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Can a stock CB 750 bottom end handle an increase i power of 25%

You can but a waste of time. Your exhaust setup alone will likely cost you 20+ hp. due to practicality issues. Somebody makes a gear drive to replace the DOHC primary morse type hy-vo chain but at a cost of $5000 an engine, and he will tell you you are crazy if you pass your idea to him of 2 engines.
 
Because you insist on 8 cylinders. Try to work that out to where the exhaust still flows well enough to not cost any power. On all smaller engines like motorcycle the exhaust is a power increasing method and part of the engine power is directly attributed to it, but on an 8 it becomes a nightmare trying to do it. If you take the most direct route and simply run pipes to all then you lose that 20%, 10-15 for sure. Not hard to lose 20.
 
Flat plane V8s have no trouble scavenging exhaust pulses efficiently on each side, as each side fires evenly , every 180 degrees. CB750 fours are essentially one side of a flat plane V8.
I can go four into two into one on each engine, then maybe connect the two ones with an X pipe.I don't see a loss.
 
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Firing at an even 180 has nothing in common with scavenging every 180, two absolutely different things. When you split the exhaust streams in a 180 four into the standard two sides with 1-2 and 3-4 paired up they are NOT exact and a whopping energy loss happens on both sides on the 360 degrees with no impulse happening at all. Yes, all Japanese 4-2 exhausts pretty much are substandard in that respect, they are designed mostly for good looks over power. A 4-4 makes more.

The firing evenly must be accompanied by even exhaust events or the potential for extra power is lost. That's a true well designed 4 into 1 header only, anything else costs power.

X-pipes are garbage, they screw up wave tuning in my book. A car trick retro-fitted to bikes to slow them down. X-pipes were originally designed for cars where the two dual exhaust pipes are located too far enough apart to do a good quality pipe join that wave tuning likes. They also fake pipe pressure hitting atmospheric but nothing does that like the real thing.

Pick up and read the book 'Scientific Design of Intake and Exhaust Systems' written back in the '30s.
 
I mis spoke. Every 180 degrees is for 4 into 1, which gives more power at redline . 4 into 2 is every 360 degrees, which gives more power in the more useful mid range. The 2 into 1 further down wind would send weaker low pressure pulses back up the pipes every 180 degrees, and might be counterproductive. I have read up on 4 into 1, and 4 into 2 into 1 since the previous post, and it seems that for more power in the mid range, 4 into 2 is better than 4 into 1 . The x pipe is overkill. Either way, I don't see a 20% drop in power. Thank you.
 
'Every 180 degrees is for 4 into 1, which gives more power at redline . 4 into 2 is every 360 degrees, which gives more power in the more useful mid range.'

No. Don't know where you got that. Actually, likely I do but it's wrong. I can make upper end or mid range power with either one of those setups. Each one tends to lean one way but they absolutely do not have to. I've built 360 degree systems that made power at over 10,000 rpm and 4-1 180 that makes power at mid range to pull your face off and dropped in power at higher rpm as intended. I can tell you why the 2 timings go where they do but it's not a high or mid power thing, rather the overall useage of the system to best effect. You can do the exact same with a single cylinder engine.

X pipe is NOT overkill, it's a crutch for a system that is not optimum. They actually weaken pulses.

And what about the standard Japanese 4-2 that comes on most bikes with fire at 180, fire another 180 and then dead for 180 and dead for the next 180 too? Using 1-2 and 3-4 paired up. It's not either one of those.

'I don't see a 20% drop in power.'

No wonder, your exhaust ideas are sadly mistaken. I've run the smallest engines you can think of (Yamaha Twin-Jet 2X50 cc., 9.5 hp. output, modified to 18 hp. do the % calculation there!) to the biggest that go in a car (700+ cubic inch, 1500+ hp. in a 200 mph pro stock race car) and messed with exhaust on all of them, I'm considered pretty good on the subject by those that know me. I build expansion chambers for 2 strokes as well. I can make a 4 stroke setup on a stock DOHC that picks up 10% just by removing the fiberglass packing around the muffler core. BTDT. I raced a Kawasaki 400-3 modded from 40 to 66 hp., the exhaust was the reason I got it so high. Stock engine hp. at 7000 rpm, mine shifted at 10,000.
 
