[Immer54]

Hello All,

I'm doing a resto/mod on a 1981 CB400T, which is nearly completed. I've got the standard exhaust set up on it 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. I've asked around on other forums and 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/calc_exhaustlength.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 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? I know that here in Germany, I can get a used MIG Welder for $50, and pretty much any TIG capable machine is 4x that cost.

I've attached a picture of my project so folks can see what I'm working with. Thanks for the help in advance!

 

[J-T]

Can't much address the other questions but the H-box under your engine is what Honda calls a power chamber. Here's some info on it.

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

And here's a recent discussion on the subject:

https://www.hondatwins.net/forums/6...sion/124294-converting-dual-two-into-one.html

 

[66Sprint]

Removal of the under engine chamber WILL substantially REDUCE the overall performance of the engine (unless you compensate for the change)......
There are MANY, MANY more factors involved in designing an exhaust system than the two you mentioned......
Remember, the system is already a compromise....Improvements in one area of the rev range will result in a detrimental effect at other rev speeds......
Based on your questions, I doubt you will be able to out-design the Honda engineers, but who knows, you could get lucky...

(Please forgive the sarcasm, I DO understand the aesthetic need to be rid of the chamber, but in the past, those who have opted for "looks" over functionality are invariably disappointed by the bikes performance loss......Looks better, sounds better, but doesn't run as well....)

 

[Immer54]

Can't much address the other questions but the H-box under your engine is what Honda calls a power chamber. Here's some info on it.

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

And here's a recent discussion on the subject:

https://www.hondatwins.net/forums/6...sion/124294-converting-dual-two-into-one.html

Both of these threads were very helpful. Thanks for the info!

 

[Immer54]

Removal of the under engine chamber WILL substantially REDUCE the overall performance of the engine (unless you compensate for the change)......
There are MANY, MANY more factors involved in designing an exhaust system than the two you mentioned......
Remember, the system is already a compromise....Improvements in one area of the rev range will result in a detrimental effect at other rev speeds......
Based on your questions, I doubt you will be able to out-design the Honda engineers, but who knows, you could get lucky...

(Please forgive the sarcasm, I DO understand the aesthetic need to be rid of the chamber, but in the past, those who have opted for "looks" over functionality are invariably disappointed by the bikes performance loss......Looks better, sounds better, but doesn't run as well....)

Engineering gut check noted! Thanks Sensei.

 

[drydreamer]

Mig welder, do you have other needs as well< you will find a mig very very useful bit of kit and well worth the money.

Mine ( now over 28 years old ) has paid for its self many many nay times over and used for all sorts, welding up rotten cars and exhausts, some structural steel work and even removing sheared off brake belled nipples and sheared off mudguard bolts in alloy fork legs.

I have "repaired" many a front header pipe and silencer for bikes using scrap exhaust tubing from fast fitters skips, but ask first.

You will develope your technique with practice.

practice makes perfect.

Measure you pipe diameter and get some lengths of scrap pipe from the fast fitters.

Some diameter pipe is perfect to slide onto the header and around the curves.

You will be glad you got a mig, mine is a hobby type 120 amp max using 0.6 wire and pub gas, which seems to be the best compromise and economics.

I would love a tig ( i have used gas for years ) and am saving up for one. I would also love oxy acetylene but that set up if out of my reach money wise, i have a number 1 tip ready for the day though, and braze rod and flux.

 

[dtsmjr8dan]

Do not use braze rod on exhaust. It WILL fracture from the heat/vibration. Tig, mig, or gas weld. The original pipes are double wall (two different diameters with an air gap between). I hope this helps. BTW, many have tried to match Mr Honda's engineers. Some have come close (after many hours and many bucks). But , it's your bike. You can have a 'look' , or a bike that performs well.

 

[JamesPal]

Check out these guys: Cone Engineering. There already has been a lot of comments on this thread. Stock Honda exhaust systems are hard to beat. Honda has put a huge amount of effort into the design for efficiency and noise. Earlier twins were not subjected to the EPA noise restrictions. Sometime around 1977 Honda started responding to the EPA regulations proposed for 1978 and exhausts got more restrictive. Here a good discussion of Exhaust Theory. Good luck and keep us posted.

 

[drydreamer]

If mig welding, observe and OBEY all safety warnings, cdi's, stators and all ( thats ALL ) electronics are fragile and will not like mig's up there backsides!!! ECU's, abs and any other fragile expensive stuff like alternators etc, are best removed for their protection., even arc can kill electronics so be warned!!!

Some will say that tosh, get it in writing so when you fry stuff on there advice, they will pay the replacement cost bill.

Better to be safe that sorry.

 

[RansHondas]

What about TIG precautions?

If mig welding, observe and OBEY all safety warnings, cdi's, stators and all ( thats ALL ) electronics are fragile and will not like mig's up there backsides!!! ECU's, abs and any other fragile expensive stuff like alternators etc, are best removed for their protection., even arc can kill electronics so be warned!!!

Some will say that tosh, get it in writing so when you fry stuff on there advice, they will pay the replacement cost bill.

Better to be safe that sorry.

drydreamer,
Just purchased a TIG welder and, having MIG'ed for years understand the fragile nature of the electronics you mention above. Does the same apply for TIG?

 

[drydreamer]

its an electric arc so yes, don't take the risk as fried electronics are REAL EXPENSIVE!!!