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Shortened Silencer

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Has anyone else did it? I have a 98 RM250 with a fatty and a stock silencer. I did some research on RMatvMX on the FMF shorty and i was reading the reviews and some smart @$$ kept posting that cutting the stock pipe down would do the same thing. so I gave it a shot. I cut 5 inches out of my silencer and repacked. noticed a difference on the bottom end for sure and gets on the pipe on easier when u pull the trigger. saved me a $115 for now :lol::thumbsup:

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Cool. Should post up a pic if possible. Just curious is there really an advantage to a shorty silencer? I've been considering a buying an PC R304 silencer and PC Works pipe to move my power up into the higher RPMs. I currently Have a PC platinum 2 and R304 shorty.

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the silencer manipulation . is better if you want louder and less torque!(possibly) I went back and forth a couple times from stealth to stock and nobody is gonna tell me that 'shorter is deeper'. To go to the extreme, take off the silencer altogther and see how 'deep' that is! That firecracking sound is immensly deep!

Basically, the silencer is an expension chamber after the expansion chamber of the pipe does all its work which is why there are so many different pipes. It 'may' increase top end by letting exhaust out faster, but again, that's controlled more by the pipe and if you didn't have to rejet or change the air screw, cutting a silencer really doesn't to anything, but piss people off............

To this I'll also add a recent experience even though it's with a blaster as my kid wanted to take off the stock silencer, heavy as hell, and put on my old rm250 stock silencer. The tone didn't really change much and the top end went up, however, to get the torque back on the bottom the air screw needed manipulation as does his gas formulation, now. He also has to change his style of throttling a bit, too. That may or may not be applicable as there is no PV on that bike as far as I know (it's not mine so I haven't looked) and the pipe is stock, too, though all I'm saying is that if you didn't have to change other stuff with the chop and didn't notice higher top end and loss of torque, the chop did nothing but make it louder as all your performance is being controlled by the pipe.

Edited by wrencher2s

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Considering that FMF has been engineering and building silencers longer than many of us have been riding I would kind of take their word for it when they claim that their shorty silencer boosts low end and mid range power.

http://www.fmfracing.com/Mode/Category/61

And with that fact, I would say that when WR400Fdunk shorten his silencer and experienced a low end boost, he was totally correct in his impression and offering a mod that would be worth repeating.

Good job WR400Fdunk!, positive gas on the way! :thumbsup:

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shorter the silencer the more aggressive the powerband..my 2c worth, i a point i guess, im sure a baked bean can sized one wouldn give you a neck breaking band lol

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Ok i really got to beat on my bike today. as a skeptic myself I didnt understand or buy into the though that a shorter exhaust would actually do anything but make the bike louder.. and I was thinking that the difference in "Core diameter" would be "boosting low end" not the length of the silencer.. but after deciding to break the hacksaw and rivet gun out and give it a try.. and to my suprise it made a "noticible" difference in snap out of the corners and response when short shifting.. when lugging out of the woods in second gear the bike requires no trigger pulling :thumbsup: to get on the pipe like before and when in 3rd the bike doesnt fall off the pipe when bouncing through heavy roots and erosion on steep long uphills.. all and all i would recommend anyone to try it its easy and in my case worked pretty well for my bike.. thanks for reading

good stuff :lol:

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I have no idea why but this works in reverse with a 4 stroke. shorter the silencer the less back pressure and you end up with less grunt and more top end. a 2 stroke seems to be more snappy while less overall exhaust length. i have heard of people cutting a few inches out of there expansion chamber to modify power curve.. i have a $200 fmf fatty moto head pipe.. im not getting near it with a torch or saw. i figured if i messed up the silencer it would "force" me to have to buy a brand new shorty or powercore 2 (key word "force" is the word i would have to use on the wife.. as if i HAD to buy a new one or my bike would explode :thumbsup: ) i figured it was a win win.. because if i messed it up i would end up with a pretty new silencer if i did a good job on it would i could live with it.

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after listening to the bike most of the HIGH pitch sound comes from the expansion chamber being audible due to the factory silencer being much quieter.. the deep tone exists but is over powered by the pinging and cracking in the expansion chamber. shortening my silencer took about 5 inches of core length and packing out.. bike tone isnt necessarily and DEEPER per say, only more audible :thumbsup: so the deeper more powerful sound is enough alone for me to do this mod. not that theres anything wrong with that "0 M. F. G! this thing is about to FN explode" sound we all have grown to love so much :lol:

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I guess you can keep reading the advertising all you want and depending on the pipe, it may give you more 'bottom end' though it's hard to believe cause if that were the case, a free flowing straight pipe would be awesome. WRONG! Which is why there are some many different pipes. Read reviews on the FMF turbine series and they say the same thing as the shorty. I guess it could be that any FMF silencer does in fact do everything! Restricting exhaust pressure increases torque on the bottom........and different silencers on different pipes will give different results.

