Topic: CB650 Horsepower!

[Sam Green Racing]

Not sure Brad, it was posted by Ibsen in Norway.
What does strike me is, in the first part of the post he quotes 2 motors power at the crank but the lower tuned dosn't seem to loose anything when transfered to the rear wheel. ???I recon the figures may be out a tad.

Sam.

[mlinder]

OK mlinder, I see you waiting but are you not going to guess at its top end speed ?

Sam.

I'm over-guessing, but without actual CD, I can't give an absolute.
Motorcycles are very bad aero.
Even a very good one would be lucky to hit .7 to .75
To hit 160 mph, One would need something like 125hp at .7 or .75

I suppose a lowered bike, showing maybe 6 square feet, with your bike making 114bhp (which is what yours makes, according to my math), and a drag coefficient of .6, you could reach something like 167-168.

[Sam Green Racing]

Ok mlinder, I was waiting for this one.
The third bike is a 1994 GP RS125 Honda with 1999 body work.
Now you would think that the drag coeffcient of the much more modern bike would be better, but is it.
The RS now bored out to 140cc makes 45bhp and weighs in at 71kg=156lbs to give .288.
This bike will be lucky to see the same speed as the 67bhp CR750 135mph.
Is it posible that the drag coefficent of the older bikes, the CRs was better(I don't have any figures)than the modern RS125.
If you look at the power to weight ratio of a ZX12R that is better than a Busa why is the Busa faster.
In theory the RS with its .288 should be faster than the factory CR with its .242.
Figure that out.

Sam.

[mlinder]

Ok mlinder, I was waiting for this one.
The third bike is a 1994 GP RS125 Honda with 1999 body work.
Now you would think that the drag coeffcient of the much more modern bike would be better, but is it.
The RS now bored out to 140cc makes 45bhp and weighs in at 71kg=156lbs to give .288.
This bike will be lucky to see the same speed as the 67bhp CR750 135mph.
Is it posible that the drag coefficent of the older bikes, the CRs was better(I don't have any figures)than the modern RS125.
If you look at the power to weight ratio of a ZX12R that is better than a Busa why is the Busa faster.
In theory the RS with its .288 should be faster than the factory CR with its .242.
Figure that out.

Sam.

I thought you said that the 3rd weighed the same as the 2nd, which is why I came up with those figures.

Thats an easy one to figure out though. Drag increases exponentially. bhp per lb means jack #$%* for top speed. drag coefficiency and hp are all that matters.

[Sam Green Racing]

I didn't mention the weight of the bikes, just my own weight.
I wondered how you came up with 114bhp.
When I said the bikes are used for flyers I slipped in the word aerodynamics and you picked up on it, well done.
Going back to the point being made, although it helps to have a good power to weight ratio, if the front of your motorcycle is the shape of the front of your house, yer goin nowhere

Well worked out mlinder.

Sam.

[mlinder]

I didn't mention the weight of the bikes, just my own weight.
I wondered how you came up with 114bhp.
When I said the bikes are used for flyers I slipped in the word aerodynamics and you picked up on it, well done.
Going back to the point being made, although it helps to have a good power to weight ratio, if the front of your motorcycle is the shape of the front of your house, yer goin nowhere

Well worked out mlinder.

Sam.

I thought you were referring ot the 3rd bike in this:

The mid powered one is the factory CR750 of Ralph Bryans, it weighs the same as ours but produces 96bhp and was timed at 161mph in practice for the Daytona race in 1970.

Even a brick wall can reach some speed. Drag doesnt affect much until higher speeds are reached. the whole exponential thing.

Theres a term used for a 'spot' in a graph showing exponential increases, It's called an elbow. Its the spot where things start to look more vertical than horizontal on the graph. Right after that spot is where things get difficult to increase speed.
If a cb550 needs 45bhp to reach 100mph (which is close to right), it would need 180bhp to hit 200mph.
Has nothing at all to do with weight (well, it has very very little to do with weight, other than the friction created by the tires which is greater with more weight). bhp per lb only tells you how quickly can get to your top speed, not what your top speed is. only hp and CDa can get you higher speeds.

[Sam Green Racing]

Doug/mlinder,

You can play about with BHP and power to weight ratio untill the cows come home, but it wont realy tell you how fast a bike is.

to give you an example I have three bikes, all have very different power to weight ratios and I weigh 126lb.

Below are the power to weight ratios and the speed of the two least powerfull, both are CR750s, see if you can fill in the missing speed.

.169bhp per lb......135mph.

.242bhp per lb......161mph.

.288bhp per lb......

Sam.

P.S. anyone can have a guess

Mlinder, I think the 1st line of my 1st post and the last line of your last post are basically the same.

Sam.

[mlinder]

Doug/mlinder,

You can play about with BHP and power to weight ratio untill the cows come home, but it wont realy tell you how fast a bike is.

to give you an example I have three bikes, all have very different power to weight ratios and I weigh 126lb.

Below are the power to weight ratios and the speed of the two least powerfull, both are CR750s, see if you can fill in the missing speed.

.169bhp per lb......135mph.

.242bhp per lb......161mph.

.288bhp per lb......

