Topic: What to do about my header dilemma?

[strangedaze39]

Hey so I bought myself some Mivv mufflers for my CB350F a while back, but they are still sitting in their box in the living room because I haven't been able to figure out what to do about the headers.

I figure my options are this:
A) Find a nice OEM pair that doesn't need any work
B) Find a beat up pair with little rust and pitting, clean them and have them chromed.
C) Find a nice OEM set of CB550 headers and use them (they fit the CB350F with a little or no slight bending)
D) Get some Stainless Steel stock bend my own headers at the shop and polish them to a chrome like shine with my wheel.

Problems and questions:
A) These are hard to find and when you find them, the owners normally don't want to part with them.
B) I'm not sure if this methods realistic. Also how much would it cost do you think?
C) Same problem as A but if I look for both I double my chances of finding a set I suppose.
D) The Mivv mufflers are polished stainless steel and look just like chrome. I'm sure I could easily polish the self made headers to the same finish but will it hold up the same?

Sorry I know thats a lot a questions, the more answers opinions on this the better. Thanks for reading.
SD

[troppo]

G`day mate
I was thinking about doing my own pipes for my 750 due to the cost of decent ones here in aus.
The fab work isnt going to be hard for me beause i`m a welder during those horrible times known as working hours lol, so i asked a few questions and one of our gurus gave me the following information that would adapt to your 350 easily

@troppo, to get you started, here's my learned summary of header design theory for the 4-stroke 4 cylinder CB750:

1) Length and width of header, collector, and megaphone sections
From experience, optimal pipe width has been shown to be around 75% - 95% of exhaust port diameter.  At this ratio forces gases to escape with added speed and allows the sound pressure waves to have some influence as well.  Wider than this and exhaust gas speed and hence heat, is a problem.  Narrower than this and back pressure and loss of power is a problem.

Collector pipe volume = sum of volumes of inlet pipes.  Hence, with 4 pipes, Collector diameter = 2 x header pipe diameter.

Megaphone section length adds to overall resonant length, and when combined with header section length, will determine the system's resonance characteristics.

2) Temperature
Break mean exhaust pressure (BMEP) is a good tool to calculate average exhaust temperature (EGTav).  BMEP = K x power (Hp) / displacement (cc) x rpm, k= 447420.  BMEP x 60 = approx EGTav.   At 6000 rpm, the CB750 may have a BMEP of 6.8, which means an EGTav of 410oC.  At 410oC sound travels at about 58,000 cm/sec.  (Pipes will blue at about 600oC)

3) RPM
Pipes will resonate at precise sound frequencies.  RPM determines the sound frequency at which each cylinder is running.  Resonance is key to pipe design, and that means we must establish at what frequency a motor operates, or more precisely at what frequency we want our pipes to resonate.  I’ve chosen 6000 rpm as the target design point.

Since a burst of sound comes every 4 strokes or 2 revolutions, each cylinder's sound frequency at 6000 RPM is therefore 50 Hz. Because there are 4 cylinders, operating at 2 opposing strokes, the combined sound frequency of the exhaust system at 6000 rpm is 50 x 2 = 100 Hz.

Also note that 6000 rpm = 0.01 seconds per revolution (inverse of rpm) or about 0.005 seconds/stroke.

Putting this together, you get; that in the interval between the power stroke and the beginning of the exhaust stroke, at 6000 rpm, sound would have traveled down an exhaust pipe, a distance of about 72.5 cm (28.5") Call this the Header Tuned Length (TL).  Actual exhaust port timing is more precise than merely assuming ½ stroke (90o), but for this illustrative example its close enough.

Pipes will resonate (that sweet sound) if their length is a multiple of their resonant frequency.  From above, TL= 72.5cm and that means 2/1 or 1/1 or 1/2 or 1/3 or 1/4 etc of 72.5cm (1/1 = 28.5”, ½ =14.2”) are harmonic.  All other lengths will dampen the sound.

4) Sound pressure and pipe length
  Sound waves have pressure - they can move the air (exhaust gas in our case).  Also, sound waves can interfere with each other and can be inverted by bouncing them off of objects.   This can be accomplished by using bends, divergent cones (megaphones), pipe length and in the case of multiple pipe exhaust systems, by timing the arrival of sound from one cylinder with the port timing of another cylinder.  At 6000 rpm, a pipe of TL/2  cm would present a wave front to the opposing cylinder at precisely this moment.  Hence placing the first bend at 36cm in a header pipe length of 72 cm would achieve maximum extraction at 6000 rpm.  Thanks to harmonics, any multiple of TL will also work, just to a lesser extent. Conversely a pipe length that is not a multiple of this length, will have adverse effects.  If actual exhaust port timing were to be used, then a slight adjustment to this length would be necessary.