Just for fun, here's a thread from our buds saying the FMFq helped his torque two posts down or so:

http://www.thumpertalk.com/forum/archive/index.php/t-761933.html

BTW, the shorty doesn't have a spark arrestor and actually you could get the same effect by packing the insulation less tightly and/or putting in a 'block' about halfway out with no insulation at all. Same thing, no cutting.

this is a list of all their silencers and 'advertising' and just we are sure, the 'shorty' is for the track and I can tell you I'm not riding in the woods and trails with the same rpms as you do on the track!

http://www.fmfracing.com/Mode/Category/52

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I gave up on reading the crap they post about there products and modified my "stock" silencer to close to there dimensions (except core diam) and in doing so It helped my bike on the low side.. bottom line is my bike has more grunt, snap, torque or whatever u want to call it or not call it.. it worked.. im not trying to advertise any specific product or make you hack up yours.. IT WORKED FOR MY BIKE do what you want to yours i could care less..

Its not like im trying to get everyone to run out and buy stock silencers and cut them short. regardless of theory.. I cut 5 inches off my pipe I have a deeper more powerful sound and the "powerband" comes in much lower and much stronger and comes in sooner. im happy with what I ended up with.. my silencer had no sparky since i have had the bike so.. and the shorty is very respected and used around here on the GNCC circuit.. i ride my bike wide open as often as I can.. woods,track, roads, railroad, gravel pits, whatever.. I have no Issue riding my bike fast in the woods.. but I do know that whatever "MAGIC" that has happend to my silencer by cutting it short helps me in and out of the tight corners followed by steep uphill climbs as well as in the tight stuff on the track

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Why you are so angry is beyond me as I'm making a logical discussion as to why one might want to do or not do this, but maybe you are mad because from your last post you just proved what I was talking about.

1.)The pipe is what controls low end torque and that info is everywhere, furthermore, regardless of silencer, when pipe manufacturers tell you their pipe is going to give you more low end, they tell you it's at the LOSS of top end. Silencer is not mentioned, that's the way it is and it is that way because design physics of scavenging dictates there is no pipe which is both a torque monster and high rpm pipe. More bottom due to the design of the pipe and how it scavenges which is TUNED at low rpms costs you high rpms. I went back and forth between a Q and a shorty and found no difference as we will examine below (pipe related ) AND in part because a shorty has a smaller circular dimension out compared to a Q and the Q is deeper no matter what you say and physics dictates that. That little stock silencer was so loud with the little out dimension it was just deafening.

2.) I still dont' think I've seen what pipe most of you were using before and you are confusing 'snap' with torque which have nothing to do with each other and I said that a shorter or NO silencer is going to enable gases to escape faster and allow faster spin up to the MAX design of the PIPE ONLY. That is not torque and logic dictates that if your pipe is permitting that much change from a silencer change it is not necessarily a low rpm 'woods/torque' pipe. And I can tell you that as I'm climbing, i want TORQUE and not some SNAP which is gonna kick in and break loose or raise the front wheel. Snap and torque are not the same AT ALL. Snap is quickness to raise rpms, torque is power across an rpm range.

3.) Now this is the best because you ADMIT you are running in the same section at higher rpms and closer to the powerband...........that is not and has NOTHING to do with torque at all and in fact you have actually DECREASED your torque curve and made it more spikey which you may like to ride with, but most don't and if you really want that old school BAM, go back to straight pipe which will be either on or off the powerband with both and all DECREASING the usable torque curve. Increasing your rpms has shifted your torque curve HIGHER and not lower like you suggest.

4.) For some pipes this will do nothing and in fact we see why because with the shorter silencer it takes GREATER RPMS which the manipulator admits to create the SAME EXHAUST PRESSURE FOR HIS PIPE! Higher rpms to make the same pressure is NOT increasing torque at lower rpms, but moving the torque to a higher rpm range which is OPPOSITE of the claims.

