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Hello All,

I'm doing a resto/mod on a 1981 CB400T, which is nearly completed. After some urging by jcw I've decided to make a move an get a welder for this project. I am going to go with a MIG welder. A- cuz they're cheap. B- cuz I have only ever used MIG. C - cuz I want to turn and burn some tools for my shop with steel (cart for tools, rolly chair, cart for welder, stand for project bikes) and doing it with TIG seems like a pain in the ass. If I get into welding components for the bike and it turns out I like it, I'll look at getting a TIG machine later. I can get a MIG welder for $50 here in Deutschland, where a TIG Machine will run me 4x that.

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.

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.....

3. MIG vs. TIG welding

For me, I'm not really in love with the stainless steel look of custom headers. Could that change? I guess. For now I'd be happy to make an functioning 2-1 header with a new muffler, all painted black. So, has anyone in this thread ever welded an exhaust with a MIG welder? Is there any major drawbacks? Have I missed anything in the above considerations?

Thanks for the help in advance!
bike2.jpg
 

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I have neither encoraged you to MIG an exhaust or start with an exhaust as your first project.

Be warned i think this is a BAD idea.


MIG welding projects around the garage is a good first step. Practice on some thin sheet to simulate tubing will tell you if MIG exhaust is practical or not. I suppose you COULD glue metal together with MIG and grind them to look decent, but thats not real welding. The problem is the lack of heat control a cheap MIG welder can provide.
 

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I salute your enthusiasm. Mig welding is pretty easy to learn, and most people will be able to lay down a decent looking run within minutes of their first attempt. The challenge comes when your work is not laying flat horizontal in front of you, or is thin. Welding vertical, out of position (upside down), thin, or tubular is when you realize there's more to it. Not trying to discourage you, but definitely practice on scrap tubing before you commit to the tubes you carefully shaped for your exhaust. I think you'll find it looks much better if you use a pulsing technique on your tubing, rather than steady trigger. Think of it like spot welding. I've been using mig for years, and have sold quite a few custom stainless steel exhausts for various bikes, that were back-purged and Tig welded. I would never use a mig for exhaust unless I had no choice.

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What he said ^^^^

Yeah, exhausts on cars are mig welded but like I said, you cant flip a car over on a table and TIG weld it and no one sees the shitty looking welds anyway.
 

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Discussion Starter #6
I salute your enthusiasm. Mig welding is pretty easy to learn, and most people will be able to lay down a decent looking run within minutes of their first attempt. The challenge comes when your work is not laying flat horizontal in front of you, or is thin. Welding vertical, out of position (upside down), thin, or tubular is when you realize there's more to it. Not trying to discourage you, but definitely practice on scrap tubing before you commit to the tubes you carefully shaped for your exhaust. I think you'll find it looks much better if you use a pulsing technique on your tubing, rather than steady trigger. Think of it like spot welding. I've been using mig for years, and have sold quite a few custom stainless steel exhausts for various bikes, that were back-purged and Tig welded. I would never use a mig for exhaust unless I had no choice.

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Is that because the MIG machine is hotter and less precise? In reading about the difference between the two, it seems like that's the major difference. Folks say that TIG allows you to dial down the heat so you don't blow out the material. Is that why you wouldn't use it for an exhaust?
 

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Sorta. With mig, one of the challenges is getting your machine set right, and that's not always easy, plus, you have to adjust your settings as you go, to compensate for the heat build up. The end of a run takes less amperage than when you began, because the metal is now heat soaked. And with mig, that wire just keeps coming out, wether you have the metal hot enough, or not. With Tig, YOU decide when, and how much, if any, to add filler, once you establish a puddle. Plus, with Tig, you have full control of heat/amperage with your foot pedal, so you can dial back the heat as you move along.

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You can control the heat input at the machine but then also make fine adjustments as you progress with the foot pedal with TIG. More heat when you start, less when you end. You can also control heat input with the speed you travel with both processes.
The main issue is the cheaper the machine, the less stable at low amps. It is the stability and control at low amps that you need to weld exhaust tubing.
Structural tubing and flat plate you can weld with any process. Stick, oxy/fuel, mig, tig.

