BHP vs BS ...

worntorn said:

I do recall that discussion, but the "We concluded" part was down to that Rohan Gang of One.

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If you look closely at that discussion, I posted that quick pic of Micks chart, and was rather neutral with what I initially said.
LAB then confirmed what I suspected - that that 850 curve was seriously suspect.

Its the totally different shapes, the 850 one especially, when they should all be a family of similar shapes that caught my eye initially.

BTW, I did attempt to redraw that 850 curve with it flipped over.
But my attempts were a bit rough, and I can't presently find where I 'filed' it.
It did look more like a member of the Norton dyno charts ...

Yes, here is an earlier even rougher version.


If you can imagine it interwoven into the 750 and 750 combat torque curves,
and the rpms at max torque being at 5000, it would fit quite well.
A member of the Commando family of torque curves, not a complete oddballl..

acotrel said:

Rohan has got a very big armchair.

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You mean a very good eye for detail.

There was a reason I was called on to do some proofreading (in the computer business),
I can spot BS at a thousand yards....

Rohan, If you are correct, the X-axis is reversed. So what should we read instead of the 3000 RPM number ? Just from riding my own bike I would expect the torque curve to be as you have drawn it, with a peak at about 6500 RPM. On my 850, I use very high overall gearing with a very close ratio gearbox. There is still a very high risk of over revving on up changes.

If that axis is reversed on the dyno read-out, it would probably indicate a software error. I wonder what else might be wrong. In a previous life I wrote a lot of software using the orthogonal polynomial curve fitting technique. I always had a way of verifying the output. Perhaps that is not so easy with a dyno ?

I think its more likely to be a problem with a transparency at the printing stage.
Having had some experience with assembling stuff for printing - when folks provide them with no writing to indicate which way it should go. !
You sometimes see pics in magazines, and even in books, with the bike mirror imaged...

It should have been picked up at the proof reading stage though. ?

Nortons claimed max torque for the 850 as 56 ftlbs @ 5000 rpms, and without changing cams that 5000 rpms is about as good as written in stone.
Thats where the cylinder filling is as good as it gets, and it takes a lot of fiddling to move it anyplace else .....

Surely if the graph was mirror imaged during printing, the scales would have been reversed in the publication ? It might be easier to accidentally reverse the X or Y values when putting the graph up on the screen or onto the printer as it comes off the dyno ? - Personally I have no interest in what max. horsepower my engine makes - torque is everything. I use the close gearbox to ride the torque curve and if the overall gearing is not high enough, you cannot feel it.

I believe that most claimed horse power rates from the day were skewed as a marketing gimmick . I could be wrong, I often am. :wink:

It has been WELL discussed here already previously that Nortons claimed 60 bhp (in the brochures)
is a real 42 to 45 rear wheel horspowers.
The Combats give a few more.

acotrel said:

Surely if the graph was mirror imaged during printing,

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The 750 graphs seem correct, so its only the 850 torque curve that has been added incorrectly to the plots...

Rohan said:

It has been WELL discussed here already previously that Nortons claimed 60 bhp (in the brochures)
is a real 42 to 45 rear wheel horspowers.
The Combats give a few more.

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Probably where I read it.

I have often wondered when I have seen figures for some bikes way above 100 BHP, what they would read if the dyno was accurately (precisely ? ) calibrated from primary reference standards for mass , length and time. I read that the Dynotek inertia dynos all use a nominal value for the Yamaha V- max as a reference. It is only one point in the calibration range. If your bike was up near the reference value, that might not be too bad. However it is up at about 130 BHP and Commandos are way below that. Where is the zero point and is the calibration graph a straight line ? The thing about it seems to be - if you are using a dyno ,you are only looking for improvements. So unless the calibration graph is absurd, you don't really have a problem until you claim bragging rights.

If the best Manx Norton turns out 50 BHP, it takes a really good (claimed) 60 BHP 650cc Triton to beat it. - Might need a tail wind ?

As you keep saying, its the torque curve, not some hp number.
Weight, and rider skill also come into it.

I find it hard to believe that in this day and age of electronics, there are not accurate dynos everywhere, besides compensating software or controllers that produce more accurate results on paper...

grandpaul said:

I find it hard to believe that in this day and age of electronics, there are not accurate dynos everywhere, besides compensating software or controllers that produce more accurate results on paper...

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I think there are it seems that dynojet ones read a little higher than various others maybe 8% depending on who you talk to. I have run my bike on several dynojet units and they all read the same. A Dyno Dynamics machine read about 9% lower.

The mythology about dynojet and the V Max is that their calculations of the mass of the roller and its acceleration didn't work out right when known horsepower bikes were run on it, so eventually they took a V Max which had been run on another dyno and calibrated the Dynojet to that.
I don't know how much of this story is true and how much is Chinese whispers, but if it's a known horsepower bike it's not that bad a way to calibrate.

I think that part of the problem is that BIke manufacturers have always quoted horesepower at the crank and dynojets etc etc take figures from the rear wheel.
A late 70s BMW will claim something like 70 BHP and if you put the bike onto a rear wheel dyno you will see something in the early to mid 50s BHP, which seems to result in losses due to gearbox and shaft drive being about 15 BHP, A Norton should lose less as chain drive is more efficient than shaft drive.

