Quieter exhaust options?

[Chuck78]

I did also pick up an FMF Q Stealth for a (Spanish-era, pre-rebadged-KTM) 2017-ish Gas Gas spark arrestor to try and fit to the KDX. I may need to graft a KDX inlet pipe onto it if I can't bend the stainless steel Gas Gas inlet stinger tubing on the Q Stealth properly.

I'd also always been wanting to try out an OEM '91-'94 KDX250 spark arrestor for fitment testing and assessing required modifications to fit a 200H/220. They are quite long and angled in the middle.
I currently have an Acerbis 035 which is very similarly shaped like a hockey stick, and made of some polyurethane/thermoplastic type material for the canister. They've been long since NLA, discontinued in the late 1990's. They've got a longer perforated baffle + Krizman turbine type spark arrestor on the INLET side. The Acerbis 035 is quieter than the NLA old FMF Q (2" longer version of an Turbine Core 2 that's at most 2-3 dB quieter than the Turbine Core 2),& also quieter than the Pro Circuit Type 296 (which is about 2dB quieter than the Turbine Core 2).

I'm not sure if the KDX250 OEM spark arrestor is a chambered design or just a really long perforated glasspack type tube, but a chambered design will be the quietest.
I'm not going back to the HEAVY stock welded steel muffler. Once they're clogged up, you mostly likely will only be able to clean them by cutting them apart and re-welding when unclogged. The substantial weight alone makes that not an option for me for an already top-heavy bike for a 2-stroke.

I'm curious how this Gas Gas FMF Q-Stealth will do once the inlet angle is adjusted. I also needed to use my plumbing) HVAC swaging tool to slightly expand the flared inlet female coupler portion.
From what I can tell, the Q-Stealth are no longer offered...?

[SS109]

The regular FMF stuff is the best I've tried but, like I mentioned before, you gotta keep it somewhat loose to really absorb the most sound.

Another possible option is getting an older E-series silencer, as it is chambered and the quietest one I've ever heard, and adapt it to your application. It hardly uses any packing material. You could also try taking a standard Turbine Core and making it in to a chambered design. Just try to replicate to some degree that of the factory E-series silencer internally.

I don't think any of the coatings available would handle the heat, sadly, and the ones that can don't help to reduce sound.

One of the problems with trying to quiet down the expansion pipe is weight. You know how the stock H-series pipe was built. What you could do is basically recreate a pseudo factory pipe type construction on your current pipe but with some of the insulating wrap and then strapping on just the outer skin of the exact same pipe over it. The outer skin would reduce sound and keep the exhaust wrap in place. It would probably add 5+ pounds but might get you to what you're after.

[Chuck78]

The regular FMF stuff is the best I've tried but, like I mentioned before, you gotta keep it somewhat loose to really absorb the most sound.

Keeping the packing wrapped loosely is a crucial step that I'm sure gets missed a lot by installers...


I think there's some fancier FMF Pillow Quiet packing iirc? I think my wife bought it for her 200 XC-W at one point, but it was more like a blanket with an outer covering vs the standard muffler packing type fiberglass insulation that you can grab a handful of and rip chunks out of due to just being a bunch of intertwined fibers stuck into each other, no outer covering. I can't recall any good comparison on that stuff as it's been a few years. I may try it again...

https://www.fmfracing.com/products/packing-015301

They call it 2-Stroke Premier Packing apparently, although the packaging calls it FMF Everlast Packing. It looks just like the 4-stroke Permier Pillow type packing, with sewn-in pleats to help it wrap around the glasspack's tubular steel core.

I don't think any of the coatings available would handle the heat, sadly, and the ones that can don't help to reduce sound.

One of the problems with trying to quiet down the expansion pipe is weight. You know how the stock H-series pipe was built. What you could do is basically recreate a pseudo factory pipe type construction on your current pipe but with some of the insulating wrap and then strapping on just the outer skin of the exact same pipe over it. The outer skin would reduce sound and keep the exhaust wrap in place. It would probably add 5+ pounds but might get you to what you're after.

