XBR engine transplant into CB250RS

[Steve H]

A number of people seemed to think the 250 cam would work well in the 500 I found it seemed to restrict it, I ended up with an XR500 cam and fitted a pin to drive the tacho drive.

[tevie54]

I must admit I nearly just drilled the XL500 camshaft but when I read you could just use the 250 one I did that. I didn't notice any difference at the time but maybe I'll try it again when I take the engine apart this winter.

[Propellor]

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

It's a forum. That's all we can do.

Look how much talking this person needs:-

http://charming.awardspace.com/otto_diesel/power-vs-torque.html

I draw your attention to the final paragraph.

[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

I have no experience of engine tuning so please forgive (and correct) any errors. Trying to place myself in the position of someone making changes to the engine with expectation of improvements in power, wouldn't I be most interested in looking at curves relating to the crankshaft output?

Thinking about the dyno in the context of comparing the engine performance of different machines, I'm not sure rear wheel power figures are the best way, and definitely not the rather abstract torque figure seen on a lot of dyno sheets. In this context wouldn't the best way to show power/torque figures be based on crankshaft output? When comparing engines isn't that the ONLY way? Not only that, but the resulting information would very easily expose any cloak and dagger marketing by manufacturers. A lot better than it does now? The only sure way of comparing, in figures, the performance difference between complete machines is a tractive effort graph?

[tevie54]

STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.

[guest564]

STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.

The extra duration will give more power. The XR camshaft has even more duration and will give a little more power, I don't recall the exact timing specs though.

[guest564]

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!         

The race bikes used to put down around 45bhp at the rear wheel without issues. There was probably just as much stress put through the chassis and forks by the race tyres than by the engine.

[Propellor]

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!         

The race bikes used to put down around 45bhp at the rear wheel without issues. There was probably just as much stress put through the chassis and forks by the race tyres than by the engine.

Thank you. That's very interesting information. The race bikes ran the stock forks, yokes and wheel rim size? Xbr engine?

[Steve H]

STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.

If you plot a graph of lift against crankshaft rotation , then my understanding is that power output is related to the area under the graph. There is also a point above which the lift has minimal effect so rather than have a peak on the graph you have a flat line capping the top of the peak off. If you are looking to increase power then increasing the area under the graph will help. The most effective way to do this is increase the duration, the other factor is to get the valve open as soon as possible so that it is open for as long as possible at the maximum flow rate, if you increase the lift (and keep the duration the same) then the valve has to open correspondingly faster, increasing the area under the graph.
In summary duration has a bigger part to play than lift.
I did run a megacycle cam for a short period of time and that was very quick, unfortunately it also ate the rockers very quickly. Hence the XR cam.

[Propellor]

STEVE H, you might be on to something there, I checked the specs of both camshafts and although lift is the same for both, the XL500 cam exhaust timing opens 10 degrees earlier than the RS, don't know if it makes much difference to power but it might.
The XR cam might be even more radical.

If you plot a graph of lift against crankshaft rotation , then my understanding is that power output is related to the area under the graph. There is also a point above which the lift has minimal effect so rather than have a peak on the graph you have a flat line capping the top of the peak off. If you are looking to increase power then increasing the area under the graph will help. The most effective way to do this is increase the duration, the other factor is to get the valve open as soon as possible so that it is open for as long as possible at the maximum flow rate, if you increase the lift (and keep the duration the same) then the valve has to open correspondingly faster, increasing the area under the graph.
In summary duration has a bigger part to play than lift.
I did run a megacycle cam for a short period of time and that was very quick, unfortunately it also ate the rockers very quickly. Hence the XR cam.

Now that makes a lot of sense. Even to a tuning dummy like me. Would using lighter valves and springs allow steeper ramps? That, in a naive way, would seem to gain effective duration without increasing lift.

Do I get brownie points or custard pie?

What do you know about torques?......

[Propellor]

• The back wheel turns a big roller
• They rev the tits of the bike and measure how fast the big roller accelerates
• From the acceleration and mass of the roller it possible to calculate the force applied by the back wheel
• They shut the throttle, the roller slows down because it now has to spin the engine
• From the deceleration rate they can determine the force applied by the back wheel and get a measure of the losses in the drive train
• They add the two together to get crankshaft torque.
• Simples

Not all Dynos work this way but a lot do.

I hear what you're saying. But they don't seem to do that. Nearly all curves I've looked at as examples seem to simply take the rear wheel horsepower and plot a torque curve against it at engine speed.

This is not the same as you said.

What you said, in principle is great. In practice they don't seem to 100% trust the "coastdown run". Not enough to categorically say "here, this is your crankshaft figure".

Simples? Should be, but it ain't.