved the problem fairly and swiftly.


    Year: 1997 Submodel: Cobra Florida

    Helpful (2)

  • 5/5

    Installation Time: Shop Install

    Installation Cost: $250-$500

    May 13, 2013

    go all out

    I have 40s on every muscle car I have and to make sure my 02 gt could compare with the sound of the 60's muscle since its legal where im at I completely changed the exhaust removed the cats and did straight pipe back with the 40's and even the tiny v8 in the new edge mustang can keep the sound of a big block


    Year: 2002 Submodel: GT Fruita, CO, USA

    Helpful (2)

  • 5/5

    December 04, 2009

    great sound

    just got these installed on my 03 gt they sound awesome just need a x pipe now


    Year: 2003 Submodel: GT

    Helpful (2)

  • 5/5

    July 25, 2016

    Flowmaster 40

    If you're like me who owns a v6, you are always looking for ways to look and sound like the big boys. Well, these plus Mac long tube headers = OMG. I've never thought that my little 3.8 could sound good. Deep rumble at idle and tons of neck breaking when driving down the road. I would recommend these two products for anyone who has a v6 and want the v8 burbly sound. Hands down the mid I've ever done to my car!


    Year: 1999 Submodel: V6

    Helpful (1)

  • 3/5

    Installation Time: Less than 1 Hour

    October 04, 2012

    Nice tone

    Sounds good. The inlet and outlet were a little bigger than the measurements on line said they would be, but it wasn't nothing a little extra welding couldn't fix.


    Year: 2001 Submodel: GT Hebron, IN 46341, USA

    Helpful (1)

  • 5/5

    July 10, 2011


    Make my car sound great. People think my v6 sounds like the GT. Thanks American muscle


    Year: 1999 Submodel: V6

    Helpful (1)

  • 5/5

    February 26, 2011

    Nice sound and feel

    Awesome sound! Get's loud up to about 2500 rpms but gets quiet once you're on the open road in 5th gear. Make sure you know your pipe size. I had to have midas custom fabricate part of my stock H pipes to fit them. 2.5 inch pipes and 3 inch tips


    Year: 2004 Submodel: GT

    Helpful (1)

  • 5/5

    November 19, 2010

    beats the ground

    I have a 93 notchback with 3 in 2 chambers and they are super loud they beat the ground!

    austin dukes

    Year: 1993

    Helpful (1)

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