I was really hoping to have some kind of spray-on or brush-on rubberized type high temperature coating to cover the pipe... Perhaps some sheets of silicone rubber wrapped around and strapoed might have to suffice for a test run? With some very slender stainless steel zip tie type exhaust insulation clamps, I believe I recall seeing something of that sort available.
This winter, or at some point, I plan to make a cage type expansion chamber pipe guard or support at least. I have the Hyde Racing pipe guard skid plate combo, but I still managed to slam on the pipe side and bend one of my new Pro Circuit Platinum 2 pipes... That makes 2 new and 3 bent pipes + 2 mangled Gnarly Desert pipes...

If I'm going to all that effort, I'll also try some sort of wrapping, but the heat wrapping fabric strapping rolls will retain water within theor woven fibers,so that's a no-go without finding any paint-on type high temperature rubberized coating.

I'll keep digging, haven't searched at all yet other than here.

[Chuck78]

This Design Engineering Inc high temperature silicone spray is specifically designed to coat and seal the fibers of exhaust header wraps, although I'm not sure how thick the coating is, or how long it'd hold up in an off-road environment...



The real issue with using the header wrap insulating tape is that it'll just want to slip and slide down to the smaller diameter portion of the pipe, so any coating on the header insulating wrap tape products would need to be a tough fiber-embedded coating.
If this stuff were thick enough of a film, I'm certain it'd help out with coating a 2-stroke expansion chamber and slightly deaden the sound waves. For $15/can, it's worth buying just to find out. I'll probe the Internet forums for others who've used this, to see if anyone can comment on the film thickness.
1mm or 1/16" thick high temp silicone sheets to wrap a chamber may be a good option as well I'm not trying to add 5lbs to the bike after dropping the dollar value equivalent of a running riding well used KDX on full titanium hardware and other aluminum weight savings KX etc type parts...





Also, I spent a ridiculous sum on picking up an NLA / NOS dB Snorkel, although being a bolt-on external silicone rubber tubing device, it's going to be subject to damage, do ultimately I'm looking to find a more permanent long term solution to this subject.
These reduce the dB levels up to 8 decibels, but I'd read KDX fitment requires a bit of work in some instances, depending on spark arrestor used. It also was stated that the Maier MX fender is best used with it, no issues there as I'm sold on their toughness despite really liking the OEM fender look.
One member here 15+ years ago said it changed their jetting requirements significantly despite mist users stating no jetting changes required, including a KDX dB Snorkel write-up on the Dirt Rider forums.

https://www.off-road.com/aimages/articl ... ticle.html

howitworks.jpg (10.75 KiB) Viewed 866 times

The NLA NOS FMF Q-Stealth chambered exhaust silencer with mods for KDX fitment might be a winner, although it is quite fat and needs modified heavily to fit, likely having the stinger/inlet pipe cut off and swapped for an OEM KDX or FMF inlet pipe. I traded my FMF Turbine Core 2 for one of my Acerbis 035 spark arrestor silencers, but I may try to find an FMF or preferably Pro Circuit muffler/silencer that's damaged, in order to use it as a donor for the inlet pipe swap.
The KDX250 OEM dpark arrestor gutted and converted to a chambered design like the E-Series, and with the interior degreased and sanded and coated with RYV Silicone type gasjet sealant might very well be an excellent solution also.... Or as you said, the OEM E-Series spark arrestor adapted.

[SS109]

This Design Engineering Inc high temperature silicone spray is specifically designed to coat and seal the fibers of exhaust header wraps, although I'm not sure how thick the coating is, or how long it'd hold up in an off-road environment...

The real issue with using the header wrap insulating tape is that it'll just want to slip and slide down to the smaller diameter portion of the pipe, so any coating on the header insulating wrap tape products would need to be a tough fiber-embedded coating.
If this stuff were thick enough of a film, I'm certain it'd help out with coating a 2-stroke expansion chamber and slightly deaden the sound waves. For $15/can, it's worth buying just to find out. I'll probe the Internet forums for others who've used this, to see if anyone can comment on the film thickness.
1mm or 1/16" thick high temp silicone sheets to wrap a chamber may be a good option as well I'm not trying to add 5lbs to the bike after dropping the dollar value equivalent of a running riding well used KDX on full titanium hardware and other aluminum weight savings KX etc type parts...

Might be an idea to spray the pipe first with that stuff, then apply the the heat wrap while it's still tacky, let it cure, then spray the outside with that same stuff sealing it all up. That would probably be the lightest setup you could do on the pipe. I think it would probably work and last pretty good.

