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10192007, 03:10 PM  #23 
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10192007, 03:27 PM  #26 
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Torque vs Horsepower
The Basics That shove in the back when you mash the throttle is your body accelerating forwards. Acceleration is merely changing velocity over a period of time in a particular direction. 0 mph to 60 mph in 5 seconds. 100 to 150 in 10 seconds. You don't have to accelerate forwards either, as going from 100 mph to 0 mph over 5 seconds is also considered an acceleration (just in a rearward direction). We'll save angular acceleration for another day (cornering forces). What causes acceleration? Well, lets get really simple. Newton's second law (a widely accepted one I might add) states: 1. Force = Mass x Acceleration (F=MA) If you subject an object to a particular acceleration, it will "feel" a particular force. Double the acceleration, double the force. Rearranging the equation provides: 2. Acceleration = Force / Mass (A= F/M) Double the force acting on an object and you double the acceleration in that direction. Double the mass, but keep the same force, and the acceleration is HALVED. More mass is always harder to accelerate. The mechanical equation for "motive power" at a single instant in time is: 3. Power = Force x Velocity (P=FV) ...rearranged we get: 4. Force = Power / Velocity (F=P/V) Lets take equations 1 and 4 and substitute them together to get: 5. MA = P/V ...rearranged: 6. Acceleration = Power / (Mass x Velocity) It is this basic equation that you should know. Your acceleration (the rate at which your speed changes) is directly proportional to how much power you are putting down, and inversely proportional to your mass and/or velocity. That is, if your power goes up, your acceleration goes up. If your mass or velocity go up, your acceleration goes down. Period. End of story. Learn it, know it, love it, suck the wind out of a party with it.... What about TORQUE????? Good question. First off, what is it? Torque is when you have a force applied on a moment arm. The moment arm can be small (screwing something in with a screw driver) or large (tightening a bolt with a socket wrench). The piston of an engine is pushed downwards during it's power stroke, applying a torque on the crankshaft. This can happen slowly (low RPM) or quickly (high RPM). In an ideal world, an engine would produce the same amount of torque at all engine speeds. This is RARELY the case, but for this argument we will assume it to be true. Imagine our fictional engine, operating at full load at 1000 RPM. All the pistons working together to turn that crankshaft. Now lets spin the engine up to 4000 RPM. Each piston is exerting the SAME amount of force on the crankshaft as before. The engine's torque has not changed, BUT the rate at which that torque is made has now quadrupled. Over the same period of time, the engine is producing the SAME amount of torque, but four times as quickly. How do we quantify this and what does it mean? The relationship between POWER and TORQUE: 7. Power = Torque / Time Double the amount of torque over a certain amount of time and the power is also doubled. Cut the time that is required for a certain amount of torque to be made in half and the power is again doubled. Looking at our engine example above, when spun at a higher speed, while the torque didn't change, the rate most certainly did. In fact, the torque was made four times quicker, or in 1/4 of the time. That amounts to FOUR times the power. Does that mean FOUR times the acceleration??? Assuming the initial velocity is the same in both scenarios, you betcha. Same torque, not the same acceleration. Interesting... The power equation that is most common is: 8. P = T x N / 5252 where P is power in HP, T is torque in ftlb, and N is engine speed in RPM. Note that this equation is identical to equation 7, just fixed to account for crazy customary units. Hold the phone...I've seen a dyno plot of my engine, and it makes TWICE the power at 8,000 RPM than it does at 4,000 RPM! When I'm accelerating, how come I don't feel twice the acceleation? Simple, While the power you are making is double at the higher engine speed, your velocity has also doubled, effectively canceling out any change in acceleration. Check equation 6. Interestingly enough, if your engine has a perfectly flat torque curve, you will accelerate at the exact same rate at all speeds in any single gear. Power and velocity go up...acceleration stays the same. Neat. Ok...I'm following you. So how can I accelerate faster at a certain speed???? How can I get more power? How can I get more acceleration? Well, without increasing the amount of torque that your engine makes (through supercharging, turbocharging, or any other form of modding) you are stuck with the acceleration that you have in each gear. Hmm...in each gear... Gearing Gearing is when you take a rotating shaft and change it's speed to better suit the application. Take first gear. Imagine trying to get your car rolling if it were equipped with a directdrive transmission (no gearing...the engine turns at the exact same rate as the wheels). Even with a gentle launch, your wheels would have to spin at about 1000 RPM just to meet the idle. Thats rediculous. This is why we have gearing reduction. The wheels rotate about 15 times slower than the engine in my car in first gear. About 10 times in second, 7 in third and so on and so forth. In sixth gear, the wheels actually rotate FASTER than the engine. This is called overdrive, but thats not important. Because my wheels rotate 15 times slower than the engine, I can keep my revs around 10001500 RPM in first and the wheels are only rotating at about 6070 RPM (one rotation per second). Back from my tangent...My first gear tops out at about 43 mph. Lets assume I have that perfectly ideal engine and my torque curve is flat. That means I'm making my peak horsepower at my peak engine speed. Or in first gear, right at 43 mph. At ANY speed below that, I'm making less than my peak horsepower. Lets say I installed a new rearend with a higher final drive ratio...say I take my stock 4.10 gears and replace them with 4.77s. My first gear now tops out at 37 MPH. My peak power is at 37 MPH, in fact, at any speed between 0 and 37 mph, my car is now making MORE power than it did with the stock gears. The same holds true for each of the next 5 gears. At any given speed, in any given gear, because of better gearing the car is putting more power to the ground. That is gearing in a nutshell. How can that little Honda accelerate so quickly with so little horsepower? Gearing. Plain and simple. Gearing. Take two cars, both with the magical "ideal engine" (flat torque curve) and identical weights: Car A  300 lbft of torque with a 4,500 RPM redline. 257 max HP @ 4,500 RPM Car B  150 lbft of torque with a 9,000 RPM redline. 257 max HP @ 9,000 RPM If both cars were geared identically, Car A would demolish Car B. But, if geared so that each car maxed its gears out at the same speeds, while Car B would be making a lot less torque, the engine speeds are so much higher. Both engines would share the exact same power curve. Both cars would accelerate at the same rate. The downside is that Car B is operating at a much higher engine speed for most of the time... I just read this whole post and I'm not sure what I learned... Power is what causes acceleration. More power = more acceleration. Period. Engines make power through a combination of Torque and Engine Speed. You can have a lot of one and not a lot of the other and still make big power. Still, when it all boils down to what is actually causing you to accelerate, its POWER POWER POWER. Sure you need torque, sure you need engine RPM, but POWER is the only factor that directly relates to acceleration.
