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'79 CB750F SS -- 4 into 2 exhaust fabrication

Luis Etchenique

'79 CB750F Super Sport
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South Ponte Vedra Beach, Florida
The process of restoring/modifying a CB750F SS which I have recently acquired includes --among many other things-- includes the fabrication of an exhaust system; I was wondering if anyone in the group has done this in the past -- and what the experience was ( or is ... ) like.

The easy route would've been finding a used unit, purchasing aftermarket, or simply buying a reproduction, but I'm taking the long way and will fabricate from scratch.

Although I see the great majority use the lighter *and proven* 4 into 1 systems ( or 4 into 2 into 1 ), I have decided to build a 4 into 2 -- simply because I seek bi-lateral symmetry. The material will be stainless steel. The 1.5" header pipes will join at two 2 into 1 merge collectors, and from the collectors I will run a 1.75" secondary pipe to each the mufflers, one on each side. These mufflers are canister-style with minimal restriction.

I definitely haven't performed any thermodynamic analysis ( I'm an economist by education :shrug: ), but I also don't seek superior performance: just adequate for an occasional ride.

I'd like to know if anyone ( even a rank amateur as yours truly ) has completed an exhaust build. Any comments, suggestion, advice, "do's" or "dont's" will be welcome.
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I haven't done the entire thing but don't need to to come up with a couple things. I know a whopping lot about exhaust and power. I've done a bunch of (back) half systems to success.

One is that giving away the amount of power one does just to get looks is, well, not smart. A 4-1 header will add a solid 7 hp. and sometimes more to these that the 4-2 will not. Of course there are some who just like to drive slow motorcycles. Wanting to do a lot of work and spend a lot of money to still get one though, I think that is a first. Well, maybe not, now I think of it I've known several people that spent big bundles to go slower but I don't think that started out being the plan.

Two, the exotics of merge collectors are wasted on a system that really is not tuned and can never be, all 4-2 exhausts with the classic 1-2 and 3-4 pairing do not space the exhaust pulses properly to get equal benefit to each cylinder paired together there unless you pair 1-4 and 2-3, making it harder to build. In the classic pairing one cylinder always suffers compared to the other one in the pair. That latter pairing is one that does not look as pleasing either.

Three, canister mufflers are for noise control only, you need a true bell megaphone on both sides to get more power, but the price is more noise. Minimal restriction means nothing when you can make a system actually increase power by restriction so low it makes a vacuum. A classic 4-2 cannot do that, again because the pair exhaust timings are lopsided.

Four, everybody says they don't care about the missing power until they drive one that does have it, then things change instantly. I've had people drive mine with the extra hp. and they have the standard 4-2 and they instantly wanted mine instead.

Five, the 4-1 is easier to build and much cheaper too, and using stainless you better be a better than good welder doing the work.

Of course, yours and do as you will...............the physics will not be denied though.
Thanks AMC -- I've been doing some reading on the subject and you're 100% correct on all points. I am extremely puzzled by the works and the dynamics of exhaust systems -- and how they defy common sense at my level.

One of the mandatory questions I have is: Since the Honda engineers merged #1 and #2 on the left side, and #3 and #4 on the right side and omitted a crossover pipe ... am I not following to some extent their engineering "logic" ? Obviously, the stock system has a level of sophistication that we tend to ignore/dismiss, but naturally they had to balance regulations, performance, and looks.

Now ... to make things a bit more complicated ( I bet you saw this one coming ... ), the exhaust I'm designing/fabricating will be *elevated* similar to the scrambler-style CL360, of course in my case with two pipes running (and subsequently merging ) on each side of the machine. The reason is exclusively looks and originality -- and the promise of "several" weeks of measuring/cutting/tacking/welding/mounting ... and hopefully some day assembling.

As far as welding, I have a guy in town who helped me in the past. He specializes building towers for high-end sport fishing boats.

At this stage, I already have all the parts I need: a collection of mandrel formed u-bends, straight pipe, reducers, merge collectors, etc., all fresh in boxes. Couple of years ago I fabricated an exhaust for a BMW R90/6 I built as an endurance racer replica ... I add a link below if you want to take a look.


