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MIG Welding an Exhaust Header - CB400T

This is a discussion on MIG Welding an Exhaust Header - CB400T within the Technical forums, part of the Caferacer.net Forums category; Originally Posted by 540nova Trialsrider, I realize that. you do, others might not....

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Thread: MIG Welding an Exhaust Header - CB400T

  1. #21
    Senior Member TrialsRider's Avatar
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    Quote Originally Posted by 540nova View Post
    Trialsrider, I realize that.
    you do, others might not.

  2. #22
    Senior Member 540nova's Avatar
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    Quote Originally Posted by TrialsRider View Post
    you do, others might not.
    If you're interested in it, you can read more about it on Bike Exif.

    Sent from my Pixel 3 XL using Tapatalk
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  3. #23
    Senior Member TrialsRider's Avatar
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    My CB350 riding days are done unless I develop senile nostalgia.
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  5. #24
    Senior Member brad black's Avatar
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    i've welded exhausts with a mig. just need to adapt your method to suit the material.

  6. #25
    Senior Member Andyshep's Avatar
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    You can learn to make beautiful welds with TIG or you can learn to grind and paint so you don't see your MIG welds.

  7. #26
    Senior Member DesmoDog's Avatar
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    Quote Originally Posted by jakejensen View Post
    I've got the standard exhaust set up on the bike currently - twin downpipes, the resonator, and twin mufflers. However, for vanity sake (looks and sound) I want to try my hand at welding a 2-1 exhaust for it. Here is a short list of the considerations I've made so far (correct or not so correct!!)-

    1. Resonator.... Ugh.
    So this one has been a head scratcher. After asking around on this forum the concensus is that they do relatively little in terms of performance. The primary contributor to the vacuum resonance that dictates performance on this bike is the air box ( or so I'm told) by providing constant temperature and pressure for the intake. The resonator's primary contribution to the performance of the bike is to add total overall length to the system. Now, if it was removed, what would be the effect on the engine's performance? I've been told pretty much zero. But I can't say from experience.
    I have no knowledge of what the resonator on this bike looks like aside from your picture, but I doubt it makes the system longer. How can it? Looking at that picture, it's too small. My guess is it's there mainy as a crossover and they took advantage of it by adding some acoustic damping also. Again a wild guess. The crossover function of it WILL effect performance BTW, but you say you want a 2 into 1 so you're not deleting the connection between the two pipes. THAT said... how much of a difference the crossover makes on a 1981 400 might not be much. When I worked at Victory we added a crossover between the two pipes on the first dyno engine and gained significant power. On a small twin? I dunno. And BTW we put the crossover where it would fit, not where the numbers said it would work best. Ok, we didn't bother calculating anything since it wouldn't really fit anywhere else anyway. Which is the point here too... Just make it fit.

    I'm not really sure what you mean by "vacuum resonance" by "providing constant temperature and pressure". You can't have a resonance with constant pressure - the two terms contradict each other. Resonant tuning relies on pressure waves. Those pressure waves are only helpful when they happen at the right time, and that depends on rpm, the speed of sound, and the lengths of the components before the airbox (or some other abrupt change in area). Yes, modern bikes put the airbox in resonance too, but I'd be surprised if the airbox on anything from 1981 was designed with resonance in mind, let alone a CB400. I don't think airbox resonance came into play for at least another ten years? IMHO that airbox is likely designed to fit the frame and hold an air filter. Honda fans will correct me here I'm sure, I'm more familiar with certain Italian bikes.

    The intake does have a much bigger effect on performance than the exhaust does, but on that bike I doubt much of anything has been designed to maximize performance over fitting the package space. Bottom line - you aren't going to significantly change the power on that bike by making calculations based on estimates using formulas you found on the internet. Did you time the cams? So how do you know when the valve events are happening? Did you port the intakes? No? Then don't lose any sleep over the exhaust. Sure, do the calcs and use the numbers as directional info but don't go nuts trying to make the theoretical ideal system fit. It won't. And it's not ideal anyway.

    2. Calculating overall length
    This site provides an automatic calculator for length of the system based on 2 criteria - http://www.nightrider.com/biketech/c...austlength.htm

    I. midrange RPMs
    II. cam angle of open on exhaust or BBDC ( Before Bottom Dead Center ).

    Now I'm not building a race engine, but maximizing horse power wouldn't be awful.....
    The link doesn't work for me so again I'm shooting from the hip here, but...

    Midrange rpm. Yep, you have to decide which rpm range you want the tuning to be effective at. And by that you're deciding where it will hurt you too. Boosting midrange equals longer lengths. Ain't gonna fit even worse than if you wanted to bolster top end.

    "cam angle of open on exhaust or BBDC"? I'm going to assume you have some sort of autocorrect going on with a couple typos there, because that makes NO sense. I get that it's a reference to valve timing? Yeah, that matters too. My guess is you haven't checked this, and don't really understand what it is (both figuratively and specifically). You have to know when the valve opens to know when the low pressure pulse should be there. So you need to know the cam timing, the speed of sound in the exhaust, and the rpm you want it to be effective at before you can calculate a pipe length. You're guessing at any of those? Then just make the pipes fit the bike and look "right".

