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fmf fatty or Gnarly?

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well im going to buy new pipes. im going with the fmf shorty. but im stuck on the fatty or Gnarly. my bike is all stock and geared 13/51 so it has good lowend. and its fast. but i dont want the gnarly and kill the topend.i dont really care about topend because i ride trails and never get passed 4th gear. i loves me some lowend! if i buy the gnarly and shorty will it kill my speed?. i dont wanna open it wide open and go about 30 mph:ride: lol. this is my first bike so im new to everything so im a nub:bonk:

but thanks for reading:thumbsup:

Edited by Elementx665

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IMO the gnarly, thicker guage steel, perfect for trails. The shorty is more for track, they make turbine cores etc, spark arrestors, just in case a ranger just so happens to test your bike... And takes no power away

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Instead of making a new thread i figured i would post here.🤣. I am also stuck between two pipes. The fatty and SST and im gonna get a FMF shorty as well. im looking to gain more topend but my mid range is beast so i dont want to kill that. which one?🤣

thanks for your opinion 👍

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For the '02 KX250, the stock pipe really is pretty damn good. It has the best overall spread of power in my opinion.

The Gnarly will enhance the low-end slightly, it's not a dramatic difference but it is noticeable. And it doesn't really kill top-end power, because there isn't really any top-end power from this engine, it's all midrange. But the engine will quit revving more abruptly once past the power peak with the Gnarly rather than getting a few extra revs to avoid a shift.

The Fatty will improve the midrange slightly with no loss up top, but it will soften the low-end somewhat. Not a big deal, but it does make for a little more clutch work in tight slow-speed sections.

The Pro-Circuit Platinum or Works loses a little more bottom than the Fatty, adds about the same midrange, and adds a little more up top than the Fatty. A nice pipe for open motocross tracks, but not so good for trails.

The Pro Circuit Platinum II is essentially a clone of the Gnarly. You'd be hard pressed to tell a difference between them.

The FMF Shorty silencer is only slightly shorter than the OEM silencer. It has a nice sound to it, but it's loud, and doesn't do squat to the power.

The Pro Circuit R304 is shorter than OEM or the Shorty. It does brighten the low-end throttle response slightly, but again, it's quite loud. Not worth it in my opinion.

Depending on what type of power you want, this is my recommendation:

For tight trails, use the stock pipe with an FMF Turbine Core or Q Stealth. Replace the stock reeds with Boyesen Power Reeds. You WILL have to rejet.

For very tight Supercross style tracks with obstacles right out of corners, use the Gnarly with a Shorty or the Platinum II with an R404, there isn't much difference between those two set-ups. Again, use the Power Reeds.

For more open terrain and tracks, run the stock pipe or the Pro Circuit pipe with a Power Core or 304 (not the R) silencer. Run Boyesen Carbon Pros, or better yet, the Boyesen Rad Valve.

Again, my preference is the stock pipe. It has the best overall spread of power.

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Whys that?
Steve explians it much better than I can.

Originally written by Moto814

The FMF SST pipe is an amazing pipe.

Why is it amazing? Because in just about every case, it will work WORSE than the stock pipe that came on the bike.

The SST uses a restrictive reverse cone and pressure bleed to intensify the return wave and lower the initial flow out the stinger section of the pipe. What that in turn does is increase piston crown temperatures and make jetting difficult at best.

Now increased piston crown temperatures can be compensated for to some extent with richer jetting. But, by doing that you are defeating the purpose of the pipe, and negating any power gain you may have gotten from the pipe in the first place. Why are increased piston crown temperatures bad? Well, in some cases they are not. You see most bikes are jetted horribly rich from the factory, and raising the piston crown temps in this case will "fool" the engine into thinking it is jetted leaner than it is. Which will in some cases improve power. But in most cases, it will just cause slightly better throttle response and an increase in the spooge out the back of the pipe.

