Question re corrected torque on dyno graphs

[RMC83]

Hey folks,

I have a question on torque & how it's measured & reported on the dyno graphs. I see most, if not all MRC dyno charts have the following listed; wheel power measured, engine power measured and engine power corrected, however all the charts only seem to have the torque corrected figure & do not have the torque measured figure at the wheels. Does anyone know why?

My car's just been on the dyno, she's currently kicking out 741whp & 1013Nm torque, but i'm struggling to then compare the torque figure with any of MRC's graphs. I believe my torque figure is also the measured figure at the wheels, but i'm wondering whether this 1013Nm would be an apples with apples comparison with the values MRC state & if not, are the losses the same as whp vs bhp?

Any info would be great.

Ps...for clarity...i'm not dealing with mrc. I'm over in malaysia & the dynos over here don't correct for or state engine power, they only measure whp & torque....hence the query re mrc's graphs to see if i can corrolate my results with what you guys in the uk have.

Cheers
Roy

[IanH755]

Hi Ray, the MRC graphs say "Torque (corrected)" as well as "Engine Power (corrected)" so my understanding of how the get those two "corrected" figures, which could be very wrong as I'm no expert at all, is this -

Step One - Calculating the HP - Firstly Wheel HP = (Wheel Torque X RPM) / 5252 then MRC allow the dyno tun rundown with the car in Neutral to calculate driveline drag (call it Drag HP). Then the Drag HP is added back to the Wheel HP figure to give the Engine Power (corrected) figure.

Step Two - Calculating the Torque - The Wheel Torque is directly sensed by the dyno and that figure is then used to create the Wheel HP figure mentioned above. The dyno software then takes the Drag HP figure as a percentage of the Wheel HP figure and applies the same percentage to the Wheel Torque figure to calculate the Torque (corrected) figure.

So for example using completely made up figures to make the maths nice and easy -

1332 WHP = (1000 Wnm x 7000 rpm) / 5252 then the Drag HP is calculated during rundown in neutral to be 168 Drag HP so -

1332 WHP + 168 Drag HP = 1500 Engine Power (corrected). Now we just need to work out the correction percentage which would be (100% / 1332) x 168 = 12.61%

Then to work out the Torque (corrected) figure - 1000Wnm + 12.61% = 1126nm Torque (corrected). So using those figure the MRC graph would show 1500hp Engine Power (corrected) and 1126nm Torque (corrected).

As to the reason why MRC don't show the Wheel Torque figure and only the calculated one - I guess that while most people are interested in Wheel and Engine HP figures, they only want one engine Torque figure, but thats just a guess on my part.

[welwynnick]

I was going to post something along the same lines, and then realised that I didn't really know, and was curious to see what other people came up with.

As a rule, I think the US uses wheel power and torque, so tyre and transmission losses stay out of the equation and keep things simple. Estimating flywheel power and torque is done by adding a flat rate factor like 18% for example. That's not so good, because it's different for 2WD & 4WD, auto & manual etc. It's also a just a good guess, and people always get confused by net and gross.

The loss % also varies with wheel speed - you can see this on the few dyno charts that do show transmission loss as a separate curve. It's not a straight line, but curves gently upwards. This means if you use the same loss as for power, you will probably get an over-estimated flywheel torque figure.

In Europe we tend to use flywheel figures, which is better overall but more difficult (because you have to measure transmission loss, as Ian describes). When I thought about how rolling roads do this, I started scratching my head a bit. Although MRC do state that they measure wheel power and wheel torque on the dyno, I think there's a problem. The dyno doesn't know the radius of the car's wheels. It needs to know wheel radius in order to calculate wheel torque from the tractive force.

Fundamentally I think the dyno just measures the speed and torque at the roller, which gives you the power vs road speed. If you know the engine speed, you get wheel power vs engine speed. However, the dyno only knows the wheel torque if it also knows the wheel radius. I don't think the dyno takes that into account, but this is where I go out of my depth.

Therefore if you want wheel torque, I THINK the process would have to be:

Measure dyno roller power to get wheel power vs road speed
Measure engine speed to get wheel power vs engine speed
Measure transmission loss to get flywheel power vs engine speed
Divide flywheel power by engine speed to get flywheel torque

So producing flywheel torque would be the last step in the process rather than the first, and it's often the step that doesn't get performed, as flywheel torque is usually the answer you want. Having said that, I'm sure I've seen some dyno charts from THT Performance that give wheel torque (or the equivalent of what that would be at the flywheel).

By the way, there are two ways to convert between power and torque:

Power in HP = Torque in lb-ft x RPM / 5252
Power in HP = Torque in Nm x RPM / 7121

Regards, Nick

[RMC83]

Thanks Ian & Nick.

Yes...i'm not 100% sure on the technical detail either re how wheel torque can be measured without wheel radius, however this is maybe just exactly the same value as the torque exerted on the rollers....so whatever the dyno roller sees, that essentially is your torque at the wheels.

So yeh...looking at the various dyno printouts from mrc, its clear that its always the calculated / corrected torque at the engine that is plotted along with Engine hp. Which is rather interesting, as my dyno graph has wheel hp plotted along with torque & torque value stated must be wheel torque. Which also adds up, as i can calculate what the power at max torque should be based on rpm & the calculation is correct with what is on the graph.

So it seems my car is actually producing some amount of torque...1013Nm @ 3900rpm, which if taking a 15% loss into consideration, would equate to 1165Nm at the engine at 3900rpm.

Cheers
Roy

[welwynnick]

Using the power equation, here's the quick way to get a simple answer - how to get wheel torque:

Look at the flywheel torque curve and see where the peak is - what is the engine speed?

Take the wheel power at the peak torque RPM, and multiply by 7121 / peak torque RPM.

Nick