HP vs. Torque

So someone please explain in good English the difference and meaning of these two regarding an automobile. Which is necessarily better for accel.?????

Horsepower> unit of power for measuring the rate at which an engine can perform mechanical work. It is usually abbreviated hp. One horsepower was originally defined as the amount of power required to lift 33,000 pounds 1 foot in 1 minute, or 550 foot-pounds per second. Three different horsepower values are used to quote the performance of an engine: (1) Indicated horsepower is the theoretical efficiency of a reciprocating engine, which is determined from the pressure developed by the cylinders of the engine; (2) brake or shaft horsepower is more commonly used to indicate the practical ability of the engine, or the maximum performance, which is the indicated horsepower minus the power lost through heat, friction, and compression; (3) rated horsepower is the power that an engine or motor can produce efficiently for sustained periods of time.

Torque> a force that tends to rotate or turn things. You generate a torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut. A car engine creates torque and uses it to spin the crankshaft.

Instead of using hp or torque, I would use the power to weight ratio, if you are interested in fast acceleration. The more weight you have, the more power it takes to accelerate it. You want to minimize the weight in order to maximize the acceleration. The following list shows you the horsepower and weight for several high-performance cars and one low-performance car for comparison (the info may not be exact, but close enough to get an idea of what I am saying). I listed the peak horsepower, the weight of the car, the power-to-weight ratio (horsepower divided by the weight), the number of seconds the car takes to accelerate from zero to 60 mph, and the price.


car name> Horsepower, Weight (lbs), Power:Weight, 0-60 mph (seconds), Price
Dodge Viper >450, 3,320, 0.136, 4.1, $66,000
Ferrari 355 F1> 375, 2,975, 0.126, 4.6, $134,000
Shelby Series 1> 320, 2,650, 0.121, 4.4, $108,000
Lotus Esprit V8 >350, 3,045, 0.115, 4.4, $83,000
Chevrolet Corvette> 345, 3,245, 0.106, 4.8, $42,000
Porsche Carrera >300, 2,900, 0.103, 5.0, $70,000
Mitsubishi 3000GT bi-turbo> 320, 3,740, 0.086, 5.8, $45,000
Ford Escort> 110, 2,470, 0.045, 10.9, $12,000

You can see a very definite correlation between the power-to-weight ratio and the 0-to-60 time -- in most cases, a higher ratio indicates a quicker car. If you want a fast car, you want a good power-to-weight ratio. You want lots of power and minimal weight. So the first place to start is by cleaning out your trunk!

:biggrin:

Thank you very much. That puts a lot in prospective. It also proves that one vehicle you missed...(the 2002 Vette Z06) is the best bang for the buck-405, 3,118, 0.129, 3.9, $52,000. Nice formula for acceleration figures. I like it. So HP does have more to do with accel. than TQ? Interesting...

So, are there some known tricks to lightening the load on a 4-dr. Grand Prix? Things outside of the engine, I mean...

And what about torque? Why is it put out there like something special? Obviously, it helps with the vehicle's "getup". But where does it come into play? Is torque really that feeling of being put into your seat?

Torque is the only thing driving the car forward, and HP is a measurement of how fast the engine is moving when it produces the torque.

Torque is force... Force = Mass * Acceleration. F=MA

A Semi produces 700 pounds of torque at like 2000 rpms and has only a couple hundred HP.

A motorcycle produces ~100 pounds torque at 7000 rpms and produces the same HP.

So is the truck accelerating faster? It WOULD be if the M wasn't so disproportionally big on the truck, thus lowering the A by a lot.
If they weighed the same somehow, and the truck could shift instantly through enough gears to keep the tach at about 2000 rpms, then it would slaughter the motorcycle even with the same horsepower.

Weight aside, torque produces the exact same acceleration at any speed. Car A produces 100 pounds @ 2000 rpms and accelerates through those rpms exactly as fast as Car B with 100 pounds @ 4000 rpms does through its peak torque rpms. Same torque, same acceleration - Car B, though, has TWICE the HP.

So more horsepower indicates torque at higher speeds. Typically this means higher HP cars have higher top speeds, but we're talking about engine speed, not wheel speed. With one-speed transmissions, it would be more obvious what each measurement means.

Theoretical Race:

Car1 Car2 (Each car has a one-speed trans)
------------- -------------
torque (lbs/rpms) 200/2000 100/8000
redline (rpms) 10,000 10,000
gearing(mph/1000rpms) 10/1000 10/1000 (this means 100 mph redline limited top speed)
hp very low very high
weight x x

In this race, Car1 would slaughter Car 2 off the line up to 40 or 50 mph when the torque curb on Car1 is really plummeting and on Car2 is really climbing. At 40 or 50 mph, Car2 begins out-accelerating Car1 very drastically. In fact, it reaches 60 mph first, even though Car1 is still in front (an argument for 0-60 times not being as important is time/distance measurements). Car2 will eventually take the lead and easily reach 100 mph at its redline. Car1 will accelerate very slowly through the high mph's, and may not even have enough torque left (enough hp left) to reach 100 mph at all. On the other hand, it is not uncommon for a small car to have enough HP to hold a high speed like 150 mph, but not have enough torque to get to that speed in the first place. Cars like the RSX compensate for this with 6 speed transmissions to keep the rpms in the high HP zones. Getting to the HