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Old 05-08-09, 05:38 PM   #81
joshmac
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Default Re: 33bhp Dyno Resutls

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Originally Posted by Liam9188 View Post
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Originally Posted by simonDyna View Post
Dyno's read rear wheel power
No they don't, otherwise it would change drastically depending on what gear you are in. In first gear, many bikes put over 1000nm of torque through the rear wheel.
Where did you read that?..

They do read power at the wheel. Do you think it's just coincidence that the rear wheel spins a roller on a dyno?
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Old 05-08-09, 05:41 PM   #82
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Default Re: 33bhp Dyno Resutls

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Originally Posted by joshmac View Post
Where did you read that?..

They do read power at the wheel. Do you think it's just coincidence that the rear wheel spins a roller on a dyno?
+1. They use an adjustment factor to calculate the power at the crank from the power reading taken at the back wheel. Except for in factory conditions they will somehow measure the crank power.

From what I gather it doesn't matter what gear you are in...the engine will still put out the same power and the gearing just affects how this is translated into speed.
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Old 05-08-09, 05:42 PM   #83
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Default Re: 33bhp Dyno Resutls

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From what I gather it doesn't matter what gear you are in...the engine will still put out the same power and the gearing just affects how this is translated into speed.
Yeah it's the gearing that affects the speed/acceleration.
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Old 05-08-09, 05:48 PM   #84
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Default Re: 33bhp Dyno Resutls

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Originally Posted by Liam9188 View Post
No they don't, otherwise it would change drastically depending on what gear you are in. In first gear, many bikes put over 1000nm of torque through the rear wheel.
Yes they do read rear wheel power.

Measured power runs either happen in top gear or one below. The reason this is done is because the lower gear ratios do not provide time to get correct/accurate measurements.

They do not put out 1000nm in first. They have little or no resistance from the rear wheel because of the lower gearing. The only thing that changes drastically is the speed of the rear wheel.

For example, if an engine puts out 100ft/lb of torque at 5000rpm it does so in every gear. Run that through a 5th/6th gear ratio of 1:1 and it will be crusing. Put it through a lower gear ratio and it will accelerate like hell. Its the same amount of torque, just being used differently through the gearbox.
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Old 05-08-09, 06:10 PM   #85
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Default Re: 33bhp Dyno Resutls

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Originally Posted by TazDaz View Post
+1. They use an adjustment factor to calculate the power at the crank from the power reading taken at the back wheel. Except for in factory conditions they will somehow measure the crank power.

From what I gather it doesn't matter what gear you are in...the engine will still put out the same power and the gearing just affects how this is translated into speed.
In factories, engines are run on test benches before being fitted. A fueling/exhaust apparatus is connected and the engines are run. Power readings are taken right off the crank.

It is easier to do on pre-unit engines ( ie; older british bikes, Harleys etc ).

Found this link to the Superflow engine dyno benches.

Last edited by simonDyna; 05-08-09 at 06:17 PM. Reason: added link
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Old 05-08-09, 07:17 PM   #86
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Default Re: 33bhp Dyno Resutls

F = ma
P = Fv

Different gearing has no effect, except for possibly probably differing transmission losses.
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Old 05-08-09, 07:43 PM   #87
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Default Re: 33bhp Dyno Resutls

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F = ma
P = Fv

Different gearing has no effect, except for possibly probably differing transmission losses.

Exactly.

And the gearing residual/parasitic losses will come down to the number of touching parts/cogs or the weight of the drivers and driven cogs.

And another thing to add to the overall equation is atmospheric pressure/humidity.

Last edited by simonDyna; 05-08-09 at 07:44 PM.
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Old 05-08-09, 08:02 PM   #88
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Default Re: 33bhp Dyno Resutls

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Originally Posted by simonDyna View Post
And another thing to add to the overall equation is atmospheric pressure/humidity.
Engine temp, air temp, fuel density +101 other factors!
Weight of the cogs you're spinning though? Constant mesh box innit. And only a factor on an inertial dyno.

Anyway, we are going around in circles. We know it's dynojet dynos that are full of s*** and there are a million ways to play with the power readout.

Adjustment factors for crank power are one of my pet hates. It's nothing more than a guess. What's more it's a meaningless, misleading guess which is no use to man nor beast unless you just want to brag down the pub!
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Old 05-08-09, 08:28 PM   #89
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Default Re: 33bhp Dyno Resutls

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Engine temp, air temp, fuel density +101 other factors!
Weight of the cogs you're spinning though? Constant mesh box innit. And only a factor on an inertial dyno.

Anyway, we are going around in circles. We know it's dynojet dynos that are full of s*** and there are a million ways to play with the power readout.

Adjustment factors for crank power are one of my pet hates. It's nothing more than a guess. What's more it's a meaningless, misleading guess which is no use to man nor beast unless you just want to brag down the pub!
I'm generalising with the reference to gearboxs. Whilst there will always be contacting, the number and size of driven cogs at anyone time will differ.

I agree crank torque/power is meaningless to a certain extent. But if you want to tune high performance engines, you need the crank output so you can see what the losses are at the back wheel and then look to minimise the losses.
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Old 05-08-09, 08:47 PM   #90
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Default Re: 33bhp Dyno Resutls

High performance/ any performance. You would want to reduce these losses at any level of tune.

Anyway, you can't know these things by taking a flying guess based on the first percentage you read in the Sunday sport. If you are using an engine dyno, fair one. My point about guesswork "crank power" figures based on RW figures being utterly useless boswallox still stands.

Hence the problem in this thread.
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