5) Speed of exhaust gases and pipe length
  Gases will speed up through a smaller diameter pipe and slow down in a larger diameter pipe.  Correspondingly, pressure will rise in a smaller diameter pipe and drop in a larger pipe.  Ideally, as the sound front hits the gases it speeds them up precisely when needed, and a reverse sound front slows them down precisely when needed.

The speed of sound is generally much faster than the speed of escaping exhaust gases.  Stroke of a CB750 is 63mm.  Since all the gases must escape within the span of time the piston moves this distance, we can determine the speed of the escaping gases.  The volume of gases = 63 x 61 stroke x bore = 184.1cc (under pressure).  Under the same pressure, that volume would take up 23 ¼ cm of a 1 ¼” I.D. exhaust pipe.  At 6000 rpm, this volume of gases have about 0.005 seconds per stroke to escape, hence the speed is 23 ¼ / .005 cm/sec = 4,650 cm/sec.  This would only be true if there was no pressure differential in the exhaust pipe.  However, since gases are highly compressible, the actual speed is a fraction of this rate calculated as a ratio of pressure (about 50% actually), in any event, a far cry from the 58,000 cm/sec that sound travels at.  Seen another way, the exhaust gases could have travelled an average of about 5”- 9" down the header pipe when the sound from the next cylinder hits them and forces them to pause.

Summary
CB750 pipes tuned to 6000 rpm
Single pipe dimensions 1 3/8" O.D. - 1 1/4" I.D up to 1 ½” O.D. – 1 3/8” I.D.
Actual Header Length = resonance frequency multiple of TL = 36 cm in our example
Collector diameter = 2 1/2” to 2 3/4”
Megaphone length = resonance frequency multiple of TL = 72 cm (for example)
Total exhaust system length = 108 cm (43”)

So there you have it, 4-stroke exhaust pipe design in a nutshell. Grin

Hope this helps mate
Cheers
Troppo

[Hush]

What does a MIVV muffler look like?

[okie]

MIVV looks like this:

http://forums.sohc4.net/index.php?topic=23960.msg332621#msg332621

[strangedaze39]

Thanks troppo! That was very helpful!! What kind of metal were you planning on using? Thats my main concern, but I honestly didn't even think of most the performance/design info you posted. I was just going to copy a pattern from one of my old beat up set that is a equal length design (that would be so cool).

Thanks okie for posting the link for me, it's actually really hard to find pics of Mivvs it seems like. BTW okie did you decide to hold onto your spare set of headers? Captusa decided he was going to hold onto his for any future project he had in mind.

Any advice or thoughts are welcome. 

[troppo]

i was in two minds about the material, go with mild steel and get them chromed (or maybe just paint them, cheaper lol) or go with stainless but that wouldnt be the cheapest material to use.
I might try a mild steel for the first set i try, that way if i stuff up it wont cost me much

[okie]

As you can see, from your current situation, MIVV mufflers are worth nothing without good headers to install them with.  The only way I will sell my headers is WITH the mufflers. 

Have you seen this eBay auction?

http://cgi.ebay.com/ebaymotors/Honda-CB350-CB-350-Four-Header-Pipes-1972-VINTAGE_W0QQitemZ330265033517QQihZ014QQcategoryZ35596QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

[strangedaze39]

Hey Troppo, I was thinking about the same 2 metals. The SS would be expensive it self, but maybe the chrome plating costs would make the prices closer in the end. I also could see SS lasting a lot longer then the other metals but it might not matter much since even the chrome would last 10+ years.

@ Okie, thats fine it wouldn't do much good buying them with the mufflers that I already have. Thanks though. Also those headers you linked look non salvageable, but I don't know much about chroming pipes and if chrome platers could take that pitting out. I'm going to have to disagree and say mufflers are worth a LOT without headers, I payed $400 for mine and I feel like I got a steal. Unless you talking about use wise, but like you already know you might have a wait a while to find ideal headers but it's a worthy wait imo.

I'm going to call some platers today hopefully that will help me figure out what I should do.
Thanks for the reply's