Home > Tech Stuff > Tutorials Tutorials This is the technology page of the web site and is dedicated to a series of "edu-torials" (for lack of a better word) that may help you in your quest for speed with your 5.0 liter Mustang. In it we address several aspects of racing that sometimes are unclear. For instance, we're starting out with horsepower versus torque (hot topic). Other topics may include suspension settings and data acquisition for drag racing. So be sure to check back to this page regularly! For more information on building high performance engines, check out the books available at the SlowGT store.
Now for the biggie. Would you rather receive a $500 paycheck every week or a $1,000 paycheck every month? Yep, you're right. Although the individual paychecks (torque numbers) are smaller at $500, they arrive more frequently (at a higher RPM) so that you have more buying power (moving power). So next time you wonder about horsepower versus torque... think about getting your current paycheck, just twice as often.
The amount of torque applied to the rear tires is calculated by multiplying the engine torque by the overall gearing. Car A is driving the tires with 6,960 ft-lbs of torque and Car B is driving it's tires with 9,990 ft-lbs of torque; both at 10 MPH. In this case, at 10 MPH Car B is applying 43.5% more torque to the rear tires (and 43.5% more accelerating force to the ground) WITH THE SAME ENGINE TORQUE! What's the difference between the two cars in this example? Power and gearing! Even though the two engines produce the same maximum torque, the fact that Car B produces it at a higher RPM means it's making more power (see previous article). Although this example is limited to one particular MPH value, it applies to all speeds. If you can gear the car properly, the car making the most power (all other things being equal) is going to accelerate more quickly (at all speeds) and is most likely to win. [ Top of Page ]
Any object (a Mustang included) is accelerated by applying a force to it. In this case, torque from the engine is applied through the rear tires to the ground where it then becomes a force accelerating the car. As we stated above, for all speeds that the car travels during the run, we want the maximum possible force applied to accelerating the car. This means that we want maximum rear wheel torque applied to the ground for all speeds during a quarter-mile run. In other words, we want to maximize the number you get when you multiply the rear wheel torque (which becomes a force where the tire meets the ground) times the speed at which the car is traveling (distance/time) for all speeds you travel during the run. Because force times speed defines power, we have again decided that we want to maximize power applied to the ground by the rear tires throughout the run; at ALL speeds you travel during the run. [ Top of Page ]
Suppose you want an engine that puts out 600 ft-lb of torque. Sounds pretty impressive, doesn't it? Well, let's see. Suppose we removed your engine, put a 3 foot breaker bar on the input shaft to your engine and I sat on it. Correctly assuming that I weight about 200 lbs, you would then have a force of 600 ft-lbs (3 feet x 200 lbs) applied to your drive train. However, that is all that 600 ft-lbs is - a static twisting force and it says nothing about whether you actually moved your car; or, more importantly, whether you moved your car quickly. If I sit on the breaker bar all day long without moving it, I've applied a lot of torque (force) for a long time, but I didn't move your car. So saying that 600 ft-lbs of torque is being applied to your transmission input shaft doesn't tell you anything about whether you've moved your car. OK. Let's say I sat on the breaker bar and it actually moved; it rotated so that I was lowered toward the ground. Now we have done some work. We moved something. Work is defined as force times the distance through which it acts. Or more simply force times distance (f x d). We can view this as the force applied by the rear tire to the ground in accelerating the car multiplied times the distance the car moves. Let's say that I'm strong enough to turn the breaker bar one full revolution while applying 600 ft-lbs of torque (probably not that easy, even with a 3 foot breaker bar). Well, now we're moving the car; we're doing work. But how quickly do you think we'll cover the quarter mile with me turning the transmission input shaft? Maybe in a day or so? That's not impressive. So to this point, we've decided that defining the amount of torque applied to your drive train is not enough. What we really want to do is to move the car and do it quickly. We want to accelerate a car through the quarter mile as quickly as possible. That is to say we want apply a force (engine torque applied through the tire to the ground) to move a car (with motion, the force becomes work) and move it quickly (travel a distance in a short amount of time). In short, we want to maximize the value [(force x distance)/time]. This happens to be the definition of power. We want to maximize power. We want to apply a force to your car (torque applied to the ground through the tire) and have that force actually MOVE the car (the force acting over a distance now can be called work) and we want to do it very quickly; in a short amount of time (work divided by time equals power). In the end, we want to maximize the average power applied by the rear tires to the ground throughout the quarter mile run. Doing this requires torque, but the most important number is power; average power applied to the ground throughout the run. In the next installment, we'll look at this issue from a slightly different perspective and hopefully shed some more light on the subject. [ Top of Page ]
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