Torque of Car Engine

Torque is very important to people who are interested in buying a new car, especially people who are interested in larger pickup trucks and sport utility vehicles. Vehicles which are going to be used for towing large loads or climbing steep hills, torque is often more important than horsepower. Scientifically, torque is the moment of force along an axis. In automobiles with internal combustion engines, torque is related to the twisting force that is applied to the crankshaft. This force is caused by the pressure that is produced from a burning fuel mixture. The pressure sequentially forces each piston into its bore. The pistons revolve around the crankshaft, which powers the vehicle. This means that torque helps a car take off from a stop and pull weight up a hill. The torque rating determines how much force an automobile can apply to the road.
Torque is proportional to the stroke which describes the diameter of rotations around the crankshaft. The stroke depends on how far the connecting rod journal is offset from the centerline of the crankshaft. When the journal is farther from the centerline, the stroke is longer, which produces a greater torque. Racecars have a very short stroke which results in a large amount of horsepower at high RPMs, but a small amount of torque is generated by the engine. But the torque is not important to racecar drivers since they begin their races already in motion, so there is no taking off from a start. On the other hand, Diesel engines have a very long stroke, which is why most construction vehicles use a diesel engine.
Many people base their opinions of car on the amount of horsepower it has. But, the Torque rating tells you more about the engine. Horsepower is more relevant to top speed, not necessarily acceleration. The feeling of your eyes going into the back of your head when you are accelerating is the feeling excessive torque produces. Again, a muscle car and a dump truck can have very similar rating is both of these categories, so, dividing these values by the vehicle weight will give a more accurate value for the acceleration and performance of a vehicle. A torque rating is defined in foot-pounds. Torque ratings are usually high for a big and powerful truck or SUV, and low for a smaller sedan. High torque engines respond easier to the accelerator pedal, while low torque engines usually need to start in a lower gear and increase the revolutions per minute, in order to attain the maximum power. Some of this torque that is produced does not get all the way to the wheels. Some of it is lost in the transmission. Different amounts of power are needed for rear-wheel, front-wheel, and all-wheel drive vehicles. You can easily calculate the horsepower of an engine with a simple conversion: (Torque x Engine speed) / 5,252 = Horsepower. SO an engine which puts out 300 foot pounds of torque at 4,000 rpms would produce 228 horsepower. ({300 ft-lbs x 4,000rpm}/5252 = 228.48)
An engine reaches its peak torque range at different rpms. This can help a person choose the right car. For cars being used around town and on the highway at moderate speeds, they should have a peak torque range of 1800-2500 rpms. For more heavy duty work, such as towing and carrying heavy cargo, a peak torque range of 4000-5500 rpms would be more suitable.



Sources:
http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/fpte5.htm
http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/fpte5.htm
http://ezinearticles.com/?Torque-and-Horsepower-Ratings---How-They-Affect-Performance-and-Their-Relevance-in-Comparing-Models&id=838515
http://www.giant.net.au/users/wight/Ignition%20Timing%20Background.htm