OK, let me just ask, If I don't change the exhaust plumbing on either engine, where does the 20% loss come in? 'Cause, I didn't plan to make one system crossing both engines. Planned on leaving them as Honda made them.
 
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Only 10% or so cost then, what you would gain improving the stock setup. I assumed you were going to build your own and most efforts there are less than stellar.

Even so, you haven't dealt with the physicality issues nor the aesthetics of doing what you say now. I'm telling you, two of the four systems will be where your rear wheel is unless you add 200 more pounds lengthening the contraption. You keep saying two trans and two engines but everything else is smoke and mirrors so far. You are still not dealing with major issues. Simply getting two complete exhaust systems will be a major act of congress, they are commonly rusted out to be junk even at low mileages and look like new until you look close. So, where to get? You seem to double your cost with every post.
 
I'm telling you, two of the four systems will be where your rear wheel is unless you add 200 more pounds lengthening the contraption.
There is no reason I can't separate the two engines enough to clear the rear suspension with the inboard pipes.

amc49;31014 said:
Simply getting two complete exhaust systems will be a major act of congress, they are commonly rusted out to be junk even at low mileages and look like new until you look close
So, where do YOU get your exhaust systems when you buy an old bike?
 
'There is no reason I can't separate the two engines enough to clear the rear suspension with the inboard pipes.'

Obviously you would say that. You haven't gone there just like with every other detail and what I am talking about in your shortcomings of planning vs. reality. I can think of several from an engineering point of view. You only re-design as a forced kneejerk reaction to something you could have easily planned to not be a problem. What happens when you can't switch brain off of 8 cylinders and 2 transmissions.

You DON'T separate the engines further to do something as stupid as that. Do it, and see why. You run both exhausts to the outboards of the pair. Meaning one-off custom pipes. Hey the machine layout is yours.

I don't use OEM exhaust systems, I change them for something else on everything I build. They give away entirely too much power.

Look here. I am tired of this game and your feeding off of me to no avail, you just bury yourself further and further.

Build it. Come back and show me and I will kowtow down and worship you.

I get frustrated just posting to you for no reason, and my posting is now showing my lack of patience, I should have stopped a while back just to stay nice.

Do whatever it is you are going to do, you need a big smack against that wall of reality physics to readjust you. I shouldn't have to do that.

Yours and do as you will.
 
Thank you! I'm not playing a game, just trying to get some information. Please don't be upset if I don't take all of your advice.You have helped me in many ways, for instance, I now think I will move the engines up to on either side of my seat., because they need to be wider apart than my original design. A jackshaft for each will allow me to pivot the swingarm without taking into account where the front sprocket is, and to bring the two rear drive chains in to space them inline with the rear wheel sprockets. I appreciate your help pointing out flaws in my original design so that I could alter.
 
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OK, I think I've found a way to do away with the differential and one transmission, and tie the two engines together with the primary chains, without putting extra power through either engine. Split the transmissions off from each engine,and store one. Install high strength primary chains from each engine to the outer ends of a jack shaft. put another primary sprocket in the middle of the jack shaft, and run high strength
primary chain from it to the primary shaft of the solitary transmission (might need a transmission from a larger bike.) The jack shaft will keep the two engines in sync, and with only one transmission, the problem of being in two gears is eliminated. Still eight cylinders, but one transmission with five ample gears.
 
Yes, you did. Thanks.
I DO listen.
One CB750 transmission, or one from a more powerful bike?
I don't really want to spend the money for that race transmission.
 
Install high strength primary chains from each engine to the outer ends of a jack shaft.
I probably will only need high strength chain from the jack shaft to the transmission. The other two chains will only carry the original power from each engine, along with keeping the two engines in sync, although, the trike WILL be heavier.
 
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You HAVE to provide some sort of damping between crankshaft and jackshaft. With engine harmonics you WILL break stuff if you run direct from crankshaft to another shaft. No matter how well you have the two separate engines tuned they will fight each other because they will NEVER produce the exact same power. You will have forces in the chains and driveshaft that fight each other and cause increased wear, breakage and power consumption. Is it doable, probably, is it practical not at all. Have fun.
 
Thank you. I can put something like a cush coupling near the middle of the jack shaft. Don't want to much cushioning, because I want the two engines to stay sync'ed as closely as possible.
 
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