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Posted: Fri Apr 06, 2007 11:01 pm 
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opto wrote:
Hang on thats not what you said above, now which is correct ?


That is exactly what I said above.

Gearboxes, diffs and wheels all multiple and divide the engines torque depending on there ratio and size. By connecting it directly to the axel you eliminate one variable but that doesnt change the fact that the torque value has been messed with.

Ill give an example. Take a car, put the gearbox in direct drive (4th for a T5, 3rd for a btr) which eliminates the gearboxes effect, then say it has a 3.23 diff.

Say the engine really makes 300nm of torque at 4000rpm. The dyno will see the axle spinning at 1238rpm with 969NM of torque. Replace the 3.23 diff with a 4.11 diff and the dyno will see the axle spinning at 973rpm with a torque of 1233NM of torque. Because power is based proportionally on the speed (axle rpm) and torque, the power is the same. Both dyno graphs will have the same power line, the same shape torque curve, but the torque values will be different. as you say you then need to mess around with the ratio difference, but there are other complications that make it very inaccurate.

 

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Posted: Fri Apr 06, 2007 11:18 pm 
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I see what you are saying and understand your point about the output torque and I agree that the curve will not be correct due to the ratios, whether they are gearbox/transmission and differential, however you can obtain peak torque values.

What other factors would need to be considered in the driveline ?

I know there are losses in the driveline but I was under the impression that they would be insignificant.

 

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Posted: Fri Apr 06, 2007 11:27 pm 
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You can use it to find where you car makes peak torque but the value isnt reliable.

The best way to work out peak torque is to look at the shape of the 'torque' line on the dyno graph. Note the RPM and the power at the peak torque, then use the following formula to work out the engine torque (less drivetrain loss) at this point.

Torque(NM) = kw * 9536 / RPM

This method removes gear and tire size factors.

Drivetrain losses are still significant. That is largely why a 165kw xr6 makes 115kw on a chassis dyno. There are many other variables though, an auto gearbox for example will have nearly 5% more loss than a manual.

 

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 Post subject: Re: NM Torque figures
Posted: Fri Apr 06, 2007 11:35 pm 
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WagonDad wrote:
What NM Torque figures have people got on their standard or modified i6 engines.


Okay we did get off subject.

By calculation Approx 280 Nm, By Graph utilising ratio supplied 320Nm at the Flywheel, modified I6.

Thanks for the info Stockstandard, good discussion.

 

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Posted: Fri Apr 06, 2007 11:41 pm 
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ahh crap, it is actually:
Torque(NM) = kw * 9550 / RPM

too much rounding off in my imperial->metric conversion...

 

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Posted: Fri Apr 06, 2007 11:50 pm 
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So what's an accurate method of obtaining a torque reading short of an engine dyno and dodgy graph reading?

 

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Posted: Fri Apr 06, 2007 11:52 pm 
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That formula I posted above is the closest you will get. The number you get from that can be described as "the amount of engine torque that makes it to the back wheels" and is comparable to how rwkw relates to fwkw.

 

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Posted: Sat Apr 07, 2007 12:03 am 
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Very interesting, the reason I asked is I've just had a dyno run done on a Dyno Dynamics in standard shoot out mode, achieved 139 rwkw and 300nm. Just wanting to know if the NM figure is about right.

 

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Posted: Sat Apr 07, 2007 12:09 am 
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Does that mean to get the true torque reading you have to strap the engine straight onto a dyno?...well not literally but you get what I mean right?

sorry to go off-topic.

phong =P~

 

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Posted: Sat Apr 07, 2007 12:15 am 
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to get accurate power or torque at the engine you need to use an engine dyno. And yes you literally bolt the engine to the dyno



Image

 

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Posted: Sat Apr 07, 2007 12:26 am 
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Isn't there a formula where you convert Torque to HP and vice versa? After all HP is the rate at which the torque is being applied.
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Posted: Sat Apr 07, 2007 12:28 am 
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WagonDad wrote:
Very interesting, the reason I asked is I've just had a dyno run done on a Dyno Dynamics in standard shoot out mode, achieved 139 rwkw and 300nm. Just wanting to know if the NM figure is about right.


You can check by using the formula above or post your dyno graph and Ill check it.

btw: The formula isnt an approximation, it is the actual mathematical relationship between power, torque and rpm.

xcabbi wrote:
Isn't there a formula where you convert Torque to HP and vice versa? After all HP is the rate at which the torque is being applied.


it is the same forumula that I just posted

hp = torque(in ft-lb) * RPM / 5250
kw = torque(in nm) * RPM / 9550

It just changes a little if you are using the old hp/ft-lb or kw/nm for power/torque.

 

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Posted: Sat Apr 07, 2007 1:01 pm 
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My Dyno sheet for what its worth. Red is before (132.4 rwkw). Blue is after tune (136.4rwkw) Green is with airbox lid lifted (138.9rwkw). Also did another run with the car anchored so it wouldn't move forward but not pulled down no pressure on the tyres at all, got 154 rwkw.

 

 

Attachments:
138.9 Dyno.jpg
138.9 Dyno.jpg [ 108.04 KiB | Viewed 23 times ]

 

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Posted: Sat Apr 07, 2007 3:22 pm 
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Taking the rpm point 3700rpm, and saying you have about 114rwkw

114kw * 9550 / 3700rpm = 294nm

so yep, looks like your dyno operator has been nice enough to give you a corrected torque line instead of a measured torque line.

 

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Posted: Sat Apr 07, 2007 5:57 pm 
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Thanks stockstandard, Just trying to understand where my car is lacking because other cars with similar RWKW and similar mods are running a good half a second quicker down the quater. Oh well the search continues.

 

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