Torque on a Chassis Dyno

[NickS]

I don't get the chassis dyno readings for power vs torque. There is a loss from flywheel to wheels in terms of power, this seems to be pretty consistant across a wide range of vehicles. What I don't get is the torque. I have read lots of posts where people say that they got in the 200rwkw region and in the region of 500nm???

Is there not the same loss in torque from flywheel to wheels?? In fact I am almost certain that I have read some cars showing torque levels from a stock motor that are higher than the factory quotes I don't get it ... the power figures all make sense, the torque figures almost always seem to be WAY to high. Can someone enlighten me please.

Edit; just found an example. A stock SV8 on Sonny's dyno pulling 520nm ... the LS1 in an SV8 has 470nm from the factory Confused, very confused

[SCiFiRE]

Torque on a chassis dyne varies alot, and you can change it just changing diff gears and such, so its almost not worth reading into torque figures on a chassis dyne.

[Ryzz]

Torque on a chassis dyne varies alot, and you can change it just changing diff gears and such, so its almost not worth reading into torque figures on a chassis dyne.

Agreed, Torque gets multipled by your wheel size, diff gears, what gear your in, etc. Use it just as a measurement of increase, not as a "this is my torque", if that makes sense!

[Stevotski]

I thought that Dyno's actually measured torque then converted that to power?

Power = Torque x rotational speed

[Dacious]

I thought that Dyno's actually measured torque then converted that to power?

Power = Torque x rotational speed

They do - the problem is torque which is twisting force is affected by how the force is applied.

For example - put a 1 foot torque wrench on a bolt and set it to a certain measurement. Do a bolt up until it clicks. Now loosen it, and put your hand on the bar (steady - not that one!) half-way down and torque it up again. Much harder to do up isn't it?

The same force has been applied at the bolt - but roughly twice the force has been applied by your arm when reducing the bending moment or effective length of the torque wrench.

Dynoing a car in 1st will result in a much higher torque reading than in fourth, because you've made it easier for the engine to turn the dyno by increasing it's leverage over the drum through reduction gearing.

Larger rolling radius wheels/tyres will raise gearing and reduce torque readings given equal traction. Larger numerical gearing or smaller wheels/tyres lower gearing and will increase torque readings by increasing the engine's effect on the dyno drum. Putting narower tyres of the same radius with harder compound and more air will maximise readings by reducing drag providing you can get traction. Dynoing with softer wider tyres at lower pressure will cost power by increasing drag. One reason why manufacturers use engine or driveline dynos, reducing variables during development.

In effect, putting lower gears on a car will have the same effect as increasing torque - same as you using a three foot bar to increase your leverage on the bolt. The power of your arm doesn't change, but the greater leverage multiplies the effect and increases the torque you can apply.

That's why dyno readings are probably best used for comparisons between before and after mods and not 'mine's bigger than yours' pissing contests.

[NickS]

Thanks for the replies guys, makes more sense now

[Ryzz]

That's why dyno readings are probably best used for comparisons between before and after mods and not 'mine's bigger than yours' pissing contests.

Awesome explination there mate. Couldnt agree more regarding the pissing contest!

[lucas]

Yup agreed with everything said thus far. I'd only (have some) trust from a torque number pulled from an engine dyno.

[awddynotodd]

I don't get the chassis dyno readings for power vs torque. There is a loss from flywheel to wheels in terms of power, this seems to be pretty consistant across a wide range of vehicles. What I don't get is the torque. I have read lots of posts where people say that they got in the 200rwkw region and in the region of 500nm???

Is there not the same loss in torque from flywheel to wheels?? In fact I am almost certain that I have read some cars showing torque levels from a stock motor that are higher than the factory quotes I don't get it ... the power figures all make sense, the torque figures almost always seem to be WAY to high. Can someone enlighten me please.

Edit; just found an example. A stock SV8 on Sonny's dyno pulling 520nm ... the LS1 in an SV8 has 470nm from the factory Confused, very confused

Nick,

The Torque number on a Chassis Dyno can be difficult to understand for an operator, let alone the general public, so I'll try and explain in simple terms.

