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

No more than a standard chain driven CB 750,using a double row chain, except when in 1/2, 2/3, 3/4, or 4/5. The advantage of close ratio should be worth it.
Chart.JPG
 
Not going to comment on the chart as there are several things wrong with it. Your posting it shows you do not see them.

I can see you will never understand what close ratio does to you until you drive one. I have. The only reason anybody ever uses them is to get gears spaced to narrow powerbands, you have misapplied that to wide and a massive waste doing so. The trans matches the engine NOT the driver.

And pointing at the larger issue, you try to mod by complication only to keep your idea intact, any good engineer knows to throw away bad ideas, you just cast more concrete around them.

Yours and do as you will. I hope you don't ever waste money on it. You have lost track of (indeed not caring about it at all) the most important thing any driver of a new device is most interested in....driveability. I'll bet the thing shakes like Beyonce when done too, you will create a massive end to end rocking couple there. It should break parts.

I say that having put a GM 454 in a mid-engine Volkswagen (good idea) and a GM 350 in an expensive Pininfarina Italian sports car (very BAD idea). The latter would run at 180 mph on aircraft runways but was NOT a good project in overall results. I called the idea a fail but the guy who owned it didn't know enough to know any better.

You need to find a wrecker Yamaha V-Max and use the engine from it, the engine sound you want and enough beef you will never break it and enough power to go to the moon.
 
Not going to comment on the chart as there are several things wrong with it. Your posting it shows you do not see them.
It's an RPM to Speed chart, not a horsepower or torque chart. All I added was the 1/2, 2/3, 3/4 and 4/5 ratios in red, and the yellow shift lines. The rest I pulled off the internet. What do you believe is wrong with it?
 
While I love the CBX, the midrange is non-existent in one. All rpm but nothing else. They overheat easy as spit too. Drove one for a while. It either is at lightspeed or way less. Too heavy, too complicated, it breaks very easy too. I've ridden all brands and there is no brand loyalty to trip me up. The V-Max engine is easily 2X stronger and so is a Suzuki 1100. Either one will easily hit 80 ft. lbs. torque when built to a purpose, the CBX comes nowhere close. The big Kawasaki is a rock solid engine too.

The chart you have there has low gear as getting to 54 mph at 9500 rpm and no way. The fifth gear is wildly optimistic too, looking at close to 150 mph there, the engine will not go any faster than maybe 120 and that downhill.

I raced AMC cars for many years, and one of the biggest takeaways I got from that was that you can take a high torque low hp. engine to make big horsepower, if the engine is weak on torque (CBX) it is NOT a good candidate for big horsepower; it will be self limiting. The same rule applies to double engine anything, they often increase complexity with no equal payback in useable power. The friction losses doubled kill you. How we took lowly AMC 315 hp. 390 inch motors to usually stomp the crap out of 450 hp. 440 Chrysler and even 426 true Hemi cars. Those big Mopars waste 100 hp. simply keeping the engines turning; the internal parts are super heavy. I built those types too and saw it. Wasted energy. You complicate when forced, you don't go looking for it as you can always make things worse thinking you are helping. Listen to 'Boss' Kettering of GM back in the day...........'parts left out cause no problems and cost nothing'. He knew that the more complex you make a thing the more trouble you have with it.

Complexity just for itself is a mistake. Look at the latest Nissan VC variable compression engine that is also turbocharged. WHY did some idiot put the equivalent of two crankshafts and rod sets in an engine that could have left all of that off to simply run the boost up a bit more??? It was supposed to increase fuel mileage but guess what, it didn't. BMW beat them using a standard layout 4 simply paying attention to details. The extra parts ate Nissan alive in wasted friction. Like the CBX only worse.
 
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Look at chart and pick say low gear normal driving to upshift at 4000 rpm, a normal sedate rpm rollout number. Go directly right like a black line does there, you intersect your low rpm yellow line of your first 'half-gear' at around 3300 rpm. Do the same for 2nd gear, or closer to 3400-3500 rpm.

Those are UNUSEABLE power bands there, they are only 700 wide MAX. And so on further up the scale. You WANT the engine to drop more, it is easily flexible enough to do so at those speeds and you are throwing the engine and all its' power away doing that. You need at least 1500 rpm of good useable power in a single gear power band and most writers think 3000 on simple every day slow normal driving and 4000 if they can get it. Or they consider that the machine will become a solid chore to have to drive and I agree, having driven a few messed up bikes that had no powerband at all. They SUCK. And you WANT to go there.

This is what I am talking about when I say you refuse to contemplate good sense. Well, maybe I haven't, but I am now, the chart makes it clear. You want to shift every second of every day and all day for the rest of your life on this device. Shifting at every 700 rpm? Incredible. Even the race 125 engines I used to build at 11000 rpm and 25 hp. never needed that, the powerbands dropped from 3000 wide to maybe 1500. And you never stopped shifting those either, you want to double that work here. And it IS work once you start doing it that much, even if only pushing a button.

