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Flowmonster Performance Muffler 3 in dual inlet & outlet 12599-FM 409 Stainless

Flowmonster Performance Muffler 3 in dual inlet & outlet 12599-FM 409 Stainless

SKU: 12599-FM
Regular price $94.45 USD
Regular price Sale price $94.45 USD
Sale Sold out


The Flowmonster series of absorption style mufflers deliver a moderate tone for the enthusiast looking for a deeper, more aggressive exhaust note than most OE (Original Equipment) mufflers, unlike the racey exhaust note associated with "chambered" style mufflers.

Interior construction features 3.50 inch "straight-through" perforated 409 stainless steel tube surrounded by high-temperature sound absorption packing material which helps to deliver a deeper, mellower exhaust note. The internal design is reversible allowing the flexibility to be installed in either direction to best accommodate fitment with the intended vehicle. The fully MIG-welded 409 Stainless Steel material is embossed with the "FLOWMONSTER" logo on the case.

  • 409 Stainless Steel construction for superior corrosion resistance and good looks
  • Interior construction features high-temp interior sound absorption material delivering a deeper, mellow exhaust tone
  • Perforated High-Flow straight-through tube design for increased exhaust flow
  • Moderate increase in interior tone over most OE mufflers
  • A stainless steel performance muffler at an economical price
  • Internal Design is reversible allowing flexibility to be installed in either direction
  • Sold Individually
  • This is a Universal Fit muffler - Not a Direct fit. Some adaption may be required for proper fitment on some applications
Body Height (in) 5
Body Length (in) 22
Body Width (in) 11
Finish Natural
Fitment Universal
Inlet Connection Type Pipe Connection
Inlet Diameter (in) 3
Inlet Position Dual
Material 409 Stainless Steel
Muffler Series Flowmonster
Muffler Type Absorption / Glasspack
Outlet Connection Type Pipe Connection
Outlet Diameter (in) 3
Outlet Position Dual
Overall Length (in) 28
Reversible Yes
Shape Oval
Sound Level Moderate Sound
Type of Muffler Absorption
Part Number 12599-FM
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Policies and FAQs

Return Policy

We accept returns on products within 30 days of delivery. Return shipping is the responsibility of the buyer unless we, the seller, are at fault.

Fault reasons include but are not limited to:
  • Buyer was shipped the wrong product
  • Buyer was shipped a damaged product
  • The product page had incorrect fitment and the part does not fit the Buyer's vehicle
The Buyer will not be refunded outbound shipping unless the Seller (Modern Day Muffler & Brake) is at fault.
In cases where the Buyer purchases and ships the return please note that if the Shipper (ex. USPS, FedEx, UPS, etc.) loses the return we will not be able to offer a refund as the Buyer must contact the Shipper and make a claim.
All products must be returned in original condition, meaning that the product must not be damaged, installed, or missing pieces/parts. If the product is not in original condition a deduction will be issued of up to 100% of the purchase price of the product.
Opened boxes, plastic bags, and other packaging are not considered damage (excluding single use items such as, but not limited to, liquids, software, and other components that cannot be reused/resealed).
When we, the seller, are at fault, please Contact Us for a return label, products returned with a shipping label not provided by the seller will not be refunded to the buyer. Please also note that our return labels are issued with the dimensions and weight of the original shipment, if the product is being returned in any box/package besides the original please Contact Us with the dimensions and weight.

Returns must be sent to this address:
Modern Day Muffler & Brake
774 Broad Street
Wadsworth, OH 44281
Products returned directly to a manufacturer or the incorrect address may be issued a deduction or denied a refund entirely.
If a product was delivered outside the 30 day return window or is no longer in original condition please contact us as we may be able to issue a refund in store credit and/or with a deduction. If this is the case please Contact Us as these situations are handled on a case by case basis.
If there are any questions, concerns, or doubts about return eligibility please Contact Us and we would be more than happy to assist you.

This page and policy were last updated: July 14th 2023


This is the line that runs from the front of the vehicle to the rear. On most vehicles it connects to a junction then you will have rear axle lines. On a few cars the intermediate lines will connect to the rear wheels. It is recommended to change the axle lines if you are replacing the intermediate lines.

Our kits include all of the solid metal lines for that section of the vehicle.

We are happy to help! Please help us to help you by going to our return form and filling out the form as best as you can in order to request a return.

  • Modern Day stands by our parts and is happy to accept returns within 30 days of purchase. Parts must be in new condition.
  • If a part arrives damaged, or is defective, Please report it to us immediately. So we can handle it for your promptly.

Yes but you will need to order in advance. All of our items are not stocked at our location. So please place your order online and we will notify you when it is ready for pick up.

Unfortunately we cannot offer single lines outside of kits

Yes, all are covered by the manufacturer's standard warranty. Most warranties cover the cost of parts. Any installation or other fees are non-refundable.

Online we accept PayPal and all major Credit Cards. If your are picking an item up we gladly accept cash.

We can only ship to your confirmed address in PayPal or your billing address if you pay with a credit card.

Orders typically Arrive within 3 to 10 days depending if it is a stocked item or something we have manufactured at time of sale.

Yes please send us a message through our custom orders page.

Yes, but items sent out of the continental U.S. may incur additional charges based on their destination

The left and right sides on a vehicle are determined as though you were sitting in the driver's seat. The left is the driver's side, and the right is the passenger's side.

Check it again tomorrow. It takes up to 1 day for the carrier to give status and a delivery date. UPS and FedEx provide delivery dates, USPS usually does not but they take no longer than 5 business days.

Yes! Thank you for your military service!

Still have a question or concern?  Please contact use our contact us form!

Yes! Thank you for your military service!

Warranty Information

Customer service is our top priority and because we value your time and energy we ask you to visit our Contact Us page and reach out so that we may assist you in finding the correct warranty for your product. Please include your name, the name on the order, the order number, and the SKU of the product to ensure the quickest and most accurate response to your inquiry.

Product Notices

*NOTE: Product images may not reflect actual product appeareance.

Customer Reviews

Based on 1 review
Auric.R B.
Monster muffler

After a lot of asking , finally got a invoice that can be used for customs ,wished that they understood me earlier,but i finally got it thru customs and had to pay 111.50 dollar for shipping overseas so it was not cheap to get it , but super happy has an awesome sound and hope this will last a while before all the fiber cloth burns out in the exhaust , time will tell