can someone help me understand exhausts/mufflers?

[dean owens]

so i posted that i'm going to be making a cafe racer. one of the things i'm toying with is an undertail exhaust. but i don't just want to straight pipe it so i've been looking at everything i can on mufflers to see if i could make my own.

here's what i know....

seems most of what i've seen on here (slip ons) are pretty much just straight pipes. do they actually do anything? does the steel wool and fiberglass (assuming that's what it is) do anything?

mufflers on cars are actually two separate perforated pipes next to each other that clip the sound waves. they actually help to cut down on noise and create back pressure.

back pressure is good for the engine. (will i get enough of that by running the two into one?)

what i don't know....

do motorcycles use actual mufflers? if i'm making something what will be the best for a motorcycle. keep in mind that i like my neighbors and don't want an obnoxiously loud bike.

is there a formula for the size of the muffler based on the size of the engine?

is there anything wrong with putting the muffler in the "middle" of the whole system? it looks like it'd be easier for me to put the muffler directly under the seat and then have about a foot and a half of pipe to work with to make it look like i want.

i have some more questions but we'll go with this for now. please help. i've been googling for about two days. i know a lot more than i did. but i'd like to know more before i move ahead.

thanks

 

[jtarkany]

A mufflers primary job is noise suppression, this also has the effect of more back pressure.

Slip-ons and full aftermarket exhaust systems primary job is to increase the exhaust gas flow without concern for noise suppression thus providing maximum peak horsepower. However, the reduced back pressure that increases the max horsepower at higher rpm's has the effect of reducing torque at lower rpm's

The keys to realizing this horsepower increase are:

• making the path as straight as possible
• using a consistent diameter throughout the system
• eliminating obstructions such as catalytic converters
• dynotuning

here's what i know....

seems most of what i've seen on here (slip ons) are pretty much just straight pipes. do they actually do anything? does the steel wool and fiberglass (assuming that's what it is) do anything?

Yes, they do reduce noise and provide back pressure vs. straight pipes. The biggest factors in noise suppression and back pressure with regard to slip-ons (aka glasspacks on a car) is the type of fiberglass, how tight or packed it is and the size of your stainless steel core, ie. 1.75" ID vs. 2.5" ID (I used 1.75" in my homemade pipes to keep them quieter than the Two Bros which use 2.5")

do motorcycles use actual mufflers? if i'm making something what will be the best for a motorcycle. keep in mind that i like my neighbors and don't want an obnoxiously loud bike.

Yes, the FZ6 OEM Exhaust is a muffler very similar to what you would find on a car.

is there a formula for the size of the muffler based on the size of the engine?

I don't know but here is an article that might help shed some light on it: http://bmwmotorcycletech.info/InExTuning.htm and here is an actual calculator linked to the article: http://www.mez.co.uk/mezporting/exhaust_length.html

is there anything wrong with putting the muffler in the "middle" of the whole system? it looks like it'd be easier for me to put the muffler directly under the seat and then have about a foot and a half of pipe to work with to make it look like i want. thanks

No, just be sure to insulate it.

 

[Wavex]

Muffler - Wikipedia, the free encyclopedia

Some are straight pipes (the louder ones ), some not. It depends on the purpose and noise laws...

 

[RJ2112]

so i posted that i'm going to be making a cafe racer. one of the things i'm toying with is an undertail exhaust. but i don't just want to straight pipe it so i've been looking at everything i can on mufflers to see if i could make my own.

here's what i know....

seems most of what i've seen on here (slip ons) are pretty much just straight pipes. do they actually do anything? does the steel wool and fiberglass (assuming that's what it is) do anything?

mufflers on cars are actually two separate perforated pipes next to each other that clip the sound waves. they actually help to cut down on noise and create back pressure.

back pressure is good for the engine. (will i get enough of that by running the two into one?)

what i don't know....

do motorcycles use actual mufflers? if i'm making something what will be the best for a motorcycle. keep in mind that i like my neighbors and don't want an obnoxiously loud bike. (1)

is there a formula for the size of the muffler based on the size of the engine? (2)

is there anything wrong with putting the muffler in the "middle" of the whole system? it looks like it'd be easier for me to put the muffler directly under the seat and then have about a foot and a half of pipe to work with to make it look like i want. (3)

i have some more questions but we'll go with this for now. please help. i've been googling for about two days. i know a lot more than i did. but i'd like to know more before i move ahead.

thanks

(1) The FZ6, and almost all OEM motorcycle exhausts, use an actual muffler design. They also frequently use balance tubes or 'equalizers'.... most under engine 'pre mufflers' are balance tubes.

(2) A muffler routes the exhaust gases back and forth in a Z, to extend the time the shock wave can be absorbed by the packing material. The length of the chambers plays into the resonance of the design. Bikes with big pistons make bigger explosions, and need more muffler material to absorb sound. Twins and singles use bigger mufflers than multis of the same displacement due to this. Less, louder, explosions require more damping than more little ones. Think on M80, compared to 4 packs of ladyfingers.... roughly equal energy in both; but when you set it all off at once, it makes a louder BANG.

(3) A muffler affects the backward wave that 'reflects' to the cylinder...... Any time there is a change in pipe restriction, there is an impedance mismatch, which creates the wave. The combustion process is just as sharp of a rapid expansion of gases as a gunshot. The shock wave you hear as the 'report' of the gun, is exactly the same as the sharp blast that goes down the pipe. You get an 'echo' from the impedance change. You also get echoes from the air box, through the intake valves.

The backwards wave's arrival at the cylinder, if timed correctly, will stop the intake charge from exiting the cylinder, even though the exhaust valve is open. More charge in the cylinder when the valves close than would 'naturally' occur, and you get more power than you normally would. (Valve timing, duration, and overlap all play into this as well.)

That's why air boxes are resonant cavities. You can tune them like a trombone slide works to make the desired 'tone'. When the tone is right, you make extra power. The exhaust pipe is also a resonant device. If you don't plan on messing with the valve timing, put the muffler roughly where the first impedance change on the OEM system is.

The reason you can replace the muffler on the -6 without truly F*ing up the power band, is due to it not being the first impedance in the exhaust. The muffler surely has some impedance, but the catalytic converter has a much stronger effect. It's more than 3 feet closer to the cylinder than the muffler input pipe.

1100 feet per second is equal to 750 MPH.... supersonic speed is 714 MPH at sea level. This is why most rifles make such a loud 'crack'ing noise when they fire.... the bullets are usually @ mach 2 or above. You're hearing the sonic booms of the bullet's passage. 1100 fps is also equal to 13,200 inches per second. Which means the shock wave goes down the pipe at a bit less than 75 microseconds and inch..... whether the object moving down the cylinder is a piston, or a bullet.

 

[geetarhero]

just remember there is a fine line between too much back pressure and too little.

back pressure is directly related to torque numbers, but too much and the engine wont breathe out enough and will reduce your timing creating a loss in hp.

dyno tuning and toying with it is key to finding the perfect balance. never a given formula