FMF Shorty VS Stock YZ250: Sound Test
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- เผยแพร่เมื่อ 26 ส.ค. 2024
- I did a side by side comparison on my 2002 YZ250 of a brand new FMF Shorty, and the stock silencer that had worn out packing. In the video you can hear a difference in sound, but there really wasn't a difference in power.
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The performance difference between a shorty silencer and regular length silencer works like this; The overall length of pipe (the small diameter tubing that goes into the silencer right to the end tip of the silencer) determines power curve. The shorter distance (shorty silencer) causes the engine to loose power or "fall off" power at a lower RPM then a longer silencer. This is called "falling off the pipe"). The longer overall length (stock silencer or FMF regular length silencer) will cause the engine to hold its peak power through higher RPM, before "falling off the pipe". So why run a shorty? In some conditions the shorty will provide a bit more snap out of tight corners on a super tight track, where high RPM is not required (generally low speed tight tracks). Here is a good example; I had the shorty on YZ250 and when exiting a turn onto medium length uphill double, my engine fell off the pipe mid way up the jump face, causing me to have to shift to 4th gear just prior to the jump face. I switched to my regular length FMF silencer, and now could leave the bike in 3rd gear, and it would continue axcelerating right through and off jump face. Thats the difference between shorty and regular length. Also its important to keep the packing tight in a silencer as this provides a stronger pulse back through the pipe and the engine makes more power. In my opinion, the only people who should entertain a shorty, are basically advanced pro riders who need to tune RPM for given corners and track conditions.
Sorry guys I had a typo in this if anyone is tuning by it - please read again.
Great explanation!
That just isn't true, the small diameter pipe as you call it really is called the stinger.It is there just to bleed off the exhaust gases out of the expansion chamber, some go-kart chambers don't even have the stinger at the end of the chamber, it can be in the middle of the chamber sticking out at 90 degrees A two strokes power characteristic can be tailored by the design of the expansion chamber, the silencer plays no part in making or losing power other than if it is too restrictive it will choke the engine to death due to high back pressure. The real way that a two stroke exhaust works is that a slug of gas erupts out of the exhaust port as a positive pressure wave, when it reaches the diverging part of the chamber (The bit that is getting wider) then the slug sees this as the open end of a pipe. Due to a law of pressure wave dynamics (and some law in acoustics) when this happens a pressure wave of the OPPOSITE sign is sent back up the pipe, in other words a negative pressure wave, a partial vacuum.When this reaches the exhaust port it actually pulls fresh charge up from the crank cases via the transfer ports and into the exhaust. (this fresh charge is not lost, it is just stored in the exhaust for a fraction of a second).
When the slug of gas that hit the end of the diverging part of the pipe not all of its positive pressure was lost, some of this pressure carries on down the parallel section of the chamber until it hits the converging section of the chamber (the end cone), again due to a law of pressure dynamics a reflected wave of the SAME sign travels back up the pipe (a positive wave). When this positive wave hits the fresh charge stored in the pipe it rams in back into the cylinder! So what's the point of doing all this you might ask, well by doing this you can make an engine run at over 100% volumetric efficiency (VE), in other words a 125 engine may in fact inhale 150cc of fresh charge on each cycle so the engine performs like a 150cc engine. One way to describe all of this is "harmonic supercharging". Getting the timing for all of this two work is the tricky bit, a single chamber can not work at 2000 RPM AND 12000 RPM! The RPM range in which the pipe will work is determined by its length and the shape of the diverging and converging sections. A flat plate rather than a cone at the end would give a massive positive pressure wave bit at only one single RPM so it would not be much good!.....In a nut shell, that's how a two stroke exhaust works!
Sidecarbod - You are correct that is exactly how a tuned expansion chamber works and you explained it perfectly except for one detail; The small diameter pipe (stinger) absolutely HELPS to determine where the power curve drops off (in this case, long stock silencer compared to a shorty). Short stinger and the engine will fall off the pipe at a lower RPM then a long Stinger. Have seen this in realtime on many dyno pulls. On the engines I have run over the years, I have had custom pipes built that will pull power through to 12,000rpm, and others that will pull power only to 9600 RPM. Among the many other variables designed into the pipe to control the power curve, the stinger length is an important component. The more "high tech" and "edgy" the pipe is (built for absolute maximum hp) the more critical the stinger diameter and length become. I was trying to simplify the explanation in the post, because in motocross, unless a rider is having a custom tuned pipe built (I have had custom tuned pipes built for a 125) the silencer is really the only tuning piece left.