Most people are aware that Power(HP) = (Torque(ft/lbs) x RPM)/5252, and both an Engine Dyno and a Chassis Dyno use this formula to Calculate Power, the difference is though, an Engine Dyno uses actual RPM(Crank RPM) and Torque measured on a Load Arm attached somewhere to the Engine Dyno Cradle in it's calculation, whereas a Chassis Dyno uses Roller RPM and Torque measured on a Load Arm mounted on the Retarder that is coupled to the Knurled Drive Rollers.
Now the reason why typically you see higher "Torque" numbers on a Chassis Dyno is due to gear multiplication, (remembering that the torque is being measured down in the Dyno Bed),so any gear multiplication between the Engine and Rollers will increase Torque (less any losses).
A typical scenario is (using your LS1 example of 470nm), if we use a 1:1 ratio in the gearbox, we still have 470nm (we won't assume any losses in this example) out the back of the gearbox, the torque then goes thru a ratio of for example 3.46 in the diff, so now we have 1626nm at the AXLE.
The next step is the Tyre to Roller, but in this case it is a gear reduction due to the Tyre being larger that the roller diameter, so we would see a Torque reduction, a typical reduction using Sonny's dyno (and most other Dyno's used in Australia which have rollers in range of 217-218mm Diameter) would
be the Tyre Diameter divided by the Roller Diamter, so if we use a figure of 650mm for the tyre and 217mm for the dyno roller we end up with a figure of 1:2.995, so we divide 1626nm by 2.995 giving us 542nm measured at the roller, remember this example is assuming no driveline loss,so the measured Torque would be less than 542nm.

Some people have eluded to that using different gears effects the Torque, this is true, but what evens things (Power) out is, if you are in lower gear on a dyno, yes you will produce more Torque, BUT, the roller will be rotating at a slower speed according to the different gear being used, so the same Power will be produced at a given Engine RPM.

Another difference that no-one realises is that a different Diameter Roller also changes the Torque reading, so a dyno with 217mm rollers that is showing 100RWHP at 100kph would be measuring 215ft/lbs and the ROLLER RPM would be 2443RPM, so those that want to do the maths can verify the 100RWHP.
On a Dyno that uses 270mm Rollers (such as Sam's and PowerTorques), if one of these dyno's was showing 100RWHP at 100kph, then it would be measuring 267.3ft/lbs and the ROLLER RPM would be 1964RPM.

Rather than looking at Torque on a Chassis Dyno, it is better to look at the Motive Force, which uses the Roller Diameter in it's calculation, so dyno's with different Roller Diameters will show the same Power and Motive Force.
Motive Force is also known as Tractive Effort.
Motive Force is the Linear Force at the Roller Diameter pushing the car forward.

A few things to remember when looking at Dyno Graphs:

1.The Torque Curve (shape) will be the same regardless of gear used or Roller Diameter.

2. The Torque Curve will be the same as the Motive Force (tractive effort), except it will be a higher number.

3. Look closely at the scale on which the Torque/Motive Force/Tractive Effort is displayed, it is very common to see a Tractive Effort plot of around 3000-4000newtons being plotted on a scale of 0-13000newtons, all this does is make the "Torque Curve" look flat, because itis being squashed down on a large scale.

4. If a vehicle has made more power at the same Road Speed with identical gearing after some mods, then it has to of made more Torque, so do we need to see Torque?

A better approach all round is look at the Average Power, or even better, the Integral of the Power, this is the all of the Power added together that is under the curve, and not be too concerned with Torque Numbers.

[Ryzz]

Awesome post mate. I think this just proves why you are the king when it comes to all things Dyno. Gives me an even better understanding, would never have thought into roller sizes making a difference, etc. Thx!

[Sid447]

Nice explanation awddynotodd,

Sorry I missed it by a week or so.

Talking about the graphs: It would be very easy to have the graph laid out with both the power and the torque line if a customer wanted, as both are directly related. If the computer can plot the (corrected) rear wheel power, then it can also plot the torque by doing hp x 5252 / rpm.

Regards, Steve.

[monarocv8z]

in regard to the differing torque figures on the sv8 doesn't depend it on the dyno..? i read somewhere that dynos have a "shootout" mode which actually produces a different torque/hp figure, correct me if im wrong..

[YLD57L]

You get torque multiplication through change of gearing. Power remains the same. Makes you wonder why people are always on about peak engine torque readings or torque being the king of speed huh?

[NickS]

You get torque multiplication through change of gearing. Power remains the same. Makes you wonder why people are always on about peak engine torque readings or torque being the king of speed huh?

I think people raise torque as an issue when there is A LOT of it from very low down in the rev range. Having 250 kw at the wheels isn't much use if it's at 5000+ rpm and you hardly ever use it. I would rarely get over 3000 rpm on the road so having 500+nm from under 2000rpm is a lot more usful than having lots of power but only if you thrash the thing.

[YLD57L]

Having that engine 'torque' as you say down low means you actually have a lot of relative power down low. And probably a flat power curve, which gives you good acceleration from the get go.

If you altered your gear ratios (understandably harder for your Coupe' 4) and have a cam'd engine instead of a stroker say you would get comparable acceleration but you engine will be reving harder. Wouldn't sound as meaty as your stroker though.