After all that you may be thinking: "well this doesn't sound so bad, why are they saying this pipe is no good?" Good question. Here's the answer. Because of the restrictive nature of the pressure bleed from the pipe, it does not evacuate as efficiently as it needs to. This combined with the piston crown temperature increase yeilds jetting that is too rich. So now you have a pipe that "fools" the engine into thinking it is leaner than it really is, but also makes it much richer than it was previously. Here in lies the second problem. It makes jetting a nightmare. If you jet it by feel, you will be jetting richer, and compounding the too rich condition even more, making the bike run like a slug. If you jet it leaner, you will be increasing piston crown temperatures even more resulting in an engine siezure. Catch-22 anyone?

If you are buying a pipe for your bike, get ANY other pipe. Or better yet, keep the stock pipe. The late model (1999-2005) KX250 pipes are pretty good stock, and provide a good low to mid powerband. If you want more over-rev, go with a Pro-Circuit Works or Platinum pipe. If you want more low end go with an FMF Gnarly, or Pro-Circuit Platinum2.

I have personally experienced these jetting dilemmas with friends' bikes and the SST. I would never run one.

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👍🤣🙂 WOW thanks for that. So now i guess im going to get the Fatty pipe and the R-304 silencer.🤣:lol:

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Just curious. Why do you want a different pipe? Just because? Are you really fast enough that the stock power curve is holding you back? Or do you feel that you would benefit from relocating the power a bit?

If you want a pipe to "fix" what you feel is a flaw in the stock power curve, porting is a much better solution, and cheaper than a pipe.

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I have the Gnarly and can't tell from stock I would go pro circit or stock I just got a deal I couldn't refuse.

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Just curious. Why do you want a different pipe? Just because? Are you really fast enough that the stock power curve is holding you back? Or do you feel that you would benefit from relocating the power a bit?

If you want a pipe to "fix" what you feel is a flaw in the stock power curve, porting is a much better solution, and cheaper than a pipe.

porting sounds expensive. and yeah i want more top end. i can tear through the gears but once im in6th. it just dies off even when its pinned. 👍

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run the pc works pipe and r 304 shorty thats wat i had on my kx before it became a 500

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If you are a woods rider and don't like beat up pipes then the only choice is the FMF Gnarly or the Pro Circuit platinum 2. Their resistance to trail abuse is very good.

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porting sounds expensive.
A good port job is often cheaper than a pipe, and will give you far better results. You can have the power curve tailored to exactly what you want.

and yeah i want more top end. i can tear through the gears but once im in6th. it just dies off even when its pinned. 👍
A KX250 only has 5 gears?

It's a motocross, bike, they are neither tuned for nor geared for "speed", they are engineered for maximum acceleration in short bursts from turn to turn. If it's more speed on top you're looking for, lose a couple of teeth on the rear sprocket.

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A good port job is often cheaper than a pipe, and will give you far better results. You can have the power curve tailored to exactly what you want.

A KX250 only has 5 gears?

It's a motocross, bike, they are neither tuned for nor geared for "speed", they are engineered for maximum acceleration in short bursts from turn to turn. If it's more speed on top you're looking for, lose a couple of teeth on the rear sprocket.

No i ride a kx100. and yeah it does have very very short powerful burst. how many teeth should i lose?👍

Edited by Kawi100

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Oops, sorry, confused you with the OP.

The KX100 engine responds VERY WELL to porting. The hot ticket is to start with an 85 cylinder (smaller exhaust port), but that is an expensive route. But even the 100 cylinder benefits a lot from a good port job and head work.

If you insist on a pipe, go with the PC. A member here named Sappers is an old pro at modifying the KX85/100 engines, hopefully he will chime in here.

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well for now id like to update my silencer. then move to port work or pipe. Have you heard of the DEP silencer?( i think ill make a post) I've found a site to buy it off, but i dont even know if Americans can buy things from it. everything they sell is in pounds £. 👍:banghead:

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well for now id like to update my silencer. then move to port work or pipe. Have you heard of the DEP silencer?( i think ill make a post) I've found a site to buy it off, but i dont even know if Americans can buy things from it. everything they sell is in pounds £. 👍:banghead:
If it's on the internet, you can buy from it. But if it's overseas (pounds is British currency) then you are going to pay international shipping charges, which are likely to cost more than the silencer.

Dep Pipes has a U.S. distributor, and I've never heard anything bad about them.

http://www.deppipesusa.com/

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