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Author Topic: New honda cb 125e  (Read 15384 times)

Offline DohcBikes

  • Posts: 2384
  • Fuck You.
Re: New honda cb 125e
« Reply #10 on: Jan 27, 2016, 12:36:50 »
I spent an hour or so grinding down the weld where the inner pipe is joined to the outer pipe right near where it connects to the head. Took about a mm off all round.
Good work. If you notice a power increase from this mod, it probably needs a bigger pipe.
burning bridges sometimes light the most productive paths

Offline urastus

  • Posts: 75
Re: New honda cb 125e
« Reply #11 on: Jan 28, 2016, 01:14:30 »
That will help if you make the exhaust more free-flowing;I wonder why they always install such a small diameter head-pipe on exhausts for the 125? The dual-wall pipe .. it would be nice if they only used the outer part.I'm trying to find a muffler shop(I hope they will work on this motorcycle pipe.. many 'auto only' muffler shops will not)not far from me that will bend a new head-pipe out of 1.5" stainless steel pipe to make my little CB125S breathe a bit better;I'm installing the XL185 engine to it w/ a ported head and it needs more.

You can go too big. I read an excellent article on this site re tuning (or fabricating) the ideal exhaust for your engine based on things like cc per cylinder and size of exhaust valve, and where you want your torque. There is an equation to figure out the length of the exhaust.

I've heard the term "back pressure" since I was a kid - doesn't describe anything. To summarize part of that article: as the exhaust pulse travels through the exhaust pipe, it causes a vacuum to form behind it. This vacuum then helps drag out the next exhaust pulse. So, you don't want the exhaust pulse exiting the exhaust BEFORE the next exhaust pulse.

I hope I've understood it correctly.

The new pipe is bigger diameter. If I got a header pipe fabricated, I'd base it on the size of the inner pipe.

I apologise if you already know this. And, I found the article: http://www.dotheton.com/forum/index.php?topic=64318.msg734257#msg734257
« Last Edit: Jan 28, 2016, 01:41:45 by urastus »

Offline grcamna5

  • Posts: 1060
Re: New honda cb 125e
« Reply #12 on: Jan 28, 2016, 02:22:48 »
That's some heavy reading there urastus and I just caught a piece of it;I would think it's important to tune your exhaust to the length that matches the powerband and where you want that rpm to be..
I was reading a bit on the internet about the 'Bucket racers'/builders in Australia and it gave some interesting reading of what a couple builders said about optimum size dia. & length of the head-pipe and where the powerband will work best. I'm still learning and would like to know a bit more.
Do your Best,no one can take that from you.

Offline urastus

  • Posts: 75
Re: New honda cb 125e
« Reply #13 on: Jan 28, 2016, 06:07:34 »
That's some heavy reading there urastus and I just caught a piece of it;I would think it's important to tune your exhaust to the length that matches the powerband and where you want that rpm to be..
I was reading a bit on the internet about the 'Bucket racers'/builders in Australia and it gave some interesting reading of what a couple builders said about optimum size dia. & length of the head-pipe and where the powerband will work best. I'm still learning and would like to know a bit more.

 :) I'd be interested to hear what you do in your project, especially intake and exhaust.

Offline grcamna5

  • Posts: 1060
Re: New honda cb 125e
« Reply #14 on: Jan 28, 2016, 11:41:41 »
:) I'd be interested to hear what you do in your project, especially intake and exhaust.

I have 2) engines I'll be running/swapping between that both have the same stroke(57.8mm)and they are a 1979' XL185S & an 83' XL200R.I have both the cylinder heads on them specially ported by a porting expert in MA. I'll be running either the XL185 stock camshaft or the XR200R in my 185 & setting it up for high-midrange torque & cheap fuel w/ a low 9.6:1 compression ratio piston and a Keihin mod.PD65 carb. w/ atomizer plate from a 1977' XL125S and I want to have a muffler shop that's friendly toward bending a 'mototrcycle' pipe bend me a 1.5" stainless steel pipe;I know that muffler shops don't have the ability to make the long,sweeping bends that come on many of our stock factory header pipes but still hope for a few 45 degree bends rather than 90 degree 'sharpies'.I will go by the recommendation of that bucket racer of between 450-500mm long and 36mm i.d. which is about what the 1.5" O.D. pipe has. I imagine right about 480mm(the CB450 dohc stock headers taper from 32mm i.d. at the port & open up to 39mm i.d. at the outlet w/ one long sweeping curve and are780mm long and that CB450 dohc twin has very similar bore & stroke to my XL200R x 2 but much larger valves and ports) in length should be good.. once the end is welded on to the muffler part I'll just 'go-with-it' however it runs.. so Yeah I want to do a Lot more thinking on the length of the pipe and what will work Best  ::) I'm beginning to think a little longer will be better for more midrange..
I plan on using that SS custom headpipe on both engines and the XL200R will have an XR200R piston w/ the XR200R camshaft and I'll want the SS custom headpipe to be longer on that one.. possibly 490mm will work best for both engines..
I'm just trying to decide on the length of the pipe from the engine exh. port to the end that fits into a Honda HM319 stock muffler for a 1972 CB450 dohc.
« Last Edit: Jan 28, 2016, 12:36:04 by grcamna5 »
Do your Best,no one can take that from you.

