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FMF Factory 4.1 Slip-On Exhaust

Retail: $374.99 - $649.99
$149.77 - $589.99
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FMF Factory 4.1 Slip-On Exhaust

Product Description

The FMF Factory 4.1 slip-on exhaust is the same exhaust that professional racers use to bring home championships around the world. FMF constructed the Factory 4.1 slip-on exhaust to offer the lightest weight slip-on on the market. The technology in this slip-on exhaust cannot be found in any other exhaust. The proprietary Chamber Core technology allows for smoother and more efficient exhaust flow to maximize the power output. Since the Factory 4.1 slip-on exhaust is meant to be used for competition, FMF made this pipe ready to take to the track straight out of the box. The FMF Factory 4.1 has been designed to allow for easy maintenance with a removable sleeve for repacking and it offers the ability to fine tune the performance of the exhaust with the addition of tuning inserts and the option of a spark arrestor to give you no boundaries.

  • The lightest system available.
  • Boosts low-mid torque and offers a broader powerband with more over-rev.
  • Patented Chamber Core for optimum performance.
  • Engineered for compactness and centralized mass for improved handling.
  • Exclusive mounting design.
  • Easily serviced for re-packing.
  • Slip-on mufflers are available with a titanium or stainless steel midpipe.
  • Customizable with performance tuning inserts and spark arrestor module.
  • Meets AMAŽ sound requirements on most models.

Detailed Specifications:

  • Type: Slip-On.
  • Construction: Carbon fiber.
  • Midpipe: Titanium.

Note: For closed course competition use only. Not intended for street use.

  • Customer Reviews
  • Ask an Expert
  • Also Offered For
1-10 of 29
Ease of Installation
Location: Miami, FL, USA
The best pipe there is

the fmf slip-on exhaust is the best exhaust out there i put it on my raptor 350 and it is amazing.the bike has a noticeable increase in speed horsepower and torque.the pipe has a very deep sound to it without the spark arrester and with the spark arrester it is very quiet.Thank you motorsport team for the great advice of the pipe and evrything else.

Ease of Installation
Location: mountaintop pa
07 polaris outlaw 525

I put this pipe on my sons outlaw and it is outstanding. 10 minutes to install and it sounds like a hoard of bees , may be loud for some but I like it Haha neihbors don't but who cares. the power is instantly noyicable.only one pet peeve , when not on the gas it has a peculiar whistle which makes it sound like something is loose and rattling on it.highly recommend if you don't mind loud

Ease of Installation
Location: Terrell, TX, USA

I installed the Factory 4.1 on a very strong running 07 Honda CRF450R and what a huge improvement. Combined with a FMF Mega Bomb header this thing is a beast. installation was a simple 15 minute task, the fit is perfect. Awesome product.

Ease of Installation
Location: Eugene, OR
5 Star Muffler From FMF

If you are looking for an exhaust, this slip on application is a great addition to your bike. I installed it on my 06 YZ450F. Would buy another one in a heart beat if need be. Installation was a breeze with the slip on, the blue anodized finish is TRICK, and the muffler is holding up really well after numerous rides. RIDER BEWARE!!!!! This is a louder pipe designed for closed course riding. If your riding area is stringent on decibel ratings, think about some of FMF's quieter mufflers such as the new Q4. The new ones look much better than the ones I saw when shopping for mufflers and believe me, they will not get you tickets that end your ride!

This muffler is trick!
Ease of Installation
Verified Purchaser Featured
Location: Orange County CA
Hits Hard!

Paired this up with the Megabomb header. Night and day difference! Hit much faster than stock and noticeable mid range increase. Used this set up on 2011 YZ450F

Ease of Installation
Verified Purchaser

This product combined with the power bomb header completely got rid of the throttle snap bog on my 2007 crf250r. Added tons of power on bottom and mid, would recommend to anyone lookin to get rid of the stock duals

Ease of Installation
Location: rathdrum idaho

i had just sold my go kart and used the money i had earned to buy this and the megabomb header...i would say that is not necessarily worth the 600 dollars only because im not a pro rider i am sust starting to race. but it dafinately helped my CRF 150R in the manner of my 2nd gear holeshot, it kickes butt now...

Top 500 Contributor
Location: Peralta, nm
Big Difference

I have been running another brand of pipes on both of my CRF250Rs for just over a year now. I put a power bomb header with a 4.1 muffler on my '09 and WOW what a difference. Not only did it lower the sound, but the power increase was very real from bottom to top.

Location: Southern Illinois
Good, Quality Exhaust

I put this slip on exhaust on my 2005 Yamaha YFZ 450. It went on without any problems. FMF also gave a larger main jet with this exhaust. I contacted FMF and they said the larger jet should keep the air fuel mixture correct but if it is not perfect to turn the air/fuel screw located on the bottom of the carb out a half turn. I put the jet in but did not mess with the air/fuel screw and my atv runs great. There is a big difference with this slip on exhaust and I recommend it to everyone.