You CAN'T make me understand that, to do so would mean I have to think like you and I don't. It makes no sense and I don't go to places like that. One of the highest things considered in any new machine is how easily driveable it is and width of powerband figures in there hugely, doing anything to shorten it is ALWAYS considered bad, read those magazines you took that chart from, they will tell you in a second. All through the '70s some of the highest accolades were over any machine that could use the HIGHEST gear to begin with at a dead stop with only a small amount of clutch and it would pull off from a start to go with no trouble at all. They raved about that trait if a machine had it. Why hotrodding a machine to shorten the powerband always then takes more from the driver to use it. You want to do that more than anybody I have ever run into.

Again, no ill will or intention toward you, yours and do as you will.
 
Normal driving would be 1/1, 2/2, 3/3, 4/4, 5/5, both paddle gears together. No activation of the spider gears. The differential is there to join the two transmissions safely. The in between ratios would only be useful for full-on interstate entry. In matched gears, the differential acts just like two engines pulling one rear wheel. through sprockets on either side.
This whole thing started with my wish to join two CB 750 Fours end to end.Then I learned that I could not run them through the same transmission, and to join them on a half shaft would be dangerous, if one didn't shift with the other, so, the differential. From there, I realized that one transmission in a different gear than the other would yield a new set of ratios. That's not the main objective, but a side benefit. The main objective is to join two engine/ transmissions safely to one wheel.
 
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I asked because it may influence thinking here. You've never driven a big inch bike which will rocket you past any traffic while merging so fast it will take your breath away. Using multiple redundant trans ratios there is going backwards, I'm telling you. You never need full throttle while doing it, or more than 2 gears. The shifting is just getting in your way.

And more refusal to look at it. It's a real issue with you to the tune of going backwards. The double trans is NOT a side benefit, rather a distraction at the least and downright dangerous if the circumstances of the day lend toward it.

You don't grasp that using a 700 rpm wide powerband means you shift as soon as you shift, you never stop or let the rpm pullout, it already has.
 
I do agree that in a panic situation, trying to remember what button to push is going to end in disaster. You have to think you would be causing a hazard to others on the road if you make matters worse by pushing a wrong button. I am all for ingenuity and unique engineering but there comes a point when ideas and thoughts about being unique and different get in the way of rational and safe ways of doing things. Some things are better left on the drawing board, this is one of them. 6 pages and for an idea that will never come to fruition. I applaud your desire but the complexity is going to kill any actual build completion.
 
OK, so let's forget the extra ratios. I'll shift both together, and stay in matching gears. The main purpose of the differential is to provide a fail safe in the case of mis matched shifting.
 
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Then you have mis-engineered the whole thing. You run one engine only and a trans to match the power, or two engines and one trans adapted to both. It's already enough trouble trying to output two engines as one, you made it quadruple worse by wanting to double transmissions.

I don't see why you are so stuck on twin engines, you can get more useable power using one bigger one in today's world of massive power.
 
I want eight cylinders, and I'd LOVE to use one transmission, but it won't handle double the power. I'm really after those answers, and they have been answered.
 
Like I said, you know how to prove it. You WILL end up more enlightened than you are now. What looks good on paper is often garbage. It CAN work and be the biggest failure on the planet.

'I'd LOVE to use one transmission, but it won't handle double the power.'

Wrong. The stock DOHC 750 rates at 75 hp., modified 1200+ cc. versions of the same engine can handle 150+ hp., there is one guy pushing 200 now. Same basic trans.
 
Post#115
I'd LOVE to use one transmission, but it won't handle double the power. I'm really after those answers, and they have been answered.

Post#116
Wrong. The stock DOHC 750 rates at 75 hp., modified 1200+ cc. versions of the same engine can handle 150+ hp., there is one guy pushing 200 now. Same basic trans.

I'm sorry. When I opened this thread with the question, I took the following to say that the transmission would NOT handle the extra power. Have I misinterpreted?

Post #1
Will the transmission need beefing up?

Post#5
The stock primary chains are your weak point. They last long time in stock form but wear quickly with increased power, why there are heavy duty aftermarket primary chains, even then the chains are still the weak point.

Post #6
Isn't the same transmission used with the CB 900 and CB 1100? Do builders have the breakage problems when they install oversized cylinders? I'm new to this.

Post#9
Yes breakage problems are still an issue...especially with big bores and racing/hard riding applications. The crank and cases can take large boosts in power but the transmission and primary chains are the weak points. Mostly the primary chains. They will run for a while with large power but its a ticking time bomb.
 
The sohc chain wont last long the dohc is stronger. This thread is like beating a dead horse, same thing every day. I can only see one feasible way to do this....mate your two engines together with what ever coupler/configuration you determine you like then run it through a heavy duty baker transmission for a HD/v twin. Similar things have been done in the 60's 70's to run multiple engines in drag racing. Again its drag racing, high hp, high loads for short amounts of time.
 
The DOHC primary chain has issues at over 160 hp.

I agree with the last answer, you are flogging dead horse and with a one track mind. Part of the problem is you keep jumping from SOHC to DOHC but I may be part of that. Or, your clash in post 6 and 9 above. The engines are totally different.

Build it and find out.
 
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