XBR engine transplant into CB250RS

[xbally]

Does anyone know how easy or difficult this conversion is and what are the potential difficulties such as ignition,electrics carb,airbox,exhaust and chain alignment etc?

[tevie54]

The XBR engine will fit but as its a dry sump engine it needs quite a bit of work whereas the XL or FT500 fits a lot easier.
Check out youtube Honda CB250/500RS

[guest564]

Will the XBR engine fit? its a fair bit bigger than the XL engine and that was a tight squeeze. There used to be a lot of XL/FT engines in RS chassis, I'm sure if the XBR engine fitted it would have been done then.

[Propellor]

I wondered if the extra torque from the bigger motor might overwhelm the RS frame and swingarm?

I suppose, at the end of the day, it is gearbox output to rear wheel torque that affects the frame?  Perhaps the rear wheel torque from an xl500/RS hybrid might not be so much different from a 305cc tuned RS motor. With appropriate gearing.

Torque induced chain tension is presumably the thing which might affect the chassis? Aside from inputs from the road/cornering, which would increase with road speed?

I'm interested in making my RS a little perkier, but wouldn't really want to go much above 305cc. It would loose the essence of what (I think) I'm after.

How does a 305cc RS stack up against an xl500 horsepower wise? How much heavier is the xl500 motor?

Cheers.

[Steve H]

The 500RS I built handled much the same as the 250, the main change was that  I no longer needed to wring its neck to make good progress. The 500RS is more about mid-range and less gear changing which suits my riding style.

[guest564]

The RS chassis handles the extra power without any real issues. It seems from the other posts that the XBR motor does fit, I would guess that it wasn't a combination that was used in racing because the XBR chassis was good enough and many people were moving to custom built chassis at the time.

[tevie54]

I agree, my 500RS is so much more torquey than the 250 although on paper the 250 produces much the same BHP, about 27 albeit at the crank on the 250 and at the wheel on the 500, the torque is double, mine puts out 31 ftlbs at the rear wheel. It changes the bike into a bit of a mad wheelie bike. It does vibrate a bit more as the mass of the frame is lighter and so doesn't absorb the vibrations as well.
Ive seen a video of an XBR engine in an RS frame but its a bit of a lash up. the engine mountings are different and it needs a place for the oil tank to go, exhausts are always fabricated units as well.
I also have a 305cc engine that i'm building for my XL250S, if anyone's got any experience of these big bore RS/XL engines let me know.   

[Propellor]

Sounds very low for rear wheel torque?

[tevie54]

Well according to the Honda manual, the torque figure of the XL500 at the crank should be 30ftlbs; my RS500 put out 31 at the back wheel on a dyno but i have done a little bit of tuning so that would account for the increase. Standard 250RS's put out roughly 15ftlbs of torque at the crank so therefore that would be less by the time it got to the back wheel.
To put it into perspective Yamaha 350LCs put out 30ftlbs of torque and 47 BHP, both figures taken at the crank, a mildly tuned CB500RS should put out more torque but traditionally big singles produce less BHP than multi cylinder bikes because of the lower rev ceiling.

[Propellor]

Well according to the Honda manual, the torque figure of the XL500 at the crank should be 30ftlbs; my RS500 put out 31 at the back wheel on a dyno but i have done a little bit of tuning so that would account for the increase. Standard 250RS's put out roughly 15ftlbs of torque at the crank so therefore that would be less by the time it got to the back wheel.
To put it into perspective Yamaha 350LCs put out 30ftlbs of torque and 47 BHP, both figures taken at the crank, a mildly tuned CB500RS should put out more torque but traditionally big singles produce less BHP than multi cylinder bikes because of the lower rev ceiling.

Yes, sorry I misunderstood what you meant. With respect though, I'm still not sure the figure you mention is the torque at the rear wheel. This is a confusing aspect of what dynamometers actually measure. Again with respect, I don't believe your figure of 31ft lbs probably actually exists. I believe I can explain why I think that and what I mean by that in more detail if you want me to. But consider this:-

Unless the overall transmission ratio is 1:1 or lower, then torque will increase as measured at the rear wheel compared to crank. Each reduction ratio multiplies the torque, minus a small loss at each stage. So, in most cases, the torque at the rear wheel will have been multiplied a significant amount by the time it reaches the rear wheel. That is the whole point of reduction gears (The reduction refers to the speed).

