"Dyno Torque Figures – the truth.
On the styles of Chassis Dyno’s we see here in Australia, for example a Mainline Dynolog or a Dyno Dynamics, both of these dyno’s, calculate power at the roller in the same fashion. That is, we measure Roller RPM and Roller Torque and these 2 variables are used in the traditional Power calculation formula, either metric or imperial, you get the same result:
HP (@ roller) = (roller torque ft/lbs x roller rpm)/5252
Or
kW (@ roller) = (roller torque Nm x roller rpm)/9543
When the term Roller Torque is used, this means we are measuring Torque from a Load Cell that is attached in some fashion to the Retarder (Eddy Current Retarder/PAU) that is coupled to the Drive Rollers (the knurled rollers). The Retarder applies a braking force to the Rollers, and since the Retarder Frame is restrained from turning by the Load Cell, the force is transferred into the Load Cell which measures force in typically Kg or Pounds force. The length of the arm the Load Cell is attached to is precisely known, so with force over a given length we can determine Nm or Ft/lbs.
Roller RPM is usually measured by an Inductive speed sensor of some type, or a Shaft Encoder, or in the old days on a Vane Dyno, a Tacho generator.
These 2 inputs are used by the Dynamometer Control System to calculate the Power.
Now, because we measure Torque down in the Dyno bed from a device that is coupled to the Roller, ANY gear multiplication, or gear reduction, will influence the Torque measured at the Roller. So some variances that come into the equation are:
Torque Converter slip ratio
Transmission ratio
Differential ratio
Tyre size
Roller size
These variables all influence the Torque as measured at the roller.
Some people will then initially think, the lower the gear, the more Torque, so the more Power we will make, THIS IS NOT TRUE.
Remember the formula for calculating Power, we have Torque AND RPM, so if we use a lower gear ratio, the corresponding RPM will be reduced by the same factor as the Torque is increased, so we end up with the same power. This is a purely mathematical explanation.
The reason some of the confusion happens in regards to Torque (roller torque) on a Chassis Dyno, is there are 2 common sizes of rollers used on dyno’s here in Australia, those being 219mm or 273mm (these are raw, unmachined sizes).
The larger diameter roller will have a higher Torque reading at a given Road Speed, but have a corresponding lower RPM, the torque value increase will be the difference in Roller Diameter, I’ve done the maths for you, the 273mm roller will have 24.6% more torque. What this means is, a particular car may have 200RWKW, but it may have 500Nm on a 219mm roller, but 623Nm on a 273mm roller.
We now need to understand the Torque that gets to the roller, and why it is normally higher than at the Crank. Remember I listed all the things that will influence the Torque at the Roller, Gearbox Ratio, Diff Ratio, Tyre size etc. If I simplify it somewhat, we’ll use a gearbox ratio of 1:1 in the following example, I also am not accounting for driveline loss in this example.
Take a traditional LS1, a torque figure of 530Nm is quoted at the crank. If the Gearbox Ratio is 1:1, so we have 530Nm going into the diff, now if we have a diff ratio of 3.46:1 (this is a gear multiplication), we then have 1833Nm at the axles. (Those that have had their cars dyno’d on a Dyna Pack Hub Dyno will be accustomed to these numbers). The next step in the Torque journey is the Tyre to Roller ratio.
This is where the Dyno Roller size will affect this value. If we transfer the 1833 axle Nm thru a 650mm tyre to a 219mm roller (this is now a Gear Reduction), this ratio is 1:2.968, so we divide our 1833 by 2.968, gives us 617.58Nm on a 219mm Roller.
In the case of a 273mm Roller, our ratio is 650/273 = 2.3809, so we divide the 1833 axle Nm by 2.3809 giving us 769.87Nm roller Torque on our 273mm roller.
Remember the larger roller will be doing less rpm at a given road speed, so the same power will be present.
To simplify this somewhat, both Dyno manufacturers have a term that makes the differences in Roller sizes a non issue, this term is Motive Force on a Mainline Dynolog, or Tractive effort on a Dyno Dynamics. Motive Force/Tractive Effort is a Calculation (this is where some dyno operators get it wrong, they believe the dyno measures Tractive effort).
Motive Force/Tractive Effort is calculated as such, Roller Torque/Roller Radius, so in our example above, we have 617.58Nm/0.1095metres (radius or 219mm roller) giving us 5640 Newtons (not Newton Metres, Newtons are a linear force, Newton Metres are a twisting Force). On our large roller, we have 769.87Nm/0.1365 giving us 5640 Newtons.
Putting this into context for this thread, APS Service Centre’s dyno has 273mm rollers, so their Roller Torque figures will be approx 24.6% higher than say Sonny’s dyno, because Sonny has 219mm Rollers in his dyno, but both dyno’s would show the same Power and Motive Force.
In order to simplfy the Torque values even further, both Dyno Manufactures have what we term “Derived Torque”, which is a calculated value based of RWKW and Engine RPM, this calculated “Derived Torque” takes any gear multiplication out of the equation, so one very good way of using Derived Torque is, if you have a particular car that had 3.46 diff gears, and has now been changed to 3.7, the Derived Torque value will not be affected, whereas the Roller Torque will, due to the fact the overall gearing of the car has changed.
Derived Torque numbers will be basically your Engine Torque minus Driveline Torque loss, so owners of cars who get a Dyno Printout can more easily come to terms with Derived torque, as they accept that Torque at the wheels will be lower than at the engine, (Derived Torque is a bit of window dressing, or dumbing it down).
On a Mainline Dynolog, the user has 3 available RPM signals available to the software for it to Calculate the Derived Torque, Tacho RPM (ie HT lead, injector trigger etc), OBDII Tacho RPM, or lastly Derived RPM from the Roller Speed. The first 2 options will provide very accurate Derived Torque values, whereas using Derived RPM (which is based on a ratio of vehicle dashboard tacho against roller speed), will not be very accurate on a Automatic car due to the varying slip ratio of a torque convertor, but will be accurate at whatever point the Derived RPM figure was set. Dyno Dynamics also have a version of Derived Torque, I believe it is just called RWNm or just Torque Nm on a Graph when printed, I’m also led to believe that the DD version also only uses a derived RPM number for the Derived Torque, not an actual true Engine RPM value, but I could be wrong, and if someone can clarify this, I’m happy to be corrected.
While I suggest to users of our system to use Derived Torque on Reports given to customers, they can do as they please, and a lot just use Roller Torque. It only takes a few seconds to choose to display Derived Torque. All of our reports do report Motive Force though, so for a comparative point of view to a Dyno Dynamics Graph, compare the Tractive Effort to Motive Force on a Mainline Dynolog.
Now a something to remember, the Motive Force or Torque, has the exact same shape curve when compared on a similar Graph Scale.
Another benefit of displaying Derived Torque, if you are a Horsepower and Foot Pounds of Torque junky, instead of kW and Nm, the Derived Torque and Horsepower will cross at 5252rpm, (just like it does on an Engine Dyno)."
Taken from LS1.com
Interesting read about dynos.