Sam.

P.S. anyone can have a guess

Mlinder, I think the 1st line of my 1st post and the last line of your last post are basically the same.

Sam.

Actually, I think you're right.

[Sam Green Racing]

 

 

[nickjtc]

Not sure Brad, it was posted by Ibsen in Norway.
What does strike me is, in the first part of the post he quotes 2 motors power at the crank but the lower tuned dosn't seem to loose anything when transfered to the rear wheel. ???I recon the figures may be out a tad.
Sam.

The lower tuned motor would have delivered about 40+hp (?) at the rear wheel. I've never seen a road test report on that one. The 'higher' power one, like we got here, produced the 49+hp at the rear wheel, according to period road tests, as Ibsen says.

Maybe the lower tuned motor was for markets where there were hp restrictions for tax/insurance purposes. Ibsen should be able to fill us in on that.

[Ibsen]

There were two versions of the 650 engine. Or more correctly, they were delivered with two different camshafts and carbs. The full version had claimed 63hp, and the tuned down version had approx 50.
But it is correct that a on Dyno test back in 79 it boosted out 49,42hp on the rear wheel, while the 1977 750K was measured to 57,13:   http://www.triumf.ca/people/baartman/bike/cb750khp-77.gif

With stock sized sprockets the 650 is geared to reach 120 miles/193kph at redline, 9500rpm, in 5th gear, and properly tuned, and under the right weather conditions, and with the rider flat out, it probably will. The Cycle magazine managed a 1/4 mile timing at 12.993 @ 102.38 mph, and a top speed of 123 mph back in 1979. But over here in Europe I belive the best they acchieved was 119mph.
The weight for the Europen 650Z, with a full tank, is 496lb.

Where does this data come from? There has always been quite a few blanks. Do you know the years/designations between the two?

Sorry, but I  haven't gotten any roadtest of the tuned down version, so 50hp is the claimed hp, while the full version had 63 claimed. My guess is that the 50hp version would make approx 38~ 40hp on a Dyno test.
The 50 hp version was sold along with the 63 hp version over here in Europe. Especially in Germany where they had different tax rules. The bikes are the same, the only difference is that the tuned down version had different carbs and a camshaft with different lobes.

[nickjtc]

The 50 hp version was sold along with the 63 hp version over here in Europe. Especially in Germany where they had different tax rules. The bikes are the same, the only difference is that the tuned down version had different carbs and a camshaft with different lobes.

I thought it would have something to do with bureaucracy!

[cb650]

Should 650 and horsepower be in the same sentence ?



              Terry

[nickjtc]

Should 650 and horsepower be in the same sentence ?
Terry

Now, now; be nice to us 650 dudes!

[Pinhead]

Should 650 and horsepower be in the same sentence ?



              Terry

I guess that depends on what 650 you have. When my bike can lift the front tire in both first and second gears, I'd say, yes it has plenty of power. Also, after all of the work of changing jets, opening the airbox, and drilling the exhaust, I finally ran through a tank of gas today and I'm getting 50mpg. I was getting 40, so that's a 25% increase. I'd easily say that came with a 25% increase in power as well.

[bill440cars]

Should 650 and horsepower be in the same sentence ?
Terry

Now, now; be nice to us 650 dudes!

             I think, if you'll check, Terry "cb650" has experience with and happens to own at least 1 or 2 650s as we speak. He happens to know a thing or two about them, if you'll just inquire.

                                                          Later on, Bill

[Pinhead]

According to my Haynes manual, the engine is rated at 63 BHP at 9000 rpm. That's 9.95 cc/hp, or 0.1hp/cc. Also, it's 15.75 hp per cyllinder, each cyllinder being 156.75 cc's.

[scunny]

Bill, I think Terry was taking the Michael.

[Buber]

Take a look here
http://kz400.com/CB650/index.pdf

This is German technical approval of some sort, and it confirms that there IS a differerence in camshaft and carbs (namel jets "nockenwelle") BUT it happens at different rpms. Hence for the practical purposes i think it's the same effect. I heard that Honda did this "higher" bhp engine parameter to claim 100bhp from 1000 cm

Anyway, I don't think it really matters in real life 

[Buber]

oops, i forgot to mention - our german technical friends on this page concluded that if you change the carbs and the jets you will get the chnage in power.
So, off you go looking for those 14101-426-000 jets and carbs typ P51A  and you should get more power!

[Ibsen]

oops, i forgot to mention - our german technical friends on this page concluded that if you change the carbs and the jets you will get the chnage in power.
So, off you go looking for those 14101-426-000 jets and carbs typ P51A  and you should get more power!

And the camshaft.  Nockenwelle in German=camshaft in English.

 63hp camshaft : camshaft pn 14101-426-000, cam lobe diameter 35.6mm IN and 35.3 EX
 50hp camshaft : camshaft pn 14101-426-690, cam lobe diameter about 33.1mm

[Pinhead]

Wow, there's a huge difference in the lobes on those camshafts! I can see how they'd be 13 hp different.

[nickjtc]

As always, thanks again, Ibsen.

Nick

[cb650]

Ibsen is the man .   



                  Terry