Great product...low to mid range gains

1-10 of 29
Total Questions (30)
Questions with answers:
Questions without answers:
1-10 of 30
jesse 1 month 5 days ago Location: australia
Will this fit my standard header pipe?
i have a 2010 ktm 530exc in australia and was just wondering if this slip on fits the standard header pipe.
Expert MotoSport Staff
1 month 4 days ago Location: Portland, OR, USA
Yes, FMF Slip-On exhaust systems are designed to fit OEM headers and FMF Headers. Normally FMF will include a spacer sleeve (if necessary) that fits into the end of the mid-pipe that allows the pipe to mount up to the Stock KTM Header.
Josh05crf250r 1 month 21 days ago
Do you have a fmf slip on pipe for a 2005 crf250r?
Expert MotoSport Staff
1 month 21 days ago Location: Portland, OR, USA
Yes, This FMF Factory 4.1 Slip-On would fit your 2005 Honda CRF250R and we also have quite a few others that would fit your bike too. Just be sure to input your bike as the ride you are shopping for and the website will show you what slip-ons would fit your bike. Or here is a link that will get you there directly:;CRF250R;2005&mmy_source=topnav
JW59 Over 5 months ago
Do the FMF Factory 4.1 Slip-On Exhuast meet 96 db?
MotoSport Staff
Over 5 months ago
Currently the AMA has a 94db limit. FMF states that it meets AMA sound requirements on most models so you should be just fine if the tester follows the AMA testing procedure. The AMA will test 250 four-stroke at 5000 rpm and a 450 four-stroke at 4500 rpm.
joe59 Over 6 months ago
Does the 4.1 Slip-On Exhaust come with a spark arrester?
MotoSport Staff
Over 6 months ago
Yes, the FMF Factory 4.1 Slip-On Exhaust will come with a spark arrestor.
Over 6 months ago Location: Springfield, MO, USA
I purchased the 4.1 titanium and yes mine did i think they all do they come with quite a few goodies in the box fmf is awesome like that
OrionRX125 1 year ago Location: London
Will this fit on an Orion RX125 and is this road legal in the UK?
MotoSport Staff
1 year ago Location: Portland, OR, USA
Unfortunately I don't have anything listed for your model bike. Also, I would check with your local law enforcement to determine what is considered legal on the road before purchasing an off-road exhaust.
DuB25 1 year 1 month ago
With this slip on exhaust, will I be able to put my Pro Moto Billet spark arrester endcap on it or does it not fit or just literally can't be put on?
MotoSport Staff
1 year 1 month ago Location: Portland, OR, USA
The Pro Moto Billet End Cap does not fit on the FMF Factory 4.1. However, you can purchase an insert to fit with the Factory 4.1.
SkEwDaGo Over 3 years ago
I would like to put this on my raptor 700. How much more power will it add?
Expert MotoSport Staff
Over 3 years ago
unfortunately we do not have power specs on exhauyst systems. it varies to much even between the same make and model bike to know for sure.
Zacky Over 4 years ago
Can you do this?
If you have a dual exhaust system is it ok to put on just a single exhaust or does it have to be dual?
Over 4 years ago Location: Cream Ridge, NJ
it is alright to replace the stock dually system with this single system. instead of having the split mid pipe, it will just come right off the back like other single systems. personally, i think the dual system looks better than thesingle. as with any exhaust system, you may have to change your jetting based on location and other mods you have on your bke.
Tyman15 Over 4 years ago
What is the mid pipe for?
I don't think that my crf250r has one on the stock header, so I' a little confused. How this is going to work does the muffler just fit on the stock header without the mid pipe or do you have to put the mid pipe on to fit the muffler? why does it come with it?
MotoSport Staff
Over 4 years ago Location: P+RTLAND
I'm guessing you have at least a 04 or 05 crf250r so basing it off of that your stock mid-pipe and silencer are all one unit complete unit. The 4.1can and mid-pipe come apart so the mid-pips is required. With out it, there is nothing to connect the header to the can it's self.
Over 4 years ago Location: Cream Ridge, NJ
basically the mid pipe connects the header and the muffler. if you have a dual system, it is the piece that comes right behind the mud flap that looks like a Y. every bike has a mid pipe. if you get a plain slip-on muffler, then the muffler just conncts to the stock mid pipe
rookie Over 4 years ago
Will this fit a stock 2008 crf 250r pipe?
Expert MotoSport Staff
Over 4 years ago Location: Portland, OR
You will need the stock oem head pipe to install this Slip ON.
1-10 of 30

Dirt Bike


Honda CRF150R
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Honda CRF250R
Honda CRF250X

Honda CRF450R


Kawasaki KLX450R
Kawasaki KX250F

Kawasaki KX450F




KTM 690 Enduro R


Suzuki RMZ250

Suzuki RMZ450


Yamaha WR250F
Yamaha WR450F

Yamaha YZ250F

Yamaha YZ450F


Arctic Cat

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Honda TRX700XX


Kawasaki KFX400
Kawasaki KFX450R





Polaris OUTLAW 525 IRS
Polaris PREDATOR 500


Suzuki LT-R450
Suzuki LTZ400


Yamaha RAPTOR 350
Yamaha RAPTOR 700

Yamaha YFZ450

Yamaha YFZ450R
Yamaha YFZ450X



What It Is

Basically, this exhaust system is a miniature version of the same system that you can find for any full-size motocross bike. It is also FMF's first full system for the all-new Honda CRF150R. We tested a carbon fiber and titanium muffler mated to a titanium MegaBomb header pipe, but the muffler is also available in full titanium.


Right from the start, the increase in power and over-rev was instantly noticeable. From the bottom to the top, the bike pulls longer and harder than the stock system, and had even enough power to pull our full-size adult testers around. The pipe didn't create a massive hit anywhere in the powerband, but it did make the bike easy to ride by giving it strong, smooth power all throughout. Like always, the craftsmanship of the entire FMF system is second to none. And just slapping it on the bike increased the bling factor significantly.


It's hard to find misses with this exhaust system, but as always, the price sings. At close to $900, this system is definitely a big investment, and when considering it is probably for a bike that junior will outgrow quicker than you can say... well... outgrow, it might not be the best way to spend your hard-earned dough.

The Verdict

Every time we test an FMF exhaust system, we come away very impressed. The power increase, looks, and craftsmanship all help to elevate FMF to the top of the market. If it wasn't for the hefty price tag, this little system might have scored a perfect 10.

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