The International CBX Owners Association

no probs,
big bore kit,1351cc forged rods hi lift cams, under shim buckets, 33ml round slide race cabs, head ported and polished, drilled to 33ml right threw
bigger inlet and exhaust valves, crank balanced, dynatek coils,
1984 vf1000r front end early cbr1100xx rear end and other bits and peaces..
ps love to have a article done for the mag
cheers steve

Fantastic. Thanks for sharing. Great project!

We should probably split the thread, as I think Steve's work is worth its own headline.

I am curious how you achieved 1351cc ?? I am currently working on a "big bore" conversion and 1351 is right between a 72 and 74mm bore, provide you retain the stroke of the stock crank.
Did you bore the cases to allow for larger liners?
I was faced with this to accommodate a 72mm piston which gives me "only" 1304cc.

Also, how did you mount the original instrument pod?

Curious about your exhaust and how you have it running? Looks to be 6-1? Custom or aftermarket?

bore was taken out to 73.5ml
pipes were made up a local exhaust shop and el cheapo e-bay mufflers

That photo of the stock piston in the new bore is awesome!

Syscrush wrote:That photo of the stock piston in the new bore is awesome!

Agree, that really makes the point of how big the new bore is. Thanks for the photos Stephen.

N.

What kind of fuel do you use?

What's the compression ratio with the new setup? Going from 1047cc to 1351cc is about a 30% bump in displacement - are the combustion chambers the same volume as stock? How about squish?

Syscrush wrote:What kind of fuel do you use?

What's the compression ratio with the new setup? Going from 1047cc to 1351cc is about a 30% bump in displacement - are the combustion chambers the same volume as stock? How about squish?

Questions that run through my mind too, Phil. Looks like the piston has the same shape on top, leaving the combustion volume pretty much the same at around 21cc. Squish would also be a concern as the bore diameter extends quite a bit over the normal head opening. But for the power as stated, compression ratio should indeed be up there. Actually, a 73.5mm bore would result in a 1358, almost 1359cc total displacement and a compression ratio of 11.73:1
I have never seen it on a CBX motor, but I have seen other engines where the studs were pulled out of the cases due to excessive compression.
73.5mm bore is one of the largest I have seen for a street bike using basic stock block and head.

bike runs compression ratio of 10:1 100% octan fuel or higher if I can get it
most of the extra horse power comes from the hi lift cams, they are so aggressive we had to go to under shim buckets the seats were taken out from 28ml to 32ml
to accommodate this, no work was done on engine case to house the larger bore. just big carbs, big bores, big cams, ported balanced and polished.
cheers
regards steve

Unless the top of the piston is shaped significantly different than the stock piston - which doesn't look like it is in the picture - a compression ratio of 10:1 could only be achieved by modifying the combustion chamber and making it larger, i.e.: taking it from 21 to 25 cc. This is not some mysterious issue, it is just plain math. Now, how to accommodate a 73.5mm bore liner into a stock lower case without boring it, that is a mystery to me. I was not able to have that done with a 72mm bore liner.

EMS wrote:Unless the top of the piston is shaped significantly different than the stock piston - which doesn't look like it is in the picture - a compression ratio of 10:1 could only be achieved by modifying the combustion chamber and making it larger, i.e.: taking it from 21 to 25 cc. This is not some mysterious issue, it is just plain math. Now, how to accommodate a 73.5mm bore liner into a stock lower case without boring it, that is a mystery to me. I was not able to have that done with a 72mm bore liner.

What was the issue Mike? Con rods contacting the case?

NobleHops wrote: What was the issue Mike? Con rods contacting the case?

No. That would be a concern if you increase the stroke and the crank throw would move the rods too far out.
The bottoms of the liners reach into the lower cases and when they exceed a certain size, the outside of the liners are too large for the case bore. The stock liners have a 71mm outside diameter. I think a 67.5mm ( 3mm over) bore would be the largest you could do with a stock liner. The wall thickness would be 1.75mm. If you go with this wall thickness of 1.75mm and a 73.5mm bore, the liners would be 77mm, actually the exact same as mine ( I used 2.5mm with a 72mm bore) and that would require a bore of the cases. Unless...the Prolink cases have larger bores than a 79 block.

EMS wrote:Unless the top of the piston is shaped significantly different than the stock piston - which doesn't look like it is in the picture - a compression ratio of 10:1 could only be achieved by modifying the combustion chamber and making it larger, i.e.: taking it from 21 to 25 cc. This is not some mysterious issue, it is just plain math.

Couldn't a spacer or thicker base gasket between the crankcase & cyl accomplish that? I think that's a common mod with the turbo crowd.

bore work