Offline urastus

  • Posts: 75
Re: New honda cb 125e
« Reply #15 on: Jan 29, 2016, 05:55:05 »
I know that muffler shops don't have the ability to make the long,sweeping bends that come on many of our stock factory header pipes but still hope for a few 45 degree bends rather than 90 degree 'sharpies'.

I assumed it would be pretty easy for an exhaust guy to manufacture a header pipe. After reading your experience I remembered a long gone friend who was into drag racing. He was an IT professional but ended up making cutom headers for turbo charged cars. They were elaborate convoluted creations. Could one of these guys do it?
« Last Edit: Jan 29, 2016, 05:58:16 by urastus »

Offline grcamna5

  • Posts: 1060
Re: New honda cb 125e
« Reply #16 on: Jan 29, 2016, 09:28:42 »
I assumed it would be pretty easy for an exhaust guy to manufacture a header pipe. After reading your experience I remembered a long gone friend who was into drag racing. He was an IT professional but ended up making cutom headers for turbo charged cars. They were elaborate convoluted creations. Could one of these guys do it?

Yes,I think they sure could as long as they have the dies for small pipe(1.5" o.d.,etc.)and you know them well.  :)
Do your Best,no one can take that from you.

Offline urastus

  • Posts: 75
Re: New honda cb 125e
« Reply #17 on: Jan 29, 2016, 20:56:03 »
Mr grcamna5, I bow to your experience. I can see that I'm going through the same learning curve that you probably went through long ago :) Things look so simple when you're (meaning me) in ignorance. Although I don't think I'll ever get as "in depth" as you - too many other toys and distractions.

These links are for any other beginners like myself; all this information may already be here, although I find reading from slightly different angles helps:

http://www.coneeng.com/pdf/Designing_Building_Motorcycle_Header.pdf
http://www.coneeng.com/pdf/Choosing_Motorcycle_Muffler.pdf
http://www.coneeng.com/pdf/Speed-v-Sound-Vintage-Views-2015.pdf

There's other info there for multi cyclinder systems.

It has made me realise that I want my peak power / torque at low to mid range. The 125 seems comfortable up to and on 60km/h. I could wring its neck to sit on 80, but that just seems a waste of motorbike (and I have a 990 to satisfy my high speed needs). The 125 is still on low kms though and no doubt will loosen up more to the point I feel comfortable going harder without abusing it. Probably not though, it'll probably just run freer up to 60 :). Also I won't bother trying to mod my after market exhaust system; maybe just try to put in a couple more mount points.

Offline DohcBikes

  • Posts: 2384
  • Fuck You.
Re: New honda cb 125e
« Reply #18 on: Jan 29, 2016, 21:30:07 »
funny as hell to see you guys trying to get .34 hp more out of your 10hp engines
burning bridges sometimes light the most productive paths

Offline grcamna5

  • Posts: 1060
Re: New honda cb 125e
« Reply #19 on: Jan 29, 2016, 22:51:27 »
Mr grcamna5, I bow to your experience. I can see that I'm going through the same learning curve that you probably went through long ago :) Things look so simple when you're (meaning me) in ignorance. Although I don't think I'll ever get as "in depth" as you - too many other toys and distractions.

These links are for any other beginners like myself; all this information may already be here, although I find reading from slightly different angles helps:

http://www.coneeng.com/pdf/Designing_Building_Motorcycle_Header.pdf
http://www.coneeng.com/pdf/Choosing_Motorcycle_Muffler.pdf
http://www.coneeng.com/pdf/Speed-v-Sound-Vintage-Views-2015.pdf

There's other info there for multi cyclinder systems.

It has made me realise that I want my peak power / torque at low to mid range. The 125 seems comfortable up to and on 60km/h. I could wring its neck to sit on 80, but that just seems a waste of motorbike (and I have a 990 to satisfy my high speed needs). The 125 is still on low kms though and no doubt will loosen up more to the point I feel comfortable going harder without abusing it. Probably not though, it'll probably just run freer up to 60 :). Also I won't bother trying to mod my after market exhaust system; maybe just try to put in a couple more mount points.

I'm just now learning about this urastus;I'm going by what that Aus. bucket racer uploaded  :D
Good to see those links you posted.

Typically the short stroke(49.5mm?)engines don't have that much midrange or low end power but Do need to be 'wrung out' to give you their performance.
« Last Edit: Jan 29, 2016, 23:01:39 by grcamna5 »
Do your Best,no one can take that from you.