dustin
UAIOE
All you ever wanted to know about Horsepower and Torque.
DISCLAIMER: This article assumes that you know at least a basic working knowledge of Internal Combustion Engines, mechanical advantage, and gear ratios.
Here is an example I will build off for this article
2 identical cars, identical drivers, EXACTLY equal in weight, gearing, etc…
Car A is powered by a 200hp V6 generating 260 ft/lbs of torque, 5000 RPM redline
Car B is powered by a 200hp I4 generating 166 ft/lbs of torque, 7000 RPM redline
In a ¼ mile race, both cars would theoretically finish DEAD EVEN!
Car A would jump to an early lead, but car B would gain momentum and slowly catch up.
Let's start off with some definitions.
RPM - Engine Speed measured in crankshaft revolutions per minute
Torque - A measure of rotational force
Horsepower - A measure of torque given a specific RPM
This will all make sense, I promise
Lets start off with Torque. As above, torque is a way of measuring rotational force. One notation commonly used is Ft/Lbs. This leads in to the next key point. Torque is simply a force (Lbs) times a distance (Ft). Think of a lug nut on a car. If it takes 50 Ft/Lbs of torque to loosen the nut, you can apply it several ways. You could apply 50 pounds to a lever one foot long. You could also apply one pound to a lever 50 feet long. Any combination of Force and Distance will apply the same torque as long as F*D=Torque required. This might seem that you are getting something for nothing here, but remember, Torque is rotational force, so that 50 foot lever you will have to move the handle 50 feet to turn the nut just 57 degrees. That one foot lever would only need to be pushed one foot to get that same travel.
You are probably scratching your head, thinking what does this have to do with my engine! It has everything to do with it. Torque is the force that moves your car. When you open that throttle, you feel the torque pushing you in your seat. This is the reason Car A in the example above jumps out to an early lead.
Now, Horsepower. In terms of everyday living, one horsepower is an easy definition. One horsepower of work is equal to lifting 33,000 pounds one foot in one minute. That seems like an awful amount of power, but in reality, using the same idea as before, this can also be like lifting 330 lbs 100 feet in that same minute. This is much more realistic. Now applied to engines, this becomes a very difficult thing to measure or define. Thankfully, torque and horsepower are closely related. Horsepower is known to some simply as Torque at RPM. The formula:
Horsepower = Torque * RPM / 5252
Again, this will all make sense.
One number without the other does not give much information. For example, lets say you have a waterwheel capable of producing 32,000 Ft/Lbs of torque at one RPM. Now, this number looks absolutely astonishing, but in reality, a ride along lawn mower has more than twice the power. How can that be? When you plug 32,000 Ft/Lbs into the Horsepower formula above, you get a tiny 6.09 horsepower. This brings up the topic of gearing.
After this, hopefully all of this will make sense.
All this talk about power and Horsepower and Torque means nothing unless you look at application. An engine making 260 Ft/Lbs at 3,200 RPM does nothing to move the car if there are no gears. Most engines, come off idle at full throttle and generate both increasing torque numbers, and horsepower numbers. When the engine reaches it’s torque peak, the torque produced levels out and starts to decrease, while horsepower continues to climb. The horsepower will continue to rise as long as the RPM’s continue to rise faster than the Torque falls. This, along with the fact that torque is the force that actually moves the car, is the reason for gears. For best performance, the RPM of the engine should be within the “power band”, or between the RPM of the torque peak, and the RPM of the horsepower peak.
Let's look at a four speed automatic from Car A. First gear might be 3:1, second 1.6:1, third 1:1 and fourth 0.7:1. (These numbers came from a 4L60E automatic transmission) For a car with 29 inch rear tires, the car will travel roughly 7.6 feet for every revolution of the rear tires. This yields 695 Revolutions per Mile. Now with a 3.73:1 rear end gear ratio, and a 5000 RPM horsepower peak, this puts top speed in first gear at 40mph. Given the gear ratios, end torque placed to the road is in the neighborhood of 2900 Ft/Lbs. Second gear would top out at about 70 mph with a torque of 1551 Ft/Lbs. Third would end at 115 mph, with a torque of 969.8 lbs. Fourth would be even better at 165 mph with 678 Ft/Lbs of torque at the wheels. These torque numbers might seem high, but remember, dynamometers compensate for gearing. This is the torque produced after the gears.
Now lets look at Car B. The same transmission mated to the engine would yield a 55 mph first gear at 1857 Ft/Lbs. Second would be 101 mph with 990 Ft/Lbs of torque. Third would give 162 mph with 619 Ft/Lbs of torque. Fourth would be an astonishing 231 mph with a mere 433 Ft/Lbs of torque.
Now lets look at these numbers. At first glance it may seem like Car B doesn’t stand a chance. If you look closer, you will see that the entire time Car A is in first, it is accelerating much faster than Car B. But Car A will need to grab second sooner, and loose mechanical advantage. In that 15 mph difference between the two car’s first gears, Car B is actually out accelerating car A.
Once Car B grabs second, Car A will pull ahead slightly. But, just 15mph later, Car A mush shift into third, again cutting his torque. Now car B has an advantage and will catch up to car A. This should lend to a pretty even ¼ mile race, especially considering most cars are going to be geared to suit the engine. Car B would most likely have a steeper first and second, again cutting the difference between the two cars.