... and here is the CB in question:


Thanks again for the input AMC -- I'll start adding updates as I go, and I'll try to illustrate w/ some pics.
Doing a four is not going to be nearly so clean as a twin and that Beemer is super narrow up high anyway. Yet the muffler area there was still way too wide. Almost a straight piece of pipe on each side, in no way are you going to get that with 4 of them.

The bundling of twin pipes right at the cylinder line coming out instead of down is going to be a problem, DOHC run too hot anyway and the 750 needs an oil cooler when hot dogged for sure. Oil pressure drops 20 psi at over 90 degrees when you play with it ten minutes. But where to put cooler with exhaust in the way? The bike will likely be much wider than you think. Ever get on a CL450?

The engines bigger than 750 do use a crossover in the exhaust but it's not very efficient. The Japanese design to looks and spend inordinate amounts of cash trying to pick up what power they can in what they know is a substandard setup. Look at the pocket racers now and no longer true. Everything is 4-1.

Likely going to have to build intake too, the usual plastic airbox or rubber pods are not going to like exhaust running close at all. You are going to have to be recognizant of the heatbox you are creating right at carb level, the engine heat off the back of a DOHC is already bad enough to cook the rubber intake manifolds as it is.

Food for thought, Honda on the DOHC racebikes at one point dropped the oil coolers down a bit to cylinder level rather than upper head location and the blocking air there burned up every engine they tried to race. Where you are going to place the pipes. Consider that the OEM pipe drops to let air in between cylinder pairs in ports in the block.
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I see exactly what you mean, AMC. As far as the intake, I will be using one of those multiple-pod filter canisters for the sake of a clean and uncluttered look. That now-empty space where the airbox and battery used to coexist will be used to route the secondary pipes, zigzagging in-and-out of the frame to prevent the legs from being exposed to the hot metal -- like I did on the Beemer.

I will be adding an oil cooler but will be located --indeed-- on the front/lower section of the engine just by the existing oil filter. Besides, not much going on any more at those "lower elevations".

So the headers will be in fact tightly "wrapping" the engine right below the head -- *BUT* ... where the engine is a bit narrower since fins are smaller, which I assume is a less-hot section than the head portion. Kind of wishing upon a star kind of thing, I know ... ;-)

So far, the hardest challenge is the location of the merge collectors. I face two issues: 1) the header pipes ( each pair ) will be of unequal length; 2) the collectors themselves will be pretty close to the exhaust ports, causing havoc with scavenging, back-pressure waves, ... and God only knows what else. Essentially, all I want within the frame are the two 1.75" secondaries. On top of that, the system must be symmetrical ... of course.


Plowing forward ... stay tuned ... :bike:
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'I will be adding an oil cooler but will be located --indeed-- on the front/lower section of the engine just by the existing oil filter. Besides, not much going on any more at those "lower elevations".'

You're in optimum rock pitching territory there..........may need some kind of rock guard or screen the lower you go.
Was wondering how long it would take you to get there.

More ideas at the CB1100F.net page...........might want to consider using the 900F oil cooler setup which needs a different pan and oil pump but high volume as compared to a standard oil cooler and uses hardly any pressure to do it either. The standard aftermarket add-on cooler taps into oil filter and is a pressure hindrance. More than one thread on how to do that there.
Your rendering looks doable. If you can get those Y pipes and collect 1-3 and 2-4, that would be cool. You will have to have your bike jetted as the stock air box is no longer usable. Also make sure your "battery box" needed to relocate the battery, reg/rect and solenoid are clear of the exhaust. The work you did on the BMW is really quality. You have some good experience/tools. Get it done! Don't forget you can wrap your headers. I just did mine, and I can ride for 30 minutes, hop off the bike and grab my headers with my bare hands.....They are literally warm to touch. They smoke when at a stop sign but I love them. Get the golden header wraps.