[kdxdazz]

Kdx250sr muffler fitted to my kdx200 in the photo but as of yet untested as it's missing the rear baffle/chambers
Spent the last 2 days comparing the stock expansion chamber Vs procircuit platinum 2, after a while I didn't even want to ride the kdx with procircuit pipe because of the noise, OEM is 4.0kg, procircuit is 2.8kg, will try to do a write up at some point as I connected the wideband O2 sensor to both of them to monitor jetting changes needed

On the subject of muffler packing, I have here some OEM yz426 muffler packing from twenty something years ago when I bought it new, the first part of the wrap is fine steel wool, I think the idea is it allows the sound to penetrate in to the fibreglass

I've also tried the whole expansion chamber wrapping thing, my idea was wrapping with the heat proof wrap first then I cut strips of rubber from old car inner tubes, definitely a reduction in sound but nowhere near as good as OEM and then all the problems of water entrapment and rust

I'm flying back to Thailand this week, once I get the kdx250sr muffler up and running I'll report back on how I did the conversion and if it was worth it

[Chuck78]

KDX250SR muffler fitted to my KDX200 in the photo but as of yet untested as it's missing the rear baffle/chambers.

I'm flying back to Thailand this week, once I get the KDX250SR muffler up and running I'll report back on how I did the conversion and if it was worth it

A good friend of mine has a KDX250 which I've spent a fair amount of time working on for him, and I always wanted to take his stock spark & see how it would fit on my KDX 220, as looking at it, it appears very similarly configured.
I suspected the inlet pipe would be a slight bit larger diameter by perhaps 1mm or 2mm, but the mounting points certainly looked nearly identical, and your photo confirm, although uncertain if you've adjusted anything there.


You say the KDX250 muffler is missing the rear baffle/chambers?
Is it not a spark arrestor in the end of it? Or is the spark arrestor on the inlet side like my Acerbis?
More importantly, does it not have just a standard straight through glasspack type perforated baffle pipe?
If this is in fact a chambered muffler/silencer design, I'm fairly certain this would be the ultimate in noise reduction, although a chambered exhaust does add more back pressure and may affect performance.

The FMF Q-Stealth apparently is NOT a chambered design as I'd initially suspected, I would have had mine apart already but it is a riveted design, no bolts. The Q-Stealth is just a standard turbine core style perforated baffle tube + Krizman Bullet spark arrestor, BUT with A LOT more room for fiberglass packing inside, and that is what makes it up to 8 dB quieter apparently.

Screenshot_20250901-094151~2.png (2.99 MiB) Viewed 499 times

I'd finally found photos of a Q-Stealth disassembled as someone was converting a CR250 Honda Q-Stealth to work on their CR500AF conversion (larger baffle tube).
This brings up a lot of insight, if FMF put a massive amount of time into designing a quieter muffler, and their quieter design basically just took the FMF Q, which the FMF Q was roughly just a Turbine Core 2 but a few inches LONGER in the baffle)packing area (I have an FMF Q, their inlet pipe bends very easily due to the extra length/leverage beyond the rear mount, and lack of gusseting on the inlet pipe the way the vastly superior Pro Circuit Type 296 has the inlet pipe gusseted + thicker walled material for more strength), and basically just made a 4-stroke-type larger diameter muffler cannister/shell version so that the packing is more loosely installed and with more volume. Perhaps there is some math and science into the lengths of everything to get the sound waves to cancel out better as well?

A lot of people say the FMF Q Stealth made more power than the Turbine Core 2, and I believe it's because the glass pack baffle tube is actually SHORTER length and the spark arrestor is buried deeper into the end. I'm not quite sure what effect that has, but the spark arrestor is roughly 3 inches into the back of the Q-Stealth.
Here is a photo I took last night out in the garage, pardon the black and white, I try it out the night vision on my camera for this:

Screenshot_20250901-092036~2.png (1.07 MiB) Viewed 499 times

Perhaps there is a chamber in the rear that does something to dissipate the sound waves and cancel them out, but when you look in the inlet, you see a tube that goes several inches in before you see the Krizman Bullet type spark arrestor. This plus the larger volume of more loosely installed packing must be what makes them quieter, but they are quite large in diameter.

