Exhaust tuning?

[TygerTung]

I have a CB250RS:scooter: which I use for a post classic race bike, and it needs a new exhuast as the old one rotted out.

I was gonna make one myself, make up a two into one, but how do you go about doing the exhaust length calculations with a twin port head?! The mind boggles!!!!

Any help would be appreciated. :yes:

[xwhatsit]

I have a CB250RS:scooter: which I use for a post classic race bike, and it needs a new exhuast as the old one rotted out.

I was gonna make one myself, make up a two into one, but how do you go about doing the exhaust length calculations with a twin port head?! The mind boggles!!!!

Any help would be appreciated. :yes:

Wouldn't you just treat it as one? Just try and get both individual headers as close length as possible. That's my knee-jerk reaction but I don't know shit.

If you do make one, do post up pics! My exhaust system is pretty had-it. I'd swap you a CB250RS bottom end for one! :innocent:

[Kwaka14]

I think you need to get the volumes on all pipes in to the collector the same, so bend up your headers and then fill the shortest pipe with water (tape off the lower end) and use the volume of water in the pipe to check the volume in the others (tape the lower or upper end off the next header pipe and fill it with water and cut it off at the water level).... I do know that Suzuki have the two outside pipes in some bikes running a longer length than the inners so it may pay to ring someone like custom chambers to check....

[kneescraper]

Good question Tyger...I wont to know the answer to this too...

I always thought all the lenths had to be able to flow the same amount of volume in the same amount of time. So say...1ltr of water in 1 sec...this way you can have the lenths in different shaps to suit the application etc...

But I know nothing....please we need more info.

[Drew]

I'm a tad ignorant to the right way too, but the equall length thing must be less important than on a multi, cause you're not balancing two seperate pistons. So provided one is not constrictive, the gasses will do what they want depending on each indevidual combustion burn, (which is never the same twice), so it'll favor one valve over the other alternatively.

This was just a bunch of thoughts I had, and should probably be disregaurded as, "some boozehounds" ramblings.

[kneescraper]

^^^^^ What about back pressure, if its unbalanced wouldnt that affect the way the motor ran then?

[TygerTung]

Well it's a single, but it usually has two exhaust pipes, one down each side.

I was gonna just give it one exhaust, I can merge the two header pipes into one exhaust, but I'm not sure how you would do the calculation as this is the formula /I have thanks to motu

How about a formula??

For the exhaust pipe length use...

L= V120/N

Where
L= length of pipe
N = engine speed in RPM
V = velocity of wave in ft/sec. Velocity of the wave is a bit tricky,but if you use 1700 ft/sec you should be pretty close.

With the pipe length we are dealing with SOUND waves,called resonance tuning.The diameter of the pipe is dealing with the gas slug itself,and we want it around 300 ft/sec.Use this formula...

V = piston speed/60 X D squared/d squared

where
V = gas velocity in ft/sec
D = piston dia
d = pipe inside dia.

To find piston speed in ft/sec.speed use this...

stroke X 2 X rpm divided by 12

Clear as mud eh.I used to fart around with this stuff when I was your age,hours and hours of enjoyable fun....well,if you're into bikes that is,and I was and still am.Almost got those formulas imprinted in my brain,but still had to go look for them....the books are almost falling apart.

Oops...just remembered,these figures will give you a pipe too short to use on a bike (seen the short headers on a fuel dragster?) just double it and catch the wave next time round.
__________________


So I am unsure as to how I would relate this, maybe I could work out the area of the header pipes and have a bigger exhaust pipe of an equivilent area?

[Motu]

Just make the header pipes the same length and join the into a pipe with the same cross sectional area,or whatever size you calculate is needed.You shouldn't to think too hard to remember the formula for the area of a circle.

[TygerTung]

Yep! It's πr² I'll do some calculations and make up an exhaust, thanks!:scooter: