Topic: CB750 Automatic crankshaft

[MotoDave]

Hello all,
I just picked up a used CB750A motor because I read a while back that the primary drive in the Hondmatics was stronger (and possibly quieter?)
Instead of using two standard roller chains side-by-side the 750A uses a single wide "Hyvo" chain.  Has anyone heard that the Hyvo primary is stronger?
I saw a picture of a dissembled CR750 race motor and it indeed used a Hyvo primary.  If this is the case it would be a good candidate for lightening and balancing

[Bodi]

The HyVo chain is definitely stronger than the "standard" setup: looking at it you will see about 12 plates alternately spaced along the length. The stock setup has 4 plates, albeit thicker than the HyVo ones.
The Honda HyVo drives I've seen use a (roughly) 1:1 sprocket ratio and a jackshaft with the primary ratio in a gear drive to the clutch, the 750 has a large driven sprocket which gives the primary ratio. The wrap on the crank sprocket will be under 180 degrees, that might be a problem for a HyVo - I don't know the minimum wrap for high torque applications to avoid climbing but the HyVo chain and sprocket tooth profile is totally different from a roller chain. Honda may have had to use the 1:1 sprockets and jackshaft to ensure a full 180 degree wrap. You will have serious problems getting a driven sprocket. You will probably have to have a custom length chain made.
There are plenty of 100+HP "750" engines out there not snapping primary chains... is it worth the grief of trying to use the 750A HyVo setup?
It isn't likely to be quieter either, the HyVo has a distictive whirring sound in operation.

[MotoDave]

I knew someone out there was going to know about this.
Check out the attached pic.  It shows what looks like a Hyvo chain used in a 90+ HP CB750 motor made by M3Racing.
The primary drive ratio being different could be a big deal.  I haven't fully disassembled the motor yet.  I'll let you know what I find.

[Bodi]

Wow, nice picture. It looks like M3 have made a HyVo primary sprocket, and the but of braided line must be a chain oiler to function like the nozzle and control orifice you'll find in the 750A. If they'll sell you the sprocket you'd be good to go, just make some sort of oiler. The 750A primary shaft is a different animal from the gearbox engine, I doubt if you can use it. The primary ratio is a bit higher if I remember correctly (with a smaller driven sprocket than the 750-4) and it's a fairly big piece with a gear machined on it as well as the sprocket.

[bryanj]

The 500/550/650 uses Hy-vo and dont have a 1-1 ratio so dont see a problem