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Hp Vs Torque

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Here is some HP vs Torque info you may find interesting (most of this is from a newspaper article by Gerry Malloy in the Toronto Star on Saturday, August 30, 2003):

The fact is, horsepower ratings, perhaps the most quoted of all vehicle specifications, are almost useless as a definitive measure of a vehicle's real-world performance -- unless your real world is a drag strip. What really dictates the performance of a vehicle is the engine's torque output.

Power is the rate of doing work. 1 horsepower equals 550 ft-lb per second. In other words, for a car, power is simply the rate at which torque is delivered.

While an engine's torque can be directly measured on a dynamometer, its power is calculated. As an equation HP = Torque x Engine Speed x some numerical constant (1/5252, in imperial units). Thus, power is always a function of torque and engine speed.

Here is the full equation example that calculates HP for an RX300 engine that delivers 200 ft-lb torque at 5800 rpm:

HP = (200 ft-lb) x (5800 rev/min) x (min/60 sec) x (2 * PI rad/rev) x (sec/550 ft-lb) = 220.86 HP

Power is not measured directly, but is calculated from the torque curve, using the above equation, to create a corresponding power curve, from which the maximum power rating is derived. The real importance of that

relationship is that power and torque are directly and absolutely related, and power is a function of torque -- not the other way around.

Alas, all we have to work with in most cases are the maximum power and torque ratings, which can be misleading...very misleading.

Take, for example, two cars with identical 180 hp ratings. One achieves that peak at 7600 rpm, with a torque peak at 6800 rpm; the other achieves its power peak at 5500 rpm, with a torque peak at 1950 rpm.

The former, like many racing cars, derives its power rating primarily from the speed, not the torque side of the equation. At moderate engine speeds, in the 2000-4000 rpm range, where most on-road driving is done, it is a

torqueless wonder.

That means to make it respond even adequately, you have to keep the engine running in the top of its rev range, and that means you are constantly stirring gears. While that may be acceptable in a race car, the novelty

wears off quickly on the street.

The lower peak-torque speed and fatter torque curve of the latter example provides a more tractable package, and one with better real-world performance. That advantage would be readily apparent if you could simply

compare engine torque curves.

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He's absolutely correct. Hence the reason why my race car has 180lbs feet of torque, and 120hps. I only need horsepwr on straight aways.

I believe the reason why so many people use HP instead of Torque is because we in North America have more flat, straight roads instead of Twistees. :)

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