Well if the stinger is working as you described then it is only doing that due to the change in back pressure that a change in stinger length and/or diameter can cause. I guess then in that respect you are right, a change in the stinger and silencer will change the characteristics of the engine but it is still 'small potatoes' compared to how the actual expansion chamber can change the characteristics of the engine! Like you said though, people are more likely to change simple things first.
agreed its small changed compared to overall pipe design. But its surprising how much effect it has. Going short caused my bike to need to shift up a gear for a jump, verses going long I could let the engine rev higher and not shift. Thats pretty significant. I have relationship with a couple of custom pipe builders who are constantly doing R&D. They told me the stinger effects the wave timing going back up the pipe. Saw a 125cc 2-stroke bike making 60hp! They found something in pipe design that is the opposite of what everybody thinks, and when they applied this to the design, the supercharged effect went way up! I could share it, but to the best of my knowledge, there are only 3 people who know about it.
I actually like the stock sound better
I agree
Need the fatty to go with or stock actually does sound better .
The stock silencer needed the packing replaced that’s why it was louder
This was one of the best compression videos I’ve seen on exhaust.
ya stock sounds nicer foresure
Very Good video. Best sound comparison ive seen
Jonathan Bjørklund Thanks, glad you enjoyed it
the stock pipe needed fibers changed anyway, could of cut the stock to shorty size and repacked it would sound the same
+Nathan cata I kept the old one and repacked it in case I need a quieter exhaust for enduro racing
i'm with u
Sounds better with stock exhaust!
FMF makes it have more of a pop corn sound that all two strokes should have it sound beautiful
SOUNDS MORE CRISPY WITH THE SHORTY THATS WAT U WANT CRISPINESS JUST LIKE FRESH LETTUCE !!!
Stock exhaust has more of a pop sound
Stock sounds better
Adolf.... Du sagst es. Stock Sounds better!
For some reason or other the stock sounds better I'm gonna buy the whole exhaust system with a bills cone style pipe got stripes all around exhaust and get a bills shorty style baffle you could go with pro circuit pipe and baffle or a fmf pipes and baffles got a few too choose from gonna put it on my yz85 got a complete pro circuit pipe and baffle on my rm 250 it sounds awesome great review
go ride the dang think!!! that is a true of sound of a 2 stroke when its in that sweet spot...brrrrrrrrrrrrrrrrrrr
i like the powercore 2
very helpful!
what premix mixture are you running looks. tuff looking and sounding bike
I mix 30:1, I was using yamalube 2R, but I switched to motul 710 because it burns better, less spooge
I run Amsoil at 80:1 in my YZ250. almost Zero smoke, no carbon, plenty of lubrication.
I gained more mid and top end with a,Dynoport pipe
Is one louder than the other?
They were about the same because the stock one had worn out packing. With fresh packing the stock one is much quieter.
Boysen reeds, 6 to 9 oz flywheel wight , gut 30'' of the bottom off the barrel. That's all you need.
Well there wasn't much difference in sound really. But what matters is performance. Does it really add much bottom torque?
+shafly67 I think it improved throttle response in the low end, but not a big difference. There did seem to be a little bit more torque, but not much. It would probably be more noticeable when paired with the fatty or the gnarly.
+Premix Productions Gotta get the full system to feel any gains.
The stock one sounds MUCH better.
Thats how my Kx125 sounds
theres no difference in sound tbh but in performance ;), put one on my 2017 yz 250
The stock sounds nicer. It's crisper, maybe it's because I like 4 strokes more, but the sound of 2 strokes is just..... yum
FMF Sounds better and has a deeper big bike sound to it like a 250 should bark.
Needs rhe fatty or narly. Or keep it stock
nice vid what's the performance difference it's a 03 right
02, not a whole lot, just sounds better in my opinion.
right on thanks man nice yz 👍
NICKDOG154 thanks
Am I the only one who doesn’t understand why people want loud exhausts? If I could have an exhaust where it made little to no sound I’d pick that any day of the week so I’m not pissing off neighbors and people’s houses in riding by
Ficou top mano, passa lá no canal. Abraço
does it evan sound different i dont think it does?
It does. Its about the same loudness, just deeper sounding
The shorty makes it way more crisp and clear. The old one sounds done.
For me it's clear, I'll buy a shorty, thx for the comparison :)
HawX777 :)
his bike barely smoke..what oil ratio you're using?
30:1 But it was fully warmed up before the video
the type of oil you used?
I was running Yamalube 2R In the video, but I've started using Motul 710 Because it burns better and smokes even less. Smells better too.
There's a lot better sound coming out of that FMF...
You guys are all saying stock sounds better but it doesnt at all.
Shorty is awesome, sounds like a 125...
Can you take apart the exhaust to clean it?
+YZ 85 RACER You can take off the outside canister to repack it.
+Premix Productions oh ok thank you
Factory actually sounds better but this is not IRL
Both of them sound like my weedeater. ;)
It's to long why would they name it shorty when it's long
because it's shorter than FMF Turbinecore2
the first is better
How about you repack your stock silencer before you do a comparison. Fmf sounds so much better. Stock sounds like shit cause packing is burnt out.
stock
Or just get a 4 stroke they sound even better.