Measuring Engine Potential: The Importance of Area Under a Power Curve

[ethies]

Peak horsepower and peak torque are frequently quoted as estimation of a vehichle's power. However in reading performance testing data I've seen many instances where this is highly misleading. The Porsche 911 Turbo (with some super special so and so package) produces less peak torque, less peak horsepower, and weighs more than a Dodge Viper. However, the 911 was quicker to 60, and quicker in the 1/4 mile. The reason for this was that while the Viper had higher peak horsepower and torque, the 911 spent more of its RPM band at higher torque and power levels without having a sharp peak (flatter power and torque curves). Wouldn't then the best measurement of overall engine potential be to measure the area under the power curve or the area under the torque curve? What would the integration of work or power be called? Thanks, Eric

 

[Gza]

integrating power on time gives you work, since power is defined as dW/dt

 

[krab]

No. It's not mainly because of power considerations. Both of these vehicles have enough to break traction, however the Viper does so much more easily because it is RWD with a heavy engine in front. Compare the 0-30 times. The Porsche is way ahead. It pulls 0.98g acceleration (0-30mph in 1.4 sec), while the Viper is in the 0.6 to 0.7g range (0-60mph in 4.4sec). My little Toyota MR2, with 115 hp can almost keep up with the mighty Viper off the line because it has the engine weight placed optimally just ahead of the rear wheels.

 

[Chen]

Roughly speaking, power over time is work (the amount of energy you put into the system), and work over time is impulse (the change in momentum of the system).

 

[ethies]

Alright, krab, I'm sure that engine placement has a signifigant effect on the times; but having seen the torque and power curves, I know that the 911 is producing more horsepower than the Viper through most of its rpm band. If this particular example is troublesome simply think of two theoretical cars that have engines similar in nature to the viper and the 911 mounted similarly to each other. I probably should have thought that through better before asking the whole integration of dW/dt thing as power, but as Chen said (I didn't think of it this way) change in work over time is impulse (the change of momentum of the system) so wouldn't impulse be a usefull number for rating an engine's capability? Thanks all

 

[Gza]

and work over time is impulse (the change in momentum of the system)

I thought that force over time was impulse. Or maybe you have a different way of thinking about it involving work, and I just haven't thought it through deeply enough. I would be interested in seeing how you got that.

 

[krab]

so wouldn't impulse be a usefull number for rating an engine's capability? Thanks all

No. Given optimal traction, the only real yardstick is horsepower. To get really precise, it is the average of the horsepower over the rpm range used. More gears is better because that allows one to stay closer to the hp peak. That which is called "torque" is actually torque at the crank. It has no relevance in the calculation.

 

[Chen]

I thought that force over time was impulse. Or maybe you have a different way of thinking about it involving work, and I just haven't thought it through deeply enough. I would be interested in seeing how you got that.

Obviously you're right, I mixed it up...

 

[Cliff_J]

The average horsepower over time is one reason why the fastest drag racing cars in the world use specialized slipper clutches to allow their engines to spin at maximum horsepower and attempt to transfer as much of that as possible to the tires. Then the clutch is setup to slip most of the way down the track and the engine power is backed down to avoid spinning the tires once the clutch stops slipping near the end of the track.

At a more real-world level, guys with late-model Camaros can replace their torque converters in their cars with a higher-stall model and can instantly jump from a 13.5 to a 12.5 in the quarter mile. The higher stall allows the engine to maintain a higher average horsepower number from the higher average revs.

But the Porsche 911 Turbo has always done well for acceleration with AWD and the rear-weight bias to improve traction. Plus it handles well, go figure. :)

Cliff

P.S. In answer to the original question, publications like Popular Hot Rodding will score engine building competitors based on average horsepower and torque over specified RPM ranges.