Stupid question but had to ask-hp and torque

In real world terms what does horsepower and torque mean? I hear trq is the most important denominator but what benefits with higher trq and hp and its differences between the two? Thanks in advance. Regards. Mike

In real world terms a grossly simplified version: torque = acceleration horsepower = top speed Most people talk about horsepower but I believe the shove from mashing down on the gas pedal is more indicative of torque. For some reason, on the street or at lunch nobody seems to ask "how much torque does your car have"? I don't know why. All these numbers must be taken in the context of weight, gear ratio, torque/powerband curves, etc ad nauseaum.

Horsepower = (Torque x rpm)/5252 Torque (work) is a measure of force. Think of torque as a twisting force. Torque from an engine is the force that moves a car down the road. Horsepower is a measure of how fast the work is done. It takes more power to move the car one mile in one minute than it does to move it in two minutes. Same work involved but done faster with more hp. A car's torque is measured on a dynometer in function of rpm. Here you see the characteristic curve, from which, with the above formula, the available hp for any given rpm can be derived. The curve usually rises slowly, peaks and then sinks slowly. From this two characteristics are usually quoted: Max. Torque value (and the associated rpm) and Max. HP value (and the associated rpm). The old Porsches (911S, etc) had a very peaky torque curve. You had to rev the car up to about 4000 rpm to get anything to happen, then it really took off. This is also the characteristic of racing engines. Today, the newer models, 993 and 996, have a flatter curve, so that low-end torque is available for a better response.

Heay 993RS, Excellent explination! I especially like: "Porsche Driving is life! Anything that happens before or after is just waiting!"

What always confuses me is that torque is often quoted as the thing that gives you the "shove in the back" feeling of acceleration, although it's always power that's quoted, and in particular the power to weight ratio (bhp/ton) is almost always quoted as the ultimate indicator of performance.... <img src="confused.gif" border="0">

Greg, As you can see from the formula, torque and power are directly proportional. When people speak of "torque", they mean the torque band. If the torque band is flattish, you get a more linear acceleration as you accelerate (going from low revs to high revs). Some of the old racing cars, like the 934, had such a peaky torque band, that they acclerated like a rubber band (very little initially then whamo!). These cars would initially have trouble accelerating from a red light even when racing your wife's Rabbit. Once the high revs are hit, then they go like a bat out of hell, though...

"What always confuses me is that torque is often quoted as the thing that gives you the "shove in the back" feeling of acceleration, although it's always power that's quoted, and in particular the power to weight ratio (bhp/ton) is almost always quoted as the ultimate indicator of performance...." Well, don't fall for it! The power-to-weight ratio is a made up measurement in an attempt to come up with a parity comparison between cars. But it's not very accurate at predicting any kind of real-world performance (it's kinda like the PE in predicting stock-price performance; one step removed from the real thing) Why? Simply because quoted HP occurs but at a single RPM range, whereas when you drive your car, the RPMs change constantly. Also to expand a bit more on the torque/HP equation above, one is an integral function of the other with respect to time (torque is then the derivative of HP). Which means that if you do a freeze-frame snapshot of your trip down the dragstrip or around a racetrack, the instaneous push on your car at any given point in time is what counts. To compute this linear thrust, take torque at the engine, divide/multiply it through the gearing. Then divide out the lever-arm of 1/2 the diameter of the rear tires and you'll have a linear force/thrust at the rear axle. Plug into F=ma and you'll have your instantaneous acceleration. Compute this for each and very RPM in the range, and you'll have an acceleration graph of your car at each RPM. Integrate this acceleration graph with respect to time and you'll have velocity. Integrate one more time and you'll have distance covered. So what really matters is the shape of torque curve. The area under the curve is then total thrust you'll have in that RPM range. Tthe higher and wider the curve, the faster your car will be. So what gets you down the dragstrip or straightaway faster than the other guy is then: TORQUE !!! P.S. also if you go back to F=ma, you'll see that by lowering mass, you'll have greater acceleration as well...

[quote]Originally posted by 996FLT6: <strong>In real world terms </strong><hr></blockquote>Yowsa!

