Long Throw Crankshaft Is It Possible?

Hi, has anyone increased the capacity of their vfr? and I don't mean by the tiny amount that the big bore kits offer. I was wondering if the crankshaft from another honda v4 could be fitted, with oversized pistons of a couple of mm to create an engine with a bigger capacity. I was looking at a VF1100 crank earlier, but it looks slightly different, although the stroke would be just right. With a spacer for the cylinder casing and custom cylinders it would make for a pretty cool engine.

 

maybe the st1300? interestingly they have the same size throttle bodies, theyre both 90 degree L fours... maybe the same exact throttle bodies? i wonder how similar the blocks are. considering they face in different directions, (front to back vs side to side), use completely different transmissions, and bolt to completely different frames. who knows?

 

st1300 78x66
st1100 73x64.8
vfr800 72x48
vfr750 70x48.6

A vfr800/750 hybrid might yield a few cubes lol
or you could add a fifth cylinder to get a 936cc... piece of cake right lol

 

A crankshaft from either the st1100 or st1300 would be ace, but they way the engine is laid out is completely different to the vfr, so the crankshaft is surely to be completely different in terms of the shape of the ends of it etc. I was wondering whether the crank from something like a vf1000 or vf1000r or vf1100 could be used? They have the same layout and use similar bores for the pistons, but I'm not sure if any of them would actually fit or work with the vfr engines. If it would work it would most likely have to be done to a 750 not an 800, as from the look of this pictures I can find, (which are few and far between) the 800's crank looks like it's a completely new design to the 750's which looks a lot closer to the vf1000 and vf1100 crankshafts.

 

If you increase the height of the cylinder head with regard to the crankshaft centerline, the cam drive can become an issue. Gear drive would be a complete pain, and chain drive would require indexable cam gears to compensate for the link spacing difference.

The biggest problem with stroking a high rpm engine and maintaining the same dimensional relationships is the connecting rod length to stroke ratio. Shortening the connecting rods is a really bad idea, as the side loading on the pistons is increased, as well as the peak piston velocity. All other factors being equal, a short connecting rod is mechanically more violent then a long connecting rod. That is not too much of a concern in a low rpm engine, but a high rpm engine will find its weakest link at some point.

The only way to avoid shortening the rods are to have custom pistons made with a higher wrist pin placement, offset by the same amount as the stroke increase. Expensive and problematic.

As usual, anything can be accomplished if you throw enough money at it.

 

The 800 crank won't work; the drive pulleys go on the end, whereas the 750 has them at the middle. I doubt a 1000 crank would work either; you'd need to look at bore diameter in this scenario. Seems to me the 1000 crank would be longer due to the bigger piston diameter.

You could offset-grind the 750 crank, which would give you smaller journal diameter but a bigger stroke. Then you'd have to find rods with the smaller 'big end' diameter and the same length as the 750. You'd then have to modify the cylinders and heads to allow for the increased stroke. The same thing applies if you have an 800.

They've been doing something similar with Chevy small-blocks for about the last 25 years: a 400 crank in a 350 block, gives you a displacement of around 383 cubic inches. Not without some work, mind you.

 

If you use the crank from a V-Max, connecting rods from a Honda S-2000, and the cylinder heads from an RZ-500, that might work.... What were we talking about?:crazy:

 

There used to be a big bore kit available from a company called DYNAMOHUMM. If this company still exists, I can't find anything about them.

KC-10 FE out...
lane: :usa2: