Two Stock CB750 engines, One Transmission?
 

Will the Honda CB750 transmission handle double the power without breaking?

The two engines being phased 90 degrees apart, would be putting out one single power pulse every 90 degrees, rather than one double pulse every 180 degrees as they would if they were phased 0 degrees apart, therefor each pulse would be no stronger than a pulse from a single engine, not twice as strong, as in two engines firing in sync.

The pulses would just be coming twice as often.
Would the transmission see this the same stress as with just one engine, or would the transmission need to be beefed up?

Just a remind: the so-called "universal Japanese motorcycles" usually have an unitary contruction of engine and gearbox, so it would require a lot of re-engineering to match 2 engines to a single gearbox.

Yes, the front engine would have it's gears removed, and be connected by a chain from it's crankshaft to the crankshaft of the rear engine, perhaps between cylinders 2 and 3. (and again, 90 degrees out of phase.)

I've already watched videos featuring small motorcycles with car alternators adapted to them, with a pulley attached to their original magnetos.

I'm not sure how that relates to my original question.

The thing is, if alternating the firings from the two engines can maintain the standard pressure on the transmission, two, or more engines can be safely routed through one standard transmission. (The following illustration shows three engines phased 60 degrees apart.) Can someone show me why this won't work? Prefer actual test data.

The only difficulty I forsee is ensuring you don't end up with 2 X torque for some reason which would eat tranny.

Having said that, the pinto drivetrain survived suprisingly long after we shoved a built 302 into it. Snapped the axles on a launch.

The other question is synchronizing power output like multicarbed british cars.

Yes, the torque. Synchronizing the engines would be by chain driving them together, But you may be right about the torque. Since there is only one firing at a time, the torque should be ok at the input, but with the ability to put the torque of two engines to the rear wheel, might break the output gear. I know I could use a compact car transmission, but I would lose the ability to shift and clutch with one pedal. I would have to use one of my hands to shift.

I wager the second crankshaft is a weak spot, vulnerable to breakage.

I don't know why that would be.

I think I'll go back to parallel power plant/gear sets:
No over-stressed transmission, that way.

I'm thinking of employing some form of lean steering. Would I need to design in something like Ackerman geometry? (see illustration)

I see now that you said "two stock" not "two stroke" ... I was going to point out that they're four stroke engines but glad I didn't now

Everyone says the old Honda bikes' transmissions are indestructible. So far, in my experience, they're correct. But that's on a bike with 20 hp.

Go for it!

I have a '78 cb750 in my garage in a million pieces and waiting to be put back together. One day it will be!

What if the two engines were connected together with a planetary gear box? Might make it a lot smoother. I think that Priuses use such a thing to combine the gas and electric engines together (and it's pretty brilliant as the electric can start the gas engine, and it replace the transmissoin all together some how)

You could find a cheap open-differential rear end from a muscle car or truck. Maybe it could even be the front axle from a 4x4. Except the power would be going backwards - from the axles to the yoke. And the gears are not cut to be operated that way. Is it the same as driving in reverse though? I know that the front axle on my jeep is a "high pinion" as opposed to a low pinion, and gears are cut in reverse for this to be possible. Something to think about maybe

Of course that would add a lot of weight as they're encased in cast iron...

...and it would throw off the gearing.

I have a Gen 3 Prius, and I think the two electric motors are connected to two different elements of the planetary unit. Varying the speeds between the two gives the variable output. Yes, I thought about connecting each CB750 to different half axles of a rear end unit, and allowing each engine to be shifted independently. I came up with 11 different output ratios (see chart) (IF my reasoning is sound) but I really want to keep the engines in sync, 90 degrees apart, for the smoothness and sound.

And here's a neat thing about joining the two at an intermediate shaft with chains. If I synchronize the second engine 45 degrees apart (instead of 90 degrees apart) from each other, the sound will be like a Harley. (if that gets you off,) instead of a 90 degree V8.
Of course, with a redline of 8000 RPM, you will get a harley sound like a single harley engine at 3 times the redline, (16,000 vs 5,400)
and with the engines 90 degrees apart, you will get a CB750 sound at twice redline (16,000 vs 8,000)

I don't see info on application...a lovejoy coupler would work in a side x side setup.

Yeah, here's the diagram on how the two engines would be tied together. All three sprockets on the intermediate shaft are splined. The one clutch pedal operates both clutches, and push buttons send a signal to solenoids on both gearshift levers, left push button for upshifts, right push button for downshifts.

I bet a single clutch would work. Plenty of those motors were drag raced , I'm sure you could find heavy duty springs and better than stock clutch plates. My idea of the lovejoy coupler attaches both cranks with rubber isolation of the coupler. Problem would be crank adapters strong enough to handle to torque from the isolated engine. I would love to build or buy a reverse trike, good luck with the build. With a jack shaft setup you could put one engine in neutral at cruising speeds for better economy.

I wouldn't mess with leaning steer, might make steering heavy.
Using the jack shaft you also have the ability to add another transmission, Harley 4,5 or 6speed. Tune one engine for max effeciency at cruise rpm...