In closing, if you've got a pipe for which this will work, good for your, but 'snap' is not torque and the fact is that you haven't increased your torque at the bottom, but have increased rpms putting you closer to the powerband which is the OPPOSITE of what you think is happening because you have LESSENED the torque currve! I'm also not disagreeing that it may have worked for you which you are still angry about due to your style of riding and pipe and set-up, but I'm trying to tell people it will NOT work for all or even most as it's pipe dictated and instead of just hacking and chopping, repack the silencer with 'half the packing' in the front and then block off the back so the packing can't move OR do the opposite with packing at the back and the reason for this is it's a simple check to see if it suits you and your pipe and won't cause you $125 or more cause you chopped the shiit out of your silencer for what turns out to be nothing. For lack of a better term, you got lucky, and not everybody will and even another way is to do a limited test with no silencer at all and see how you like that and then find a happy medium that the neighbors won't like that for long!

Edited by wrencher2s

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Ok. Your post was very windy so I didn't read much of it. My bike is pulling harder before it's on the pipe. It lugs around much easier then before. Torque has nothing to do with this post. I cut my silencer and my bike is easier to ride at low rpms it also has more snap and finds the powerband much easier. I might of lost torque as your saying but I made up some unexplainable new curve of power which had not been defined yet in the horsepower books.. Save it man I really don't care. If It didn't work I wouldn't of bothered posting.

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Sorry this is all so boring to you? It was also interesting in the link that with the yz they found the BEST pipe to be the stock!

A bike or anything will lug around easier with less back pressure as will an exhaust opened car can lug in 5th gear at 25mph, but it ain't 'jumping to life' in a second. Are you running a FWW?

Torque has everything to do with this as torque is part of the 'lugging' and ability to snap and pipes are made for bottom end torque as we've discussed.

Rider said it right in what you done is made the powerband more aggressive because you've INCREASED rpms giving you more torque though that raises the sense of doing something which decreases efficiency as you need higher rpm to cover the same ground which eats gas faster.]]

You've also indicated all these improvements, but said nothing about a higher top end which is also counterintuitive.\\

Somebody was discussing how this works opposite with 4s? Not really as drag pipes on harleys and straight pipes on car DECREASE torque on the bottom end and if you want real life MX bike proof, look at the 450mx honda in the supercross and curly Q exhaust coming off the front looking like a 2s pipe.............to increase torque on the low end!

In your first post to go along with higher rpm in certain sections you admitted it gets to the powerband faster..............because your rpms are up to start with decreased pressure and you are already closer to the powerband in the first place along with the higher rpms naturally increasing torque! Higher rpms are not improving lower end 'grunt' which is 'torque' btw the and the fact is you likely aren't anywhere near the lower rpms you used to be.\\

Again, like the other guy said, you've made your powerband more aggressive which is what I've also said in that you've made it 'smaller' and more 'spikey'.

Finally, look at the dimension of the tube coming from the pipe to the silencer and what do you think is controlling the exhaust pressure as that is significantly smaller? If you 'really' want to change something, cut down the pipe to silencer interface. That is the limiting factor and that's why bottom end pipes lose top end as they can't get exhaust out fast enough at high rpms just like if you boost turbo pressure on a car, but leave the exhaust stock............that extra pressure has nowhere to go and does nothing.

Edited by wrencher2s

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Once again.. Very windy.. No fly wheel weight.. Bike has very wide and useable power bike is running normal low rpms bike has stock tranny with 14/48 sprockets.. Very wide long first gear and the rest of the gears are wide and long I don't have to shift out of second gear on the track.. Bike has almost no low end to speak of to begin with.. When I'm talking about lugging in refering to the bike almost dieing and rattling and fighting to stay running.... Just above that point.. Right where the chain stops making noise against the guides. The power in which I'm refering to has nothing to do with the powerband.. The power I gained was within the large dead lazy spot the bike spends the first 15 to 25 feet in when NOT slipping the clutch to get it on the pipe.. I don't know what you are expecting me to call it.. You have done broken out all the rule books on why everything I have typed is wrong.. But call it what you want.. The bike does better before the powerband.. My bike had great topend power and hasn't been effected by the mod. If it had lost topend or the topend had been sapped at all I wouldNt be able to pull 5th so easly with this gearing.. Bike still has great midrange and topend.. It's just not dead before it's on the pipe anymore

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Where here is what I would do if I were you which is why I bored you so......

SHUT UP AND GET TO A PATENT LAWYER! I'd go so far as to delete all my prior posts, too as you are well on your way to be VERY RICH as I'm sure we'd all buy your system if it does what you said!

You have obviously done what no other pipe or silencer manufacturer has figured out in years of testing and computer engineering.

You've just smoked them all so I'd be putting all my efforts into this endeavor and becoming a millionaire in short order because you've done what nobody else could do including eric gorr or FMF or pro-circuit and now you've done the impossible, so shut-up, delete the posts, lawyer up, and patent it and then sell it or make it yourself!