One thing not noted is that a 4into1 will give you much less ground clearance. The 4 into 2 hugs the frame rail. The 4 into 1 is like 4 inches below the frame rails. At least the standard 4into1 exhaust systems are. There is the "Sidewinder out there that has my attention. It's a 4 into 1 that contours the bottom/side of the motor for more ground clearance. Now my front forks are rebuilt, clearance isn't an issue anymore and I can actually lower my bike if wanted. It's just there are some nice right turns, that go "up-hill" on my ride into work. When I try and lean my nighthawk to scrape my knee :D my skid plate will sometimes reach the pavement. It's a minor scrape absorbed by the aluminum skid plate I fabricated, but it's there.....

Just food for thought. I know you are going the complete other direction with your custom exhaust.
'...collect 1-3 and 2-4...'

Where did you get that? Whacky. Max effort for the exact same thing as OEM.

Don't get a sidewinder, made for straight line drag racing only and you cannot turn right with one, just try it, you'll eat pavement.

If you wrap steel pipes and ride in the rain it's how you rust the pipes out 10X quicker. The wrap holds water next to the steel to boil and it erodes metal.

The OP is using SS and you DON'T want to or even need to wrap it.
AMC49 -- thanks for the CB1100f.net page referral; and I love the idea of reducing pressure loss by increasing volume.

WEZ -- collecting 1+3 and 2+4 <<although mechanically efficient>> would not only add to the complexity of fabrication and require additional space, but it would have a bit of an ackward look of "spaghetti", which I want to avoid.

The battery ( a 3lbs SHORAI lithium-iron ) is being relocated to the tail section -- under the cowling of a vintage RCB endurance racer solo seat.

The elevated exhaust system is being fabricated using 304 stainless, and it won't be wrapped -- as I commonly see on many builds. I am being careful to craft it in such a way that hiding it *would be a shame* :smoke:. The radiating heat is not a concern, since most of my riding ( which isn't much ) is on open roads and in cool weather. Keep in mind this is an excercise in design and fabrication more than engineering.
'...collect 1-3 and 2-4...'

Where did you get that? Whacky. Max effort for the exact same thing as OEM.

Don't get a sidewinder, made for straight line drag racing only and you cannot turn right with one, just try it, you'll eat pavement.

If you wrap steel pipes and ride in the rain it's how you rust the pipes out 10X quicker. The wrap holds water next to the steel to boil and it erodes metal.

The OP is using SS and you DON'T want to or even need to wrap it.

Meant 1-4, 2-3 as you stated above. So the moisture will eat the headers even if I put 3 coats of VHT before wrapping? I stripped to bare metal, 3 coats of VHT, then wrapped.
If the pipe was not chromed eventually it will eat it, but the wrap may die first. You'll see the effects then.

I've heard many a word on stripping paint but unless it was acid etched or completely glass or sand blasted it is not generally 100% clean enough to lock paint down hard all over the part.

Wrap is a cheap gimmick to hide bad pipes based on the thought that sound wave action is faster when the heat is kept in the pipe. Doesn't really work that way in reality though. I used to sell scads of it when I was in parts for the car headers. Like the cold air kits, cheap and the kids think they are 'doing something'.
As I progress with my fabrication, I come across this work by the folks at MAD Exhaust. Seems like the only elevated 4 into 2 commercially available for the CB/DOHC engine.

It doesn't quite work for me: the *idea* is similar, but my approach ( in many ways ) is quite different.


The pressure's on -- love the challenge ! :smoke:
Sounds like expected or like a Honda 350. Not big on that sound, I left it behind 40 years ago.

Pipes just keep steadily going up, they would look better to me breaking off to go horizontal (dead level) right after the pipe merge. But to each his own I guess.

The outboard carbs are going to likely boil the fuel in them as close as the bowls are there. Could make carb issues.

A lot to pay for a stoneage sound.
100% in agreement, AMC49.

The "uphill" direction of travel of the system simply doesn't work, and the sound is perhaps the worst I've heard in quite some time.

We'll see, but I think the series of bends and turns in mine will add some "complexity" to the sound waves, and should result in a bit more interesting exhaust note. When I was fabricating the BMW's exhaust, as an experiment I first tried a piece of straight pipe, and the sound was unbearable as far as ... well, just unbearably lame. The final product was like night and day.

