I dont think there is an easy analogy that will just "turn on the light". Its not an easy concept at all. I just reread this entire thread and modified my veiws slightly again on some finer points that Shadowboy and Tony and Ostrich brought up.

Can any of you computer genious's out ther make a theoretical dyno graph that will show a flat Tq curve of 300 ft/lbs from 1000 to 7000 rpm and then plot the HP line from that. And then do the opposite of a flat HP line of 300 hp from 1000 rpm to 7000 rpm and then back calculate the Tq line from that info and the put it on a graph?

 

What we see here is a linear progression of the HP value. As the TQ is constant and the RPM value obviously cannot skip numbers to a higher value, the graph is straight forward. HP=TQxRPM/5252. This is the power and tq as an engine that operates at 100% VE (see my first post in this thread) throughout its rev band.

This measurement would happen at our perfect engine's crankshaft, or on the dyno of a car that stays in a single gear.

Remember now, that this crankshaft couples to the rest of the powertrain.



This graph is representative of the forces seen at the contact patch. If the RPM value is thought of as how fast our wheel is spinning, and we know that the HP value does not change (half the shaft speed, double the TQ). Whats the problem with this graph? The engine in this example is running at the same RPM, here it is our transmission that is magical or CVT, I can't tell.

Let's apply KISS (keep it simple stupid) here. F=m*a. From the above TQ numbers we find the force by dividing the torque by the distance from the axis of the wheel to its outer most edge, a constant. We now have a force, and our theroretical vehicle's mass is constant. From this it is easy to calc our accleration, for this example our mass is 1200 and the radius is 1.5.



Now, here is where it gets interesting, in the first example, we had an engine, in the second, an engine operating at constant RPM indicating a decreasing TQ number at the wheels. Let's combine the 2.

*thinks for a few minutes on how to do this*

Lets take the HP numbers generated by the first graph, and arbitraraly select two final drive ratio's (FDR), 3 and 4, or a low gear and a high gear. Our formula looks like this then

Here is the graph of those 2



From the equation F=m*a we can determine accleration as previously calculated, and for a constant value of torque, we would have constant acceleration.


Did I screw it up? Probably, but at least I gave it a shot.

 

So does a chassis dyno measure torque or horsepower?

 

On an engine dyno, it's simple. On a chassis dyno it's a little more complicated. I think to say that a chassis dyno simply measures torque and does the math is an oversimplification. Think about it. If that was the case, then putting a car in a lower (numerically higher) gear would result in higher readings since the torque is multiplied to the rollers through the gear reduction. And that isn't he case with the chassis dyno. In order to illiminate the gear ratio variable you have to introduce time and distance, which points to horsepower.

Any other thoughts?

(maybe this should be it's own thread, but I think it fit's into the tq/hp discussion)

 

Damn dude..you fucking rock. Good stuff there.
The only question is...why is the graph on the constant HP example non linear? It should be the inverse of the TQ graph shouldnt it?

Anyways...the constant TQ graph at the top is a great example to look at. Even though HP rises sharply...the car that this motor was in would accelerate at a constant rate from 1000 to 7000 rpm.

I think utilizing HP figures are much more useful for something like an airplane that will operate usually at X rpm and stay fairly constant to get thier work done. The HP # in that case could tell you alot about the potential of the airplane. Where as an accelerating engine in a car its just much easier for me to look at the TQ in the entire rpm band to see what the acceleration may be like.

 

Depends on wether its a dynojet or something like a mustang dyno. Mustang dynos and Land & Sea dynoes have some type of load cell that does DIRECTLY MEASURE TQ. Then it uses the rpm pickup to calculate the HP figure.

Dynojet claims on thier website that they are measuring HP. But this isnt really true. All they have done is calculated a standard back at the factory based on the weight and diameter of thier drum. They probably have some type of system that applies a known TQ to each drum to accelerate it and then they can calculate the software calibration for that system.

 

Airplane or even a boat, both of which operate at a nearly constant RPM range for most industrial applications (ie large ships, ect...). However, most of the heavy industrial engines have such low engine operating speeds, it would be difficult to show on a graph what their true powerband would look like. I'm not an expert in the field or anything, but it would be interesting to see what a really low RPM tug or heavy cargo diesel motor's curve looks like.

 

Al,

Actually Dynojet is correct.

All they are is measuring horsepower.

Horsepower as a unit of how much work was done. They know how much it weighs, they know how long it took to spin it up to the speed that they know, and from there they can calculate the amount of work (horsepower) done, and where it was done.

This is why you need an RPM reading on an engine to get a correct torque reading.

how the actual force being applied to the roller is torque, but the roller is not measuring the instantaneous rate of change and the torque being applied to it. It only has to measure the amount of time it takes to spin the known mass up, and with that it calculates horsepower.