2 stroke mufflers

[rotang]

Hi I have an 02 RM250 iv been trying to get some more power from it, I heard cutting the rear muffler down 100mm will increase power is that true??

[soundbeltfarm]

Hi I have an 02 RM250 iv been trying to get some more power from it, I heard cutting the rear muffler down 100mm will increase power is that true??

im no mechanic but if that was the case wouldn't suzuki have done that on the production line?
unless its something to do with regulations.

[Krusti]

Does work. Pro Circuit even makes one. Why? Don't know.

[XTC]

Can also make them even more obscenely loud than they already are....

[Jantar]

2 stroke exhaust design is quite a complex science. Yes, you can get more oomph by modifying the exhaust, but you need to very careful as in getting that power increase you will also change the power band characteristics. The exhaust design maximises the power output at the designed rpm. Changing any of the lengths will change the rpm at which the exhaust is effective

If you shorten the header pipe, then the whole power range will occur at higher rpm, and unless the porting is changed to suit, and the carburetor size increased and intake length reduced then you will have a very peaky engine that will deliver less power at all but peak rpm.

If you shorten the distance between the difuser cone and the baffle cone, then you will achieve a higher peak rpm without altering the transfer port characteristic, but you will have reduced the overall chamber volume and hence increased the average back pressure.

Shortening the tail pipe will decrease back pressure, increase noise, and actually reduce the power output. So be very carefull before modifying a 2 stroke exhaust system.

Edit: Spelling corrected.

[soundbeltfarm]

2 stroke exhaust design is quite a complex science. Yes, you can get more oomph by modifying the exhaust, but you need to very careful as in getting that power increase you will also change the power band characteristics. The exhaust design maximise the power out put a the designed rpm. Changing any of the lengths will change the rpm at which the exhaust is effective

If you shorten the heder pipe, the the whole power ange will occur at higher rpm, and unless the porting is changed to suit, and the carburetor size increased and intake length reduced then you will have a very peaky engine that will deliver less power at all but peak rpm.

If you shorten the distance between the difuser cone and the baffle cone, then you will achieve a higher peak rpm without altering the transfer port characteristic, but you will have reduced the overall chamber volume and hrnce increased the average back pressure.

Shortening the tail pipe will decrease back pressure, increase noise, and actually reduce the power output. So be very carefull before modifying a 2 stroke exhaust system.

cripes your a encyclopaedia mate.
no your not you are a bike manual..
how do you know all this stuff?
mechanic ?
or heaps of hands on

[Jantar]

cripes your a encyclopaedia mate.
no your not you are a bike manual..
how do you know all this stuff?
mechanic ?
or heaps of hands on

Heaps of hands on when I raced two strokes (both road and motocross) back in the 70's. I also designed the expansion chambers for a couple of very successfull sprint bikes, one of which took the NZ record for its class.

[soundbeltfarm]

Heaps of hands on when I raced two strokes (both road and motocross) back in the 70's. I also designed the expansion chambers for a couple of very successfull sprint bikes, one of which took the NZ record for its class.

enough said.. you da man

[F5 Dave]

Quick grandma lesson: ‘your’ means they own it, ‘you’re’ is short for 'you are'.

That aside & back on track. We are losing riding areas quick as can be from lifestylers moving into the country & complaining about the noise from cattle. Bikes get them on the phone & keyboard even quicker.

Many have been fooled thinking their bike is faster when it is louder. Many companies know this hence the shorty muffler trend.

Fact: GP teams both road & dirt repack their mufflers super regularly.

This is to make sure the backpressure is kept constant, also the gas flow is less impeded when the packing is tight, but don’t get hung up on flow, it is a bit irrelevant. When designing a stinger section of a pipe you want to get the outlet as small as possible for peak power, although there is a point too far where piston crown temperature rises too sharply.

If you are after more power after replacing the piston & rings, ensuring the powervalve is working & replacing that airfilter which is probably clogged to heck after 3 years, then look closely at the jetting. This will likely net you more power through the range than fooling with the muffler.

Also check your squish clearance with some solder through the sparkplug. Chances are it is huge & machining the head may bring good results, though you will have to consider what gas you run.

[That Guy]

Fact: GP teams both road & dirt repack their mufflers super regularly.
.

Yes & no. MotoGP no longer need to run mufflers, having got a dispensation similar to F1 to be loud as.

Agree with you though re the rest of us need to keep as quiet as possible to keep our riding/racing areas intact.

[avgas]

a) Many have been fooled thinking their bike is faster when it is louder. Many companies know this hence the shorty muffler trend.

Fact: GP teams both road & dirt repack their mufflers super regularly.

This is to make sure the backpressure is kept constant, also the gas flow is less impeded when the packing is tight, but don’t get hung up on flow, it is a bit irrelevant. When designing a stinger section of a pipe you want to get the outlet as small as possible for peak power, although there is a point too far where piston crown temperature rises too sharply.

If you are after more power after replacing the piston & rings, ensuring the b) powervalve is working & replacing that airfilter which is probably clogged to heck after 3 years, then look closely at the jetting. This will likely net you more power through the range than fooling with the muffler.

Also check your squish clearance with some solder through the sparkplug. Chances are it is huge & machining the head may bring good results, though you will have to consider what gas you run.

a) really depends on the system, noise is heat, which translates roughly to wasted energy - however in some systems more noise, means more power to waste, means more power.
b) i thought all off roaders were reed valves setups as there is no need for sudden launches in power in the top revs, which is essentially what power valves do - shame, cos when these are tuned to perfection - power vavles are like turbos

Mate if ya want make a two stroke motor at peak performance, take it into the shop and get a full race mechanic (two stroke preferably) to look at it. They dont charge that much - and what you get back is a weapon. I did it with an RG i used to have, and he scrounged every bit of power he could out of that little motor - cos the people i was riding with at the time all had 750+ cc bikes. All the motor work cost me less than $1K, and it did better that me trying to chuck $2K worth of parts on it myself

[Motu]

What Dave said - check out the size of the stinger and you'll see the muffler is giving no real restriction,it's there for noise only.And what in hell do you want more power from a 02 RM 250 for? - I bet Shane King or any other has been could clean you up on a 125,more power doesn't make you go faster off road....

[F5 Dave]

Yes & no. MotoGP no longer need to run mufflers, having got a dispensation similar to F1 to be loud as.
. . ..

Thatguy. And what kind of 2 strokes do they run in Moto GP? They'd be the 4 stroke type of 2 stroke. eh?
There are of course 2 GP classes left running 2 strokes.

a) really depends on the system, noise is heat, which translates roughly to wasted energy - however in some systems more noise, means more power to waste, means more power.

a) What? My brain hurts. I challenge anyone to create a formula for the fanciful jibberish above.

b) i thought all off roaders were reed valves setups as there is no need for sudden launches in power in the top revs, which is essentially what power valves do - shame, cos when these are tuned to perfection - power vavles are like turbos . . ..

All the powervalve does is close lower the roof of the exhaust port to reduce the timearea. This reduces charge loss at low rpm & changes the resonance of the engine tune. The only thing that is slightly like a turbo is the exhaust pipe which acts like a sort of supercharger but instead of cramming air in from the inlet does so at the exhaust by returning over scavenged charge, but only when the frequency is correct (ie in a certain rev range).

No offence to mechanics, but by definition their bread & butter is fixing things & the majority don’t do any performance tuning. Some move into this area but be careful of what you ask for & who you trust to perform this work. Anyone could tune a MX bike for more power but the trick is to keep the power spread else it will be harder to ride and/or slower around a given track.

To add to what Motu sez. How sharp are those knobblies?