How is HP/Torque calculated?

[Steve M]

Reading torque at the flywheel with a motor on a stand is easy to understand but, putting a truck on a dyno gets me confused. With gear reduction playing into the picture, I would think torque would be way up there. Even with my 460 ft/lb motor, multiply that by 4. 10 and it's already up to 1886 ft/lbs at the rear axle. If I run in O/D reduce that by . 69 and it's still 1301 ft/lbs. I realize that tire diameter is a big factor and it's a torque reduction O/D effect. Put the truck in 1st gear, especially with the manual granny gear, and the torque numbers at the rear wheels should be pretty scarey. I've never put my truck on a dyno. All I know is it tows my 5th easier in lower gears and hauls axx empty. But it would seem the V-10 would have the performance edge even over the HO Cummins with it's ability to use direct (3rd) with a 4. 10 ratio while the Cummins has to use O/D at 70mph. Knock off 30% for the Cummins while the V-10 puts 100% out the tailshaft. Thats 450 ft/lbs at the tailshaft vs (555 X . 69)= 382 ft/lbs. What am I missing here fellers?

 

[wsmalley]

Work backwards....

Basically, a chassis dyno like the DynoJet works the other way.



You have two drums that each way something like 3500 pounds. The Dyno starts by measuring just how quickly your vehicle gets the drums spinning (from X rpm to Y rpm), and figures out the HP from that. THEN, using an external sensor to measure your engine RPM, it goes BACK and calculates torque from engine speed and the HP. It's as simple as that.



Will

 

[78repowerwagon]

The dynojet knows how fast the motor is turning and how fast the drums are turning. With these two figures it calculates final drive ratio reguardless of what gear the trans is in axle ratio and tire size.

It will virtually repeat the same curve weather dome in second or OD. The higher the gear the less choppy the graph readout.

 

[Steve M]

So if I'm understanding right, the dyno is calculating "engine torque and HP" and not the actual torque of the rear wheels turning the drum? If it were reading simple torque and HP of the rear wheels without regard to what the engine rpm is or what gear ratio it's going through, we would see some serious low rear wheel rpm torque with low HP due to gear reduction (torque multiplication) and then lower torque in steps as gears change up to O/D where we'd see higher HP but less torque? If this is the case, a CTD in granny gear with 4. 10 ratio and stock size tires and 4-Low would put some real torque to the tires at creep speed! I'd like to see the actual torque next to the formulated torque and HP on the readouts. I realize it's the motor that's making the power and the focus of the dyno readout but it'd also be interesting to see what the actual numbers are at the rear wheels.