I will pick up a KDX 250 spark arrestor here for modification then if you say they are a chambered design, that should definitely be the best for noise reduction.
I will not go back to the stock OEM spark arrestor, however, especially being so high up on the bike, that is too much weight up high for an already top-heavy bike.

[Chuck78]

Spent the last 2 days comparing the stock expansion chamber vs Pro Circuit Platinum 2, after a while I didn't even want to ride the KDX with Pro Circuit pipe because of the noise, OEM is 4.0kg, procircuit is 2.8kg, will try to do a write up at some point as I connected the wideband O2 sensor to both of them to monitor jetting changes needed

I've also tried the whole expansion chamber wrapping thing, my idea was wrapping with the heat proof wrap first then I cut strips of rubber from old car inner tubes, definitely a reduction in sound but nowhere near as good as OEM and then all the problems of water entrapment and rust

I'm very curious to see what your testing results turn out as, although the only stock pipe I have is quite mangled, I would consider running a stock expansion chamber because I'm so tired of mangling and bending every FMF Gnarly Desert and Pro Circuit expansion chamber that I run. The FMF Gnarly Woods, with it's smaller cross sectional diameter and fairly straight front lower tubing run, somehow seems to be the toughest of the three, although I do like the strongly enhanced mid-range that the Pro Circuit gives, it is definitely the fastest & most versatile of the three pipes overall when considering both torque mid-range and top end power.
I do really like the performance of the Pro Circuit Platinum 2, but it has quite a mid-range hit, a substantial surge in power. I prefer it hands down over the Gnarly Desert (PC Platinum 2 = more torque and mid range), although I feel the Gnarly Desert took a few more hard slams to bend it significantly versus the Pro Circuit, so I do suspect the Pro Circuit wall thickness is a very slight bit thinner.




This winter to make some sort of standoff brackets from the frame to work with my Hyde Racing pipe guard skid plate combo in lieu of a cage type pipe guard, although the Hyde combo guard does have a very nice protective wrap along the entire skid plate for the bottom of the pipe exposed sections up front, and a bolt-on guard to protect the area of the expansion chamber on the side which gets dented the most frequently, however, it does not fully protect it from bending when you slam down on the pipe side.

I'll be looking for both a good condition expansion chamber AND a KDX250 OEM spark arrestor muffler, as well as modifying the FMF Q-Stealth to fit, for comparison.




I'm always envious of the KTM 200 sound... I believe the expansion chamber wall thickness is greater than the KDX aftermarket pipes, AND after around '04-ish, KTM quit making factory spark arrestors, and instead just have an extremely long perforated baffle tube, considerably longer than any KDX aftermarket spark arrestor, only similar in length to the KDX250's.
The '97-'03/'04 "hockey stick" KTM mufflers were similar to the KDX250's externally but just a hair longer, and included a spark arrestor.
The KTM SX (MX/Supercross) model versions of the long muffler are slightly shorter than the off-road versions.

[kdxdazz]

The kdx250sr is a chambered design, the first part is a resonator wrapped in fibreglass, the second part is the chambers, very complicated in design, the mounts don't bolt up at all to the H series, I had to cut off one ear and reweld it in a different position and made an offset mount for the other one, I bought it not knowing it was missing the chambers but I brought it back to Australia so I could make it fit the H series then back in my bag it goes back to Thailand, I have another kdx250sr muffler lined up to use the missing chambers
I don't believe there is any spark arrestor

Part of noise suppression in the expansion chamber is the thickness of the material so there is no way around it, heavier and quieter or not, the weight on the stock pipe compared to aftermarket really is negligible given the weight is carried low but like you say the procircuit has an incredible midrange hit compared to stock, I prefer stock for it's off idle performance and like you say the procircuit just seemed to wheel spin and break traction which made it exhausting to ride, I wasn't able to test it as much as I like due to noise and disturbing neighbours

I'll test the kdx250sr muffler using the wideband O2 sensor so I can see what jetting changes are needed

Not sure if you saw my drz400 hayabysa muffler conversion video but on that bike I tried 4 different mufflers and nothing beats size and chambers, it's not that the power is less but the engine was slightly less responsive which for me is a good thing as Thai asphalt is incredibly slippery

[Chuck78]

The KDX250SR is a chambered design, the first part is a resonator wrapped in fibreglass, the second part is the chambers, very complicated in design.
The mounts don't bolt up at all to the H series, I had to cut off one ear and reweld it in a different position and made an offset mount for the other one.

