XBR engine transplant into CB250RS

[Steve H]

Is your point that there is little point is displaying a rear wheel torque value as this will be entirely dependent upon gearing. In which I would agree that the actually values are not that useful on there own but as a side by side comparison between runs they do have a value and the shape of the torque curve gives an indication of volumetric efficiency

[Propellor]

Is your point that there is little point is displaying a rear wheel torque value as this will be entirely dependent upon gearing. In which I would agree that the actually values are not that useful on there own but as a side by side comparison between runs they do have a value and the shape of the torque curve gives an indication of volumetric efficiency

Mainly I'm probing for explanations. At this point I think I understand what is happening a lot better and I don't think it changes anything I've said at all. It substantiates what I've been saying, I believe. The torque curve relating to rear wheel power, but expressed at engine speed is the issue, regarding a cause of confusion. It's abstract. It never happened. It's not rear wheel torque, but people frequently say it is. I assume that the "crankshaft torque as measured at the rear wheel" value is shown as a check on the value of the losses? Or why else is it shown? People seem to frequently misuse it.

As regards using the dyno to check on gains from modifications, then yes, we are interested in what is happening at the crankshaft. But people seem to treat the crankshaft figures with disdain. As if it doesn't really count.

But if we want to compare different machines then surely actual rear wheel torque values are the most relevant? Initially I picked up on something said by tevie54 but I also made a comment regarding an engine with half the cc but the same power value. In that context actual rear wheel values are very relevant indeed.

I'm grateful for your reply. It's appreciated. I apologise for my persistence, but i don't give in too easily! At the same time, I'm ready to acknowledge my mistakes, if I'm proved to have made them and most important, to learn from the experience.

[Moto63]

Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100  post n counting now eh

[SteveC#222]

Jeez guys all this technical stuff is making my head hurt!

Getting back to the original question, I think the answer is Yes, you probably can fit an XBR engine in a 250RS and if you did it will probably be not that much faster but may well accelerate like a scalded cat!  However it'll probably be a bit of a bugger to do so unless your RS engine is knackered or you relish an extensive winter project and are mechanically proficient you may well be better off sticking with what you have!.....I think ........going to lie down now.....

[Propellor]

Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100  post n counting now eh

Full member?

Er indoors is nodding.

Chortle.


Edit:   Hee Hee. I managed to get a bit of smut past the filter. Through the back door so to speak. Oops, there went another one. Oo er missus.

[Propellor]

Jeez guys all this technical stuff is making my head hurt!

Getting back to the original question, I think the answer is Yes, you probably can fit an XBR engine in a 250RS and if you did it will probably be not that much faster but may well accelerate like a scalded cat!  However it'll probably be a bit of a bugger to do so unless your RS engine is knackered or you relish an extensive winter project and are mechanically proficient you may well be better off sticking with what you have!.....I think ........going to lie down now.....

To be fair, I did start out my "quest" saying it was confusing. And I think I proved it is! Ha ha. I apologise if I caused any innocent bystanders to vomit.

Look on the bright side, it got me out of the short trousers category, as observed by moto63, who has some serious posting to do if he's to catch me up.

The thing which would concern me about dropping a 500 in there, especially an xbr, is that the RS frame is so lightweight. It is VERY skimpy. To say nothing of the wheel rim width, yoke proportions, fork proportions, swingarm. It looks fine for maybe 30 hp, but I'm not sure I'd want to put more through it. Not without mods. Xbr size rims. Xbr size yokes. Xbr size forks. Add material to the frame..... Doh!         

Then again, that's trying to apply logic. Sometimes you just says bo ll ocks I'm going for it. I'm not unfamiliar with that approach either.

[Moto63]

I,ll say. I,ve seen your Beemer  oo er more tea vicar

[Steve H]

The RS chassis is fine with a 500 engine. What you end up with is a 250 with a 500 engine and the corresponding advantages and compromises that it brings. I loved mine and only sold because my wife was sick of me spending so much time on it. The Dominator I replaced it with was dull by comparison.

[Steve H]

Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100  post n counting now eh

99 of which were on this thread!

[Moto63]

Don,t worry I,ll just keep posting back nice easy meaningless replies until I get my "FULL MEMBER" or shud that av read till I become a full member either way i,ll some more tea vicar thank you, oh and I,ll catch that propellor you just wait n see.

[Propellor]

Bloody hell propellor bet you,ve only persisted with it so you kud become a "FULL MEMBER" 100  post n counting now eh

99 of which were on this thread!

Party pooper.  Ha ha.

[Propellor]

The RS chassis is fine with a 500 engine. What you end up with is a 250 with a 500 engine and the corresponding advantages and compromises that it brings. I loved mine and only sold because my wife was sick of me spending so much time on it. The Dominator I replaced it with was dull by comparison.

Yes I realize it's been done. Just that I had "concerns" over the light duty components. But I duly note your comments for future reference. I wouldn't put it past myself to do something like that. Besides, if someone told me it couldn't, or even better, shouldn't be done, I'd do it just to spite em. Hee hee.

[Propellor]

Sounds very low for rear wheel torque?

I'll offer an apology as the fault may possibly lie with me, in misinterpreting what you meant. I can see that your line above is not necessarily an explicit reference to transmitted rear wheel torque. I now assume that you meant "my effective crankshaft torque measures 31ftlb at the rear wheel."

In my defence, I believe it is open to misinterpretation making reference to the rear wheel when the figure refers to the crankshaft. The only relevance to the rear wheel, as far as I can see, is that it is the place where the measurement (of crankshaft torque) took place. I told you it was confusing!

[tevie54]

Blimey, I wish my bike put out as 'torque' as you Propellor

[tevie54]

I converted my RS250 to a 500 about four years ago and I'm still doing little bits to it to improve it even now, Ive had all sorts of bikes over the years Fireblades, NC30 etc but the RS gives me the most fun. It has CBX 250 wheels a Mikuni 40mm flatslide home built exhaust plus various detail changes to frame , footrests, handlebars etc.
Power wise It'll do 100mph and still lift the front wheel in third so realistically I don't think it needs any more 'tuning'.
I might change the carb for a 34mm roundslide to give it a bit more of a progressive power delivery as its a bit abrupt in the lower gears.
I need earplugs on a run as the 'blat blat blat' starts to get tiring although I have been to France on it twice. I think Propellor is right about the frame though, it could do with a bit of extra bracing. Over the winter I'm taking it off the road to do some cosmetic restoration and a few mechanical jobs so I'll be using my XL250 for work were I'm building a 305cc version of the XL/RS engine.
Electrics are swapped from the RS, airbox has to go though, use the camshaft from the RS as this has the tacho drive in it. Not sure whether to get '500RS' stickers made for the sidepanels or leave the 250 ones just to surprise people at the lights.