Or: with Engine Cylinder Deactivation Engine-Cylinder Deactivation Saves Fuel | J.D. Power Cars
and Free Valve technology https://www.youtube.com/watch?v=OZWeNPi2XkE I could have the two engines simulate a 500 CC engine for cruising economy:

Here's the diagram:

Of course, the Leave Open valve idea would only work if the CB 750 engine is a
Non-Interference engine. Otherwise the system would have to close the valves as the piston approaches TDC. Still, the valves on the deactivated cylinder could remain fully open for most of the strokes. Even without the valve component, the fuel injection cutoff would save fuel, and be much cheaper and easier to implement.

The main problem, for me, is that by de clutching one engine, I lose sync, but since I'm sync'ing them after the transmissions, I will probably change the sync'ing every time I step on the clutches. Looks like I need to join the two engines before the clutches. SO, your suggestion of the Lovejoy Coupler, joining the actual crankshafts, may be the best way to keep the two engines constantly sync'ed, though I don't know how to gain access to the ends of the two crankshafts, one on the right side of the crankcase, and the other on the left side of it's crankcase. Maybe I can put a shaft between the two Primary Chains:

Gotcha!

?!

My last diagram may not have been clear. This is the firing order for 8 cylinders firing over the course of 6 revolutions.

This has me thinking. Even with a single 750 engine, rotating cylinder deactivation could reduce the effective displacement to 250 cc for cruising on the flat, or nearly flat, and of course the down slope. (This would work on any four cylinder) If you can insert a wedge between the cam lobe, and the rocker arm of deactivated cylinders, you can keep the valves slightly open, and lessen the resistance a fully sealed cylinder would present to pulling a vacuum on down stroke, or compressing a cylinder full of dead air on upstroke. You would have to have a fuel injection system capable of being deactivated cylinder by cylinder, and single cylinder throttle bodies and air cleaners, to prevent back firing.

You have never looked into how cylinder deactivation works. Now is the time.

Yes, I have. I've also looked into the Koenigsegg free valve system, adaptable to any ohv engine, but that's far out of my budget. There are valve systems that have variable lift controlled by extra pieces between the cam and valve stem. I know that GM's V4-6-8 engine left the valves operating normally, and just cut the spark. I know that in GM's system, they didn't rotate the inactive cylinders, and build up of oil and fuel in the cylinders caused problems. I know that Honda has some V6s that can become 3s at low load, and I don't know of any problems with them, but I suspect uneven wear in the "off" cylinders over time. Rotating through each cylinder seems to be the best way to even out wear,and prevent build up.I will be glad to look into any new material you can direct me to.

The valves are held closed. Any flow takes hp.

Where is that data found. I'm willing to read it.

This was worth a read:

Honda Worldwide | Civic Hybrid | 1st Generation Civic Hybrid

So, Honda chose to keep the valves closed, rather than open, the logic being that the air inside will act as a spring, and the energy used in compression would be mostly recovered during expansion. Leaving them open would force the engine to pump air in and out of the cylinder, resulting in lost energy.

So, it's better to deactivate cylinders by closing all valves, rather than leaving them open.

Honda's system is also extremely reliable. You basically never hear about failures with their variable cam system, which has been in most of their production cars since the early 90's.

Thank you, but this refers to deceleration. I'm interested in using reduced cylinder numbers to maintain speed. I will look into the V6 non-hybrid system.

This is more applicable, and yes, Honda closes deactivated valves. I may need to revise my thinking on leaving the valves open. I still prefer my rotating deactivation, giving each cylinder it's firing 1 out of 3 normal times.
Honda Worldwide | Technology Picture Book | VCM

Honda seems to be saying that compression and down stroke balance each other out in deactivated cylinders, but I believe each is a resistance, unless... the valves are deactivated at BDC with a full cylinder of air. In that case, spring back from the fully compressed cylinder might assist in powering compression in the fully expanded cylinder.
It may even help if the deactivated cylinders are closed halfway through the intake stroke.
only half the air to compress or expand.

It should help either way. If the valves are closed at TDC, there will be a vacuum to overcome on the downstroke, but that same vacuum will pull the piston back up with equal force on the upstroke. Conversely, if they're closed at BDC, air pressure will create resistance on the upstroke, but it will push the piston back down on the downstroke. My bet is on vacuum being slightly better, because you'll have less heat lost through the cylinder walls if there's less air, but that might be negligible in the grand scheme.

Is it possible that Honda didn't want the deactivated cylinders draw and blow to interfere with the other cylinders' intake and exhaust? Something that might be avoided by separate intakes for each cylinder?

OK, I want to thank Ecky*, Frank Lee, Piotrsko, cRiPpLe_rOoStEr, mannydantyla, and Grinder74, for helping to guide me through this exercise, and showing me how various parts of it won't work, Synchronizing the two engines without much cutting into the engine cases, and the staggered firing pattern I came up with because of the unbalanced deactivated cylinder pairs (two trying to pull vacuum at the same time). Seems the only promising idea is the combined clutch/shift pedal, and I'm not sure there isn't a better layout for that. Anyway, I'm abandoning this thread. Thanks guys.