I don't believe there is any spark arrestor

I'll test the KDX250SR muffler using the Wideband O2 sensor so I can see what jetting changes are needed

I'll begin watching out for a good deal on a used North American Market KDX 250 off-road model muffler, as modifying one of these will most definitely be the most noise reduction possible if it is a chamber design and none of the other ones are, even the Q-Stealth.
I would bet that the NLA FMF Q Stealth is probably going to be the quietest without affecting any performance however.

I was almost certain the KDX250 was a spark arrestor, but the way I see it, a chambered muffler on a two-stroke absolutely does not need any additional devices to make it a spark arrestor. It is argued that two strokes don't even need spark arrestors whatsoever, but the U.S. Forest Service just requires them regardless, 2T or 4T engine.

EDIT: comparing your photos and the stamping on the outside, versus eBay photos of US market KDX250 spark arrestors, the US market off-road model versions definitely say US FOREST SERVICE APPROVED SPARK ARRESTOR DX250D stamped into them on the rear inner portion near the outlet. Your SR model has a turn down tip also which is known to reduce the distance the sound travels since it directs the sound directly at the dirt or pavement, instead of straight out through the air. The off-road model just shoots it straight out the back with just a stub of a pipe. The off-road model can definitely have a turn down added to it however, but I'm just not certain if the off-road model is anything more than just an extra long glass pack perforated tube.
I wish I would have been looking for one of these before all of these silly tariffs, when I was using the Japanese auction sites and buying services to buy parts from Japan for dirt cheap. I don't think that is very possible at this point. We never got the SR model here obviously.

Part of noise suppression in the expansion chamber is the thickness of the material so there is no way around it, heavier and quieter or not, the weight on the stock pipe compared to aftermarket really is negligible given the weight is carried low but like you say the procircuit has an incredible midrange hit compared to stock, I prefer stock for it's off idle performance and like you say the procircuit just seemed to wheel spin and break traction which made it exhausting to ride, I wasn't able to test it as much as I like due to noise and disturbing neighbours.

I, personally, absolutely love the Pro Circuit Platinum 2 power delivery and tune, even for compromised traction technical steep slippery woods riding, although the FMF gnarly Woods is definitely the better pipe in that department, the overall power delivery is resulting in much greater peak mid-range and upper mid-range power with the Pro Circuit, which is why I really I'm interested in doing something to quiet down the expansion chamber sound.

The muffler / silencer design definitely plays a large part as well though obviously, because KTM 200 models with their OEM non-spark arrested very very long glass pack mufflers are considerably quieter than any thing I have ran on my KDX's even freshly packed, while those bikes also had FMF aftermarket pipes instead of the slightly thicker OEM versions. That alone tells me there are significant gains to be had with a better noise canceling noise suppressing silencer design, although the stock KDX double walled pipe would absolutely be the quietest expansion chamber, but I'm not sold on the performance trade off if I could possibly do something further to suppress a bit of noise from the Pro Circuit expansion chamber.
I'm fairly convinced that the metal is slightly thicker on the FMF expansion chambers, but it's still a significantly louder than the stock double walled expansion chamber. Some of the KTM models even came with an oem expansion chamber made by FMF, however most people are running the aftermarket versions out of the guys I ride with after years and years of slamming the bikes down and damaging the chambers.


Had you made any write-ups yet about your take on jetting for the stock pipe versus the Pro Circuit? Or are you just done with the Pro Circuit due to the noise?

[Chuck78]

I just did some extensive searching on the majority of my 10 minute break time at work, and could find no real info on repacking the KDX 250D off-road model spark arrestor. I do recall reading somewhere that someone said it's a real pain to do, but that's been a little while now.
I was really hoping to find some pictures to see if it is the same as the SR model just without the turn down and with a spark arrestor added, or if it is a complete glass pack tube with a spark arrestor at the end like most are.

I suppose I will try browsing the Japanese auction sites again for one of these in hopes of getting lucky, but I may also just buy the off-road model version, whichever I can get a seller to give me a low priced eBay offer on, in order to disassemble it to inspect it's design first.