And no, that's not a bunch of smart ass stuff, but if it does all that we all want it and why not pay you instead of overpriced exhaust makers who can't figure out half the benefits you've gotten!

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hey - cool it cats! your readers have learnt stuff from both of you! wr is annoyed cause he just wanted to pass on results from an experiment which worked for him, and good on him for doing so. wrencher wanted to clarify what's actually happening from what he knows about this stuff, and good on him for that too. you're both willing to help us learn about stuff, so you should both pat yourselves on the back for contributing to the kitty. As a result of both your postings, I'm very tempted to try removing half the packing as wrencher suggests, to emulate what wr did and found gave him apparently better all round power. Worth a go I reckon!

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Thanks and now we'll try to clear up some miconceptions and why there is some major problem with his quote below which nobody is picking up on.

First, a pipe consists mainly of the expansion chamber and the outlet. Look at a wood pipe and an SST and the SST is far 'smoother' with a smaller expansion chamber and likely a wider outlet or shorter to the silencer. Nonetheless, the SST is going to dictate you keep the rpms higher to be in/closer to usable power and the powerband.

Why is that?

True to what WR says is happening is because a LARGE expansion chamber gives LESS PRESSURE at low rpms because the exhaust can't fill the chamber as fast or a full. Now that makes sense because a 2s or any engine works on BACK PRESSURE and with less back pressure, you get more low end grunt because the back pressure is NOT pushing on the piston . (They claim 'scavenging' and while that may be true with a 4s, it's bunk with a 2s because it's OBVIOUS the larger volume of the expansion chambers on low rpm pipes are allowing less pressure on the piston at those rpms and has NOTHING to do with 'scavenging'. Chop that 'restrictor tube' from the pipe to the silencer and you see rpms jump and pressure drop)

That brings us to the powervalve which is 3 main parts, too, the spring, the rpm, and the pressure change.........uh, oh, did you say pressure again? Look at your bike specs and there are TWO compression ratios with the sMALLER ratio being when the powervalve opens...........IT LOWERS PRESSURE on the piston from the exhaust and that lower pressure on the piston allows turn more freely and you get the hit! Note you get the HIT at a certain RPMS which controls the powerband! Higher rpms get you to the hit FASTER! Learn and remember!

HOwever, you also have the 'spring' in the powervalve adjustment which can lower that pressure change to a no hit or boost it to the max when you get to that rpm range. YOu can, to a small degree, move your powerband UP by tightening that spring and thereby causing a greater degree of pressure to overcome that, but at the same time you may see a lag right before'

Now we look below at the quote at WR and what he says makes sense in LOWERING pressure at low rpm's for less pressure on the piston giving more grunt, however, he also claims he is closer to the powerband meaning his rpms are up! Before that we look at the problem of large expansion chamber by the admission of manufacturers and riders DECREASE top end because that large expansion chamber can't clear the exhaust pressure and holds back the piston at high rpm which is why you can't have it 'all ways'.

Its not like im trying to get everyone to run out and buy stock silencers and cut them short. regardless of theory.. I cut 5 inches off my pipe I have a deeper more powerful sound and the "powerband" comes in much lower and much stronger and comes in sooner.

(Maybe his term for 'powerband' isn't the same as 'powervalve', but we'll continue) Frankly, what he says due to when the powervalve opens which is rpm controlled is impossible! What is possible is easier spin up due to lower pressure on the piston, but that can't lower you PV settings and aside from level of hit, I really know no way of 'lowering' the powerband which is factory set. What he is feeling is that with the lower pressure he has more bottom end, but at the same time he also has more rpm which is the THE ONLY WAY you can be closer to the powerband. And I didn't quote it, but look at his response in part of his ride of rough stuff where it doesn't dr0p out of the PV...........because his rpms are up.

Less pressure allows for rpms to be up higher and get up higher and allowing the piston to cycle more easily at lower rpm, but it doesn't change the powerband/ PV and that's where the problem here is along with the fact that the length and diameter of the exit into the silencer still controls to a large degree the exhaust and pressure of the pipe which is why all pipes work nearly the same for all silencers.

And like I said earlier, don't even bother with messing with your silencer, but take it all the way off and see what happens and then decide if the advantage is going to warrant a 'silencer chop'.

Edited by wrencher2s

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That's it ~ take a cold shower and a nice shot of prozac.

We are talking about silencers here, not pipes. If you want to start a thread about pipe design then have at it, wr400fdunk just tried something and posted his results this I respect and even extended positive gas. Turning this thread into a discussion on the theory of two stroke pipe design is really nothing more than a buzz kill.

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