As far as the carbs -- I'm designing a *minimalistic* heat shield to protect the outboard carbs from exposure to the radiant heat.


If I charged myself by the hour, I'd be a millionaire by now -- ... or perhaps just flat broke ? :umm:
'If I charged myself by the hour, I'd be a millionaire by now...'

I hear that. In my car work I have at times come close to making the equivalent of $3000/hr. before, and very often $250/$500 an hour. I can't charge that, rather it is savings on work done myself, I have never paid for a car or bike repair over 45+ years.

Sound waves in the exhaust really don't see the curves or bends if the pipe is round, only severe restrictions affect them. Out and out flow can vary some though. The biggest difference in sound is when you get pipes not equal in length, your ear then picks up the timing difference as more rasp on a 4-1 header, pipes getting equal within two inches sound smoother. Put a bell on the end (megaphone) and the noise intensifies a whopping amount and gets much deeper, and on a low rpm rack of the throttle will then pick up the hard noise that hurts your ear, the meg if 7 to 9 degree diverging amplifies it. Canister mufflers simply kill all sound and even the part that makes power and why they have gone to them in all racing, the sound regulations have gone against true megaphones as they make way more noise and you lose power when you dump them to go to canister.

When you do 4-1 you get sound wave spacing of every 180 degrees in the 720 degrees it takes to fire all 4 cylinders. When you do 4-2 but with 1/4 and 2/3 paired you get 360 degree equal spacing each side. The OEM 4-2 ends up with one side firing then 180 another fires on same side and then 540 degrees happens before your next power impulse, and why you can't really tune using sound waves to get equal power out of that two pipe setup and why it sounds like the 350. You're almost hearing a single because one cylinder gets more noise out due to the long lag on one. Why a crossover pipe joining both sides can help too. 4-2-1 exhaust gives double, you get the even spaced 360 degree hits and then the 180s too and why they build torque if the pipes are tuned length.
AMC49 -- I could've NEVER imagined the physics involved when it comes to the design and engineering of exhausts systems. I've been doing some reading, and I'm in awe about its intricacies and complexities.

Been having a great time talking with my 27 year old daughter --a physicist specialized in acoustics-- discussing the propagation of sound waves within pipes. She's taught me a thing or two -- who would've imagined, right ?

Here's a cool article on exhaust technology. All I can say is I'm glad I'm not building a Top-Fuel drag racer !

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Yes, the stuff can warp your mind. All the interlocking factors involved there are staggering once you put hard science to it. I grew into the stuff while playing with 2 stroke engines when I was younger. Those get even worse with an entire other set of things done to STOP gas evacuation at the proper time.

There was a book written IIRC back in the day called the 'Scientific Design of Intake and Exhaust Systems' among other things leaning toward pre and post war developments to open a lot of that up. German scientists were cutting edge back then like Walter Kaaden.



I've even come to the conclusion you can have two forms of exhaust extraction.........one is the normal acoustic enhanced one that involves very close timings and the requisite system design and another type that is inertia only not really involving dead exact timings as it relies on mass gas inertia rather than the acoustics. They both interlink some and together you have a mess. Then you have the opposite or stuffing techniques that go on when you do NOT want the evacuation and want to stop it in any way possible. Another set of things come into play then. Then there are all the intake tract goings on as well.
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U P D A T E --

... yes, progress is SLOW, but here is where things stand as of now.

The left side headers are coming along. Although it might look simple, to get the correct radii on the bends was tricky stuff. The #2 header is made from three sections and #1 from two. Before moving on, all joints will be welded. Next step is adding the 2 to 1 merge collector, from which a 1.75" OD secondary pipe will "turn" into the frame behind the carbs -- on its way towards the rear of the bike. This secondary will subsequently "exit" ahead of the rear shock absorber into a canister-style muffler.

No cross over connection -- want to keep things clean and simple.

Naturally, the right side of the bike will mirror the left side, so fabrication should be a bit simpler.


As always ... I welcome all comments and opinions !