This of course does assume that when manufacturers take BHP readings, it is from a normal production standard engine and accessories such as alternators were retained when taking the readings, I'm told that Currently at BMW, BHP readings are supervised by outside engineers and as such are accurate (certainly magazines who Dyno test bikes nowadays get readings that are close to manufacturers claims).. But who knows what was happening in the 70s when Norton and BMW were heavily outgunned by all the Japanese factories in the claimed horsepower race.

I think what is great about the Dynojets is that they seem to provide a system of reasonably accurate comparative measurement, unlike the previous generation of water brake dynometers which might produce widely different readings depending on who was operating the thing.

Charles

Fast Eddie said:

At this years Mallory bike festival I had, unbeknown to me at the time, cocked things up with the ignition timing on my Commando.

Now let me be clear, I am no Hailwood, Williams, Rossi, etc. but I have raced a bit and at least know my way around Mallory Park.

There were some tasty bikes out in the sessions with me. A Guzzi Le Man passed me and I couldn't keep with her (yes, female rider). But she was the only rider to pass me and I rejoiced in the ultimate power that my highly tuned Commando must have been putting out, as it was clearly very fast compared to most others out there...

So I was rather surprised when I later learnt that due to the ign timing being out, the Commando was more than 10bhp down on power, putting out 45 point something versus its earlier 56.

So, despite all the bar room boasting that goes on in this game (we all do a bit I guess) a 45bhp Commando still stood out. It makes me wonder what the real RWHP of most classic bikes are, a lot less than their owners realise I guess!

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Going back to the original post, here is another dyno chart to ponder. This is from Whitworth Ranch's near stock 850 and is very similar to Duckworth's chart. It also shows Max torque very early, at just 2800 rpm then a slight decline until full torque again at around 4800. The engine makes full torque or very near full torque ( at the lowest point,94 of max %) all of the way from 2800 rpm to 5500 rpm.
The Duckworth 850 did similar, making Max at 3000 then dropping a bit as it revved, but really only losing a couple of pounds. It looks like more because of the scale, but essentially both charts show a near flat torque output over a very wide rev range.
Put overly restrictive mufflers on Whitworths bike (such as stock black caps) and the chart would look like the Duckworth chart. Restrictive pipes leave the bottom power intact and choke off the mid and top some.
But both charts show the big advantage an 850 Commando has over a lot of other engines- lots of torque down low that is held right thru the rev range. Nigel, perhaps this is why you found yourself leaving others behind, even though down some on horsepower at peak?

worntorn said:

Fast Eddie said:

At this years Mallory bike festival I had, unbeknown to me at the time, cocked things up with the ignition timing on my Commando.

Now let me be clear, I am no Hailwood, Williams, Rossi, etc. but I have raced a bit and at least know my way around Mallory Park.

There were some tasty bikes out in the sessions with me. A Guzzi Le Man passed me and I couldn't keep with her (yes, female rider). But she was the only rider to pass me and I rejoiced in the ultimate power that my highly tuned Commando must have been putting out, as it was clearly very fast compared to most others out there...

So I was rather surprised when I later learnt that due to the ign timing being out, the Commando was more than 10bhp down on power, putting out 45 point something versus its earlier 56.

So, despite all the bar room boasting that goes on in this game (we all do a bit I guess) a 45bhp Commando still stood out. It makes me wonder what the real RWHP of most classic bikes are, a lot less than their owners realise I guess!

Click to expand...

Going back to the original post, here is another dyno chart to ponder. This is from Whitworth Ranch's near stock 850 and is very similar to Duckworth's chart. It also shows Max torque very early, at just 2800 rpm then a slight decline until full torque again at around 4800. The engine makes full torque or very near full torque ( at the lowest point,94 of max %) all of the way from 2800 rpm to 5500 rpm.
The Duckworth 850 did similar, making Max at 3000 then dropping a bit as it revved, but really only losing a couple of pounds. It looks like more because of the scale, but essentially both charts show a near flat torque output over a very wide rev range.
Put overly restrictive mufflers on Whitworths bike (such as stock black caps) and the chart would look like the Duckworth chart. Restrictive pipes leave the bottom power intact and choke off the mid and top some.
But both charts show the big advantage an 850 Commando has over a lot of other engines- lots of torque down low that is held right thru the rev range. Nigel, perhaps this is why you found yourself leaving others behind, even though down some on horsepower at peak?

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OK, you've persuaded me to look through my own notes...

When I first finished my rebuild I dyno'd it and the peak torque was 50ft-lbs at 5300rpm.

Moreover, I got 46ft-lbs at 3300rpm and it stayed above 46ft-lbs all the way to 6200rpm.

If my maths is correct, that's 92% of the torque available for 47% of the rev range. Which I think is pretty good. Peak torque figures at a certain rpm can be deceiving I feel, the fact that mine gives such a high proportion of its torque so low down must make itself felt on the road.

I know this ain't 'apples with apples as my bike ain't stock. But interestingly, my bike is 'tuned' and the big carbs (35mm), high CR (10.5:1) and bigger cam (JS stage 1) are all things that REDUCE low end power according to general opinion !

Not 100% sure on this but doesn't adding CR add a little power everywhere? The other mods would tend to pull a bit away from the bottom end, but if you started with a chart like Whitworth or Duckworth chart (call em the Worth charts) then it's easy to see how you could retain that much power down low and thru the range.


Glen