Power is a constant, minus the losses at each stage of the transmission (most sources I've seen reckon on an overall loss of 10 to 12%). So it is entirely conceivable that the two engines in question, putting out pretty much the same horsepower, will have the potential to achieve the same top speed and produce the same rear wheel torque.

Am I right in saying that the link between power, torque and rpm is directly proportional? If so, then an engine producing the same power as another, but with half the torque, will have to rev twice as fast. But that is very little to do with rear wheel torque. In those two cases you'll find that the overall transmission ratio(s) are vastly different (or should be!).

Please don't take offence. I'm not out to offend, just to discuss and of course put forward my own thoughts.

Cheers.

Anyone agree?

[tevie54]

Well, as a general rule more cc equals more torque. Torque is a measurement of turning force, a dynomometer actually measures torque first then converts it to BHP, because bikes produce their power in a turning action whether at the end of a crank or at the back wheel the dyno is the only way it can be measured, once the highest torque or turning force is measured it multiplied by the RPM then divided by 5252 to get the horsepower at that RPM level. As my dad says 'torque gets you there, horsepower keeps you there'
The shove in the back you feel when you accelerate is torque not horspower

[Propellor]

Quoting tevie54.

Quote:  "Well, as a general rule more cc equals more torque."

As a general rule, you'd have to say yes. Engine torque, that is. And probably, but by no means certainly, rear wheel torque too.

Quote:   "Torque is a measurement of turning force,"

Agreed. Force x radius.

Quote:   "a dynomometer actually measures torque first then converts it to BHP, because bikes produce their power in a turning action whether at the end of a crank or at the back wheel the dyno is the only way it can be measured, once the highest torque or turning force is measured it multiplied by the RPM then divided by 5252 to get the horsepower at that RPM level"

I need to be a little careful because there are a variety of different types of dynamometer. Though we are almost certain to be concerned with the rolling road type. The inertial rolling road type? If so, then doesn't this measure the rate of acceleration of a mass and express the amount of power involved? If I'm right there, then this will give a readout of rear wheel power.

But here's where I think the confusion sets in. A lot of graphs also show a torque curve based on the rear wheel power curve, but expressed at engine rpm. This is what I think is your 31ft lbs. I don't think this is rear wheel torque though. In fact, is it ANY actual torque? Isn't it engine crankshaft torque, but taking account of transmission losses which have not yet occurred? Now that is confusing. Which is what I said at the beginning. Assuming I'm right, that is.

[guest564]

A dyno can only measure torque at the rear wheel or gearbox sprocket if its an engine dyno. A revcounter will be attached to the bike so the measured torque can be corrected to the equivalent of 1:1 gearing. The power and torque graphs show the corrected torque figure at the rear wheel and the rear wheel bhp calculated from the corrected torque curve.

[Propellor]

A dyno can only measure torque at the rear wheel or gearbox sprocket if its an engine dyno. A revcounter will be attached to the bike so the measured torque can be corrected to the equivalent of 1:1 gearing. The power and torque graphs show the corrected torque figure at the rear wheel and the rear wheel bhp calculated from the corrected torque curve.

Well, whatever else I said, I did say it was confusing!

At the end of the day, there will be as many rear wheel torque curves for the bike as there are gearbox ratios. And for the lower gears the rear wheel torque figure will be very much higher than that at the crankshaft. But there will be only one power curve for the rear wheel regardless of the gear selected. This curve will be around 10 to 12 percent less than the same power curve at the crankshaft.

Going back to tevie54's 31ft lb figure. If this is a rear wheel torque figure, which gear was the bike in? What was the torque figure at the crankshaft at the same time? What rpm was the rear wheel doing at this time? Using the formula described by tevie54 we should be able to reconcile (or not) all the above.

[tevie54]

Of course the other thing to consider is the power to weight ratio, an XBR engined RS will feel quicker than a standard XBR because the RS is lighter so acceleration will be better and i suppose pushing a lighter frame along will mean the running gear has an easier time.
I know with my 500RS the chain and sprockets seem to be lasting just as long as when it was just a 250.
Either way if you did manage to fit the XBR engine into your RS frame you would have a lightweight torquey thumper that will surprise a few big bikes at the lights.