cb750 dohc big bore help

the rod stroke ratio with cb750 rods is 1.66 and the cb1100 rods is 1.64, i think the thing im going to have to be most concerned with is pistons speeds.
 
Yes, Honda race motors were based on the 750 rather than 900 because they needed the short stroke for revs. And the 900 tends to throw a leg out of bed if revved hard.

If you have the time and inclination to build a short stroke 1000, that would be a fun project. It won't have the lazy torque of a 900 or 1100, and should feel more like a modern bike. Couple it to GSXR BS34 CV flatslides carbs for civilized manners and great responsiveness. If you are intent on going hybrid, block height shouldn't be an issue. You can either add a spacer or deck the block to make it work with whatever pistons and rods you end up using.
 
and pj, this is basically for shits and giggles, the bike its going into is going to be hardtailed and just used for around town lol.
 
teazer said:
Yes, Honda race motors were based on the 750 rather than 900 because they needed the short stroke for revs. And the 900 tends to throw a leg out of bed if revved hard.

If you have the time and inclination to build a short stroke 1000, that would be a fun project. It won't have the lazy torque of a 900 or 1100, and should feel more like a modern bike. Couple it to GSXR BS34 CV flatslides carbs for civilized manners and great responsiveness. If you are intent on going hybrid, block height shouldn't be an issue. You can either add a spacer or deck the block to make it work with whatever pistons and rods you end up using.
Click to expand...

you guys are slowly making me think of going with a short stroke 1000.
 
Ahh, I meant to mention the 900 rod 'feature' ;)
They do tend to escape when revved hard.
May be able to find a set of Carrillo at reasonable cost?
 
I find that "broken" is such a harsh word. "Added crankcase ventilation" is so much less alarming. Also known as "electrical trouble" because the electrics don't work with one leg hanging out of bed. It's the oldest excuse in the book.

It all comes down to how hard you plan on thrashing it. Redlining a 900 will break rods, but there are solutions.

Do you have a copy of the project bike that Kaz Yoshima of Ontario Moto Tech built a few years ago? It was written by Ken Vreeke and titled CB900F Project - published by Motorcyclist July 1982. They got 122hp out of that one.
 
crazypj said:
Ahh, I meant to mention the 900 rod 'feature' ;)
They do tend to escape when revved hard.
May be able to find a set of Carrillo at reasonable cost?
Click to expand...

i was planning on using cb1100f connecting rods for this reason (assuming the big end is 38mm like the 900)

teazer said:
I find that "broken" is such a harsh word. "Added crankcase ventilation" is so much less alarming. Also known as "electrical trouble" because the electrics don't work with one leg hanging out of bed. It's the oldest excuse in the book.

It all comes down to how hard you plan on thrashing it. Redlining a 900 will break rods, but there are solutions.

Do you have a copy of the project bike that Kaz Yoshima of Ontario Moto Tech built a few years ago? It was written by Ken Vreeke and titled CB900F Project - published by Motorcyclist July 1982. They got 122hp out of that one.
Click to expand...

no i do not, would you happen to have a copy of it? like i said im really just looking for a fun summer project.
 
It's not just the piston speed, even a few extra degrees of con rod angularity has a huge difference on piston to cylinder side loading, particularly as rpm goes up
It is pretty well covered online, I got into this discussion several years ago on XS650 site (I 'invented' a long rod conversion using Honda CR500 rods, Bob Bertaut posted the same thing around the same time, weird huh?)
US (447) bikes had a 10mm shorter rod (130mm) to improve off the line torque
"Euro' bikes had 140mm rod, CR500, 144mm
Cam timing varies with piston position and pressure difference, that's part of the reason the numbers are so much different for 750 and 900, the 750 has longer rod and shorter stroke
None of this stuff works in isolation, you have a total package.
Get a 125~200 single and play with that first (much lighter to get in and out of frame ;) )
I wouldn't worry about numbers too much though, just build the damn thing and do the math later to find out what you did that worked ;D
 
According to my calculations that I found in an old spreadsheet from when I had one of each (750+900), 750 hits 4000 ft/sec at around 10k and Cb900 gets there at 9000 and hits 4500 at 10k and that's close to meltdown point.

According to my old notes, 750 rod as 114.6mm ctr to ctr and 900 was 117.3, a difference of 2.7mm CB1100R HRC rods were 113mm according to an article in AMCN.

I had written at the time that big bore pistons fit either motor, but I never did check to see if that was correct.
 
Interesting.
almost 3mm longer rod with low crown pistons and spacer under the block if needed could be pretty interesting
 
Of course it's possible that I wrote that down all wrong those many years ago. I'll have a replacement 900 rod soon for a buddies crank build, so I'll double check it.
 
crazypj said:
Interesting.
almost 3mm longer rod with low crown pistons and spacer under the block if needed could be pretty interesting
Click to expand...

hmm, that would be pretty interesting. looking about about 1.8-1.9 r/s ratio
 
Yep. More fun than a barrel full of monkeys.

BTW I had a chance to measure a 900 rod this evening and it was 117.3mm.
 
Something else to add to my 'useful information, don't know when it will be needed list' ;)
38mm big end, what size small end? (15 or 16mm?)
 
Big end journals 36mm, Big end eye in conrod 39mm, Pin 15mm and that's all I measured. If you think of anything else, you're on your own. :)
 
Swivel said:
I did a CB750F with a 890cc Wiseco kit that was bored and sleeved to 67.5mm with CR's and a Yoshi pipe.It was far better than a CB900 both in RWHP and it revved quicker that a CBX-6.I was like a grenade in power delivery.It was very responsive.It was just like a later GSXR in revs and feel.It had underbucket shims and better valve springs.One of my favorite engine builds.
Click to expand...

I just bought this exact same 890cc kit from Dynoman.

What size CR's did you use. I went with the CR31's for street use.
 
teazer said:
Big end journals 36mm, Big end eye in conrod 39mm, Pin 15mm and that's all I measured. If you think of anything else, you're on your own. :)
Click to expand...

That's all I need, everything can get 'modified' if needed ;D
 
Swivel said:
Sorry I can't remember the size,The guy just had a clean set of CR's I fitted for him.They really need the underbucket shims and better valve springs to get the rev benefit.Honda fitted too soft valve springs to all DOHC 750's,900's and the CBX -6.Post a picture of your motor sometime.
Click to expand...

It may be a little while before I can have pictures everything still sitting in the boxes including the upgraded valve springs you mentioned.

Sorry don't want to highjack this thread. I am working on building a new garage.

I have a guy that used to drag race bikes and builds AMA race engines will be building the engine for me. He is currently moving to a new location but will hopefully get started this month.

Thanks
 
Found some cam timing specs @1mm lift
750
I.O. 5 deg btdc
I.C. 35 deg abdc
E.O 35 deg bbdc
E.C 5 deg atdc

900
I.O. 10 deg btdc
I.C. 35 deg atdc
E.O. 40 deg bbdc
E.C. 5 deg atdc

Seems it's worth fitting 900 intake cam with a 750 exhaust cam on a short stroke/long rod motor, piston dwell changes more at BDC with longer rod
 
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