    Anyway, it's really late and I'm rambling. Sum it up to say, if you don't KNOW the cam timing and the exhaust temps, your calculations for exhaust lengths are simply guesses. You're fooling yourself if you think putting your guesses into a formula is optimizing anything. The calcs might give you a general idea of what to look for, but you're not maximizing anything. It ain't that simple. There's a reason the serious guys use software that costs thousands of dollars a year to license to model this shit. And then go test it all on a dyno when they're "done". Then repeat as needed.
    Last edited by DesmoDog; 03-04-2019 at 12:21 AM.
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  8. #27
    Senior Member jakejensen's Avatar
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    Quote Originally Posted by DesmoDog View Post
    I have no knowledge of what the resonator on this bike looks like aside from your picture, but I doubt it makes the system longer. How can it? Looking at that picture, it's too small. My guess is it's there mainy as a crossover and they took advantage of it by adding some acoustic damping also. Again a wild guess. The crossover function of it WILL effect performance BTW, but you say you want a 2 into 1 so you're not deleting the connection between the two pipes. THAT said... how much of a difference the crossover makes on a 1981 400 might not be much. When I worked at Victory we added a crossover between the two pipes on the first dyno engine and gained significant power. On a small twin? I dunno. And BTW we put the crossover where it would fit, not where the numbers said it would work best. Ok, we didn't bother calculating anything since it wouldn't really fit anywhere else anyway. Which is the point here too... Just make it fit.
    .

    This is why I enjoy this forum so much. Straight forward answers.



    Quote Originally Posted by DesmoDog View Post
    "cam angle of open on exhaust or BBDC"? I'm going to assume you have some sort of autocorrect going on with a couple typos there, because that makes NO sense. I get that it's a reference to valve timing? Yeah, that matters too. My guess is you haven't checked this, and don't really understand what it is (both figuratively and specifically). You have to know when the valve opens to know when the low pressure pulse should be there. So you need to know the cam timing, the speed of sound in the exhaust, and the rpm you want it to be effective at before you can calculate a pipe length. You're guessing at any of those? Then just make the pipes fit the bike and look "right".
    I fucked up the acronym. The site was asking for the cam angle when then the exhaust valve is opening. I've attached a picture of the valve timing information from the service manual. That being said, I haven't done anything to the head, not even lap the valve seats, let alone porting and grinding in there. I'm not doing any performance mods like that to this project. I need to probably start with removing and measuring the valves, lapping the seats, and putting in new seals before anything else.

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    This is helpful info, because I honestly have no idea what I'm doing. (No surprise to those reading this......). I didn't know if I was overcomplicating the process or not. I watched this video no airboxes, which seems to be saying some of the same things you mention here.



    Thanks again for the info, helps me out a lot.

  9. #28
    Senior Member Teazer's Avatar
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    The question you are addressing is when does the exhaust pressure pulse start moving and therefore when will it arrive back at the valve as a low pressure wave to help evacuate the combustion chamber. Assuming that you want that low pressure wave to arrive say around TDC on overlap, you can work out the exhaust length that will work at a certain speed of sound. That means you have to know the mean exhaust temperature and you have to decide where in the rev range you want that effect to occur. At peak torque perhaps or at peak HP to boost top end. Or somewhere in between.

    There is some good software available to assist with all that. I used to use a really early version of Allan Lockheed's Engine Expert. There are new versions available or use EngMod4T for more insight into the nature of pressure waves in an exhaust duct. Many tears ago a British bike magazine addressed the issue on a twin port Rotax single and plotted the results for length and RPM. In the end they settled for two different lengths - one on each side. One boosted torque and the other HP. Obviously those two are related mathematically, but the issue is what part of the curve you want to raise. On that motor they created the largest overall increase in area under the curve by choosing two different lengths. When you optimize your exhaust what works for you may be different.

    It's the old story - start with the end in mind. In this case it translates to what are you trying to improve ie what revs do you want more power. Work from there.
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  10. #29
    Senior Member jakejensen's Avatar
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    Quote Originally Posted by Teazer View Post
    The question you are addressing is when does the exhaust pressure pulse start moving and therefore when will it arrive back at the valve as a low pressure wave to help evacuate the combustion chamber. Assuming that you want that low pressure wave to arrive say around TDC on overlap, you can work out the exhaust length that will work at a certain speed of sound. That means you have to know the mean exhaust temperature and you have to decide where in the rev range you want that effect to occur. At peak torque perhaps or at peak HP to boost top end. Or somewhere in between.

    There is some good software available to assist with all that. I used to use a really early version of Allan Lockheed's Engine Expert. There are new versions available or use EngMod4T for more insight into the nature of pressure waves in an exhaust duct. Many tears ago a British bike magazine addressed the issue on a twin port Rotax single and plotted the results for length and RPM. In the end they settled for two different lengths - one on each side. One boosted torque and the other HP. Obviously those two are related mathematically, but the issue is what part of the curve you want to raise. On that motor they created the largest overall increase in area under the curve by choosing two different lengths. When you optimize your exhaust what works for you may be different.

    It's the old story - start with the end in mind. In this case it translates to what are you trying to improve ie what revs do you want more power. Work from there.
    I've got my homework cut out for me now.

  11. #30
    Senior Member irk_miller's Avatar
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    I did an experiment with my chopper on pipe length, with some very obvious differences. Originally, I ran the pipe at over 36" from the collector. Then, I cut 8" off which was a bump in throttle response and gurgling at 4 - 5k rpms. You also had a wa-wa-wa-wa-wa wave at the longer length (which I think is a leanness, but could be misremembering) that went away with the shorter pipe. Also, the tailpiece is mig welded stainless with SS wire and a machined aluminum cap and hand built baffle. As long as your willing to take notes and make changes, you can dial in a good exhaust yourself reasonably well.

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