Again what he said^^^^
 

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... 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 thought the job of the exhaust resonator was to reflect the exhaust pulse noise such that the sounds tend to cancel out one another and make the exhaust sound tolerable.
 

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The only thing i would agree with is buying a mig to wet your feet and see if welding is something u wish to pursue. I am sure there are a bizillion cheap and expensuve TIG welders out there whose owners never really got into them.
 

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Discussion Starter #11
Thanks for the feedback fellas.

I can get into a MIG welder for cheap, like I said. So maybe I'll build the equipment around the shop first. Not having a tool cart on my first bike sucked. Tools and parts everywhere. plus my stool didn't have wheels :(
 

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These two threads were interesting on the subject. They contradict each other to some extent.

On the one hand, It helps me understand I know nothing about the why for the "H-box", or designing exhaust systems for that matter. On the other hand, the one experiment seems little if any change in performance, save for shifting the power band to higher in the RPMs.

https://www.hondatwins.net/forums/67-miscellaneous-discussion/41350-power-chambers-explained.html

https://www.hondatwins.net/forums/67-miscellaneous-discussion/124294-converting-dual-two-into-one.html
 

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Without looking too much into both threads, my take is...
1) we arent building racebikes where extracting that last bit of horsepower is consistent with the rest of the build
2) resonance chambers like the one in tge first thread are more likely made for subjective sound quality and sound dampening issues than performace issues.

My recommendations are to KISS. keep it simple. Two into one. Calculate header diameter, calculate header length, estimate post merge diameter and you have the plans for performace exhaust that is better than most.

If ypu google Wallace racing engine calculators they will have something for the length and diameter. Cone engineering will have links to recommendations on post merge collector diameters.
 

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Thanks for the feedback fellas.

I can get into a MIG welder for cheap, like I said. So maybe I'll build the equipment around the shop first. Not having a tool cart on my first bike sucked. Tools and parts everywhere. plus my stool didn't have wheels :(
I have great stools. I'll send you a stool sample.

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My take on your concern for proper header design: you can port the head, oversized pistons and valves, race cam, spending $1500 in parts and labor... for 6 hp gain.

Build something sound and aesthetically pleasing, your butt Dyno will never know the difference.

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I built this exhaust, it's over-bored, custom ground race cam, blah, blah... Is the exhaust optimized for flow, scavenging, etc? Probably not. If it is, it was by accident. Who cares, it looks good.


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Discussion Starter #17
[/QUOTE]I have great stools. I'll send you a stool sample.

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I like my stools soft and smooth. FYI.
 

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Heated garage floor is real nice, just chuck a dog bed on the floor and you're at the right height to work on the bikes.


Where you will notice if the other guy did a better job on his CB350 then you, is when you can't keep up to them going up the next big hill,
and no matter what you do :/ it's still going to have that unmistakable CB350 sound recognizable from ~a kilometre away. The best uncorked 4-stroke exhaust note imho is still a reverse cone megaphone and a resonator with a balance tube between the headers becomes more important once you add twin mufflers that offer some back-pressure.

"... save for shifting the power band to higher in the RPMs" then they possibly achieved exactly what they were aiming for, because a CB350 with only open headers will fall flat on it's face at higher speed compared to one with a well tuned intake and exhaust.
 

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I built this exhaust, it's over-bored, custom ground race cam, blah, blah... Is the exhaust optimized for flow, scavenging, etc? Probably not. If it is, it was by accident. Who cares, it looks good.


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That is one sweet CB350, but I bet it would be even lighter and slightly less prone to head shake on the big bumps if you removed one of the more then enough twin front brake discs. As it sits that bike probably has about 8 times the original front brake and double what that motors power and bikes weight can justify carrying. ... from here the rear tire now looks inadequate, you're going to be riding that front wheel a lot.
 
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