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10192007, 05:13 PM  #28 
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10192007, 06:40 PM  #29 
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Hopefully it will remain open for some time so people can learn and the myths stop spreading.
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10192007, 11:18 PM  #30 
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OP asked to keep it non technical. Basically:
at 2000rpms, you car will generate as much trust as a 325i at it's most powerful. At 3000rpms, your car will generate about as much thrust as a 230hp 328i, at it's most powerful. At 4000rpms, your car will generate about as much thrust as Porsche Boxster. At 5000rpms, your car will generate about as much thrust as an E46 M3. At 5800rpms, you will generate your peak power, slightly more than an E46 M3. Power will delcine from there, with power at 6500 being slightly lower than power at 5000. Here is a Dyno of the N54. While torque remains constant, your power curve will climb at a constant rate, as you can see from the straight line. When torque starts to drop off, so does power. Above, my representation of power at a given RPM compared to other cars was based on the N54's WHP at a given RPM, compared to another car's WHP at peak. 325i's generally generate 135140rwhp. 328i's generally generate about 180rwhp. Boxsters usually generate just about 225235rwhp at peak. E46 M3s usually generate somewhere between 265 and 275 rwhp at peak. The N54 usually generates somewhere around 285295rwhp at it's peak, 5800rpms, and then drops off from there, as you can see in the dyno graph. I hope this gives you the idea. Basically, in a drag race, you would be "dead even" with a 328i if you had it floored, and shifted at 3000ish rpms, and he had it floored at shifted at redline, provided of course, that you could shift fast enough to keep him from getting an advantage. Also, it should be noted that this speaks purely to power, and does not take into consideration the gearing of other cars, power/weight, etc. For example, even though an E46 M3 makes less power, and has a lot less lowmidrange thrust, it would still be a challenge, because of it's powerband, and the way it is geared. It makes a lot of power between 6 and 8 thousand rpms, and because the engine is so high reving, BMW could use slightly shorter gears to make the car excellerate better in a given gear, but still reach a high speed at redline. That's what a lot of lowend and midrange torque does. It means you can accelerate fast at any RPM, and faster than a lot of cars can at redline! :roundel::w00t:
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10202007, 02:04 AM  #31  
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No... torque is not meaningless. The HP on the dyno represents the amount of work done, the POWER of the engine, the torque shows where that work was done...! If that makes any sense. Some could ague that HP doesn't matter and you could choose a car based on the type of torque curve you like. They are equally important, and trying not to loose the scope of this conversation. Remember, we're keeping this laymanistic. Garrett 

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10202007, 02:14 AM  #32  
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Exactly, but you can also flip flop that equation and as you increase you RPMS faster with a lighter, highreving engine, you can acheive that particuliar cars toque figure quicker. That even though one car has more torque down low, another one might trade torque for RPMS and still end up with the same HP and acceleration. You looking at it as trying to stop a mass, think about it the exact opposite, your trying to accelerate it. So if it can push that 250lb guy around a pull from a standstill, the torque required to start him moving and accelerating him is torque, the Horsepower is the force maintaining that force. Garrett 

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10202007, 02:46 PM  #33  
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10202007, 04:02 PM  #34 
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lol, I love it when people who don't know what they are talking about pretend they do.
saying torque is meaningless, wow. I really hope you are not an engineer. 
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10222007, 08:15 PM  #35 
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Hi Everyone, First Post.
As an analogy, torque is how hard you push, hp is how many times you can push per unit of time. Also, people like torque, but what they really like is called a "jerk" ... the rate of accelleration of the rate of accelleration. :biggrin: 
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10232007, 10:56 AM  #36 
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Saying torque is meaningless is only half true. Torque is nothing without engine speed, and vice versa, engine speed is nothing without torque. Torque does not accelerate you. Ever. Torque enacted over a certain time period does. But then you are talking about power, and not torque anymore.
I tried to keep my post above in laymans terms, and frankly, its all really simple when you step back and read it twice. The fact of the matter is, you can't explain the relationship between torque and horsepower without getting slightly techincal. And, frankly, if you don't understand the techincality, you shouldn't be explaining the relationship.:biggrin:
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