Not sure I understand this thing about the area under the torque vs RPM curve. As far as I can tell, there is no relation between that quantity and anything easily measurable. The only easily derivable equation seems to be the following: Acceleration_time(from v_1 to v_2) ~ proportional to ~ Mass * Integral{from v_1 to v_2} [ dv/Torque(v) ] assuming you are in the fixed gear and the velocity v is the same as RPM x wheel radius (no tire slip). In other words, the acceleration time from velocity v_1 to velocity v_2 is directly proportional to mass (rotating mass should be counted with extra factors). It is also directly proportional to the are under *inverse* torque vs RPM (same as velocity) curve.

Duuuh...What? What I understand: It's all about torque. What I don't understand: What's horsepower good for, and why is it the first thing mentioned about engines?

Torque is a measure of twisting force. Horsepower is a measure of the rate that energy is being used or supplied. One of the functions or a car's wheel is to be a force conversion device, converting the twisting force (from the shaft) to linear force (at the contact patch and the wheel bearing). When the linear force exceeds the rolling resistance and the air resistance, the car will accelerate. The larger the force, the greater the acceleration. For the car to continue to accelerate, the force needs to be applied continuosly. Horsepower is force times speed (or torque times RPM). <img src="graemlins/yltype.gif" border="0" alt="[typing]" />

So 993 RS, If HP= TorqueXrpm/5252, aren't torque and HP always equal at 5252 RPM? If you want the best acceleration, do you shift to keep the rpm's at the torque peak, or do you shift to keep the rpms at the horsepower peak? The shift point would be significantly lower than the redline in the 996tt.AS

For best acceleration you redline. Anytime you shift you lose revs and hp. Sure, after the max hp rpm, the hp decreases at higher rpm, but it is still higher than the hp when you shift upwards...

993RS, Thanks for the answer, but I guess I'm a little slow. If torque is the factor leading to acceleration, and if the torque peak is lower than the redline (as it must be in any car with a redline over 5252), shouldn't peak acceleration occur at peak torque and not peak HP. Top speed can only occur at peak hp (with gearing to match), but it puzzles me that acceleration should as well. I remember a test of an early 80's Corvette where the testers commented that shifting above 4400 rpm did not improve acceleration. I wonder in a tt what the 0-100 time difference would be if one shifted to catch 4000 rpm in the next gear(that would be a shift at about 6000). Thanks for the input. AS

You have to bring in gearing as well in this scenario. Let's say you accelerate to redline in 3rd and from 5500-6500rpm, the torque is well past the peak and is dropping. The equivalent RPM in 4th may be 4000rpm and torque is rising towards its peak. However, you have to multiply the torque through the gear-ratios in each gear. And a lower torque @ 60000rpm multiplied by 3rd gear may actually be more than the higher torque @4000rpm sent through 4th gear. Yet again, this is based upon a certain type of torque curve. If you have a torque curve that rapidly drops off past the peak, then yes, perhaps upshifting before redline would be better. "What I understand: It's all about torque. What I don't understand: What's horsepower good for, and why is it the first thing mentioned about engines?" Ok, here's another way to look at the relationship between torque and HP. Torque is a measurement of force and HP is a measurement of that force per unit time . For example, let's picture a boxer punching a speed-bag. He hits it with a certain amount of force , say.. 200lbs per punch. If he delivers 2 blows in one second, it's a certain amount of force per unit time , like 400lbs/sec. This is power . If he delivers 4 x 200lb blows in the next second, then he has generated 800lbs/sec of power , or twice as much as before. So going back to the HP & torque equation, each RPM can be considered 2 or 3 punches per revolution (depends upon how many cylinders you have). At a certain rpm, say 2000rpm, you're delivering 6000 punches per minute for a certain amount of power . Now if you double that to 4000rpms, then you're putting out twice as many blows per minute (12000) for twice the power . Simple, eh?

Alexander, Peak Torque gives you the greatest rate of acceleration, when you go beyond peak torque you are still accelerating (but at a lower comparative rate). As mentioned above, if the torque above the torque for a given gear is above that of a lower gear after shifting, then do not downshift (unless, of course, you hit the redline). The Turbo has a pretty flat torque curve over much of the rev bandwidth...