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Chassis dyno Power and Torque figures.
There seems to be a lot of confusion over rear +whp and +fhp. I get queried nearly daily at work about what torque does the car make?
A chassis dyno measures power and torque at the wheels,
torque and power at the engine is measured at the flywheel.
Manufacturers specify flywheel torque and power figures.
The difference between chassis dyno's torque figures and flywheels dyno's torque figures can be calculated as the difference between engine rpm and dyno roller rpm.
"eg; A vehicle running on the dyno at 100km per hour rotating the engine at 2000 rpm may be rotating the rollers at 1000 rpm"
In this case as the speed is half then the torque is double, so halve the dyno torque reading to get the engine torque in this example. If our dyno torque at the rear wheels is 500Nm then the engine torque is 250Nm.
Where this becomes confusing to people is they seem to assume the gearbox is the only torque multiplier and that if the gearbox is in 1:1, commonly 4th gear, then the torque is more accurate.
Unfortunately this is not the case, there are other torque multipliers that need to be factored in, for example the differential ratio and even the tire size.
I was recently questioned regarding the validity of the high torque figures generated using 3rd gear on the dyno rather than those of 4th as it would appear people are making comparisons between vehicles based on rear wheel torque. It is true the torque figures in 3rd gear are higher in
but I don’t make it that way, the laws of physics do.
Rear wheel torque figures are not comparable from vehicle to vehicle regardless of the gear unless the gear, tire size, pressure, tie down method an diff ratio; are all the same on the same dyno.
Torque figures in 3rd will be higher than 4th, by the amount of what the difference in gear ratios is.
So if 4th is 1:1 and 3rd is 0.75:1 then the torque will be 25 % higher in 3rd than 4th, and the speed will be 25% lower if everything else remains the same.
Another example would be if two cars came in, both TD42 engines. Car one has 35 inch tyres and 3.9:1 diff ratio. Car two has 4.88 and 31 inch tyres. So at 100 kph car one is doing 2440 rpm and car two is doing 3438 rpm at the same 100 kph.
(let’s assume the guy with 4.88:1 normally offroads with 37 inch tyres but for the road and a dyno has his street legal 31’s).
So in this example the difference in ratio is more than 70%, a greater difference in ratio than the gearbox between 3rd and 4th. For the fairest comparison between these two cars then the car with the 35’s and 3:9.1 should be run in 3rd gear and the car with 4.88:1 should be run in 4th to make a closer comparison between the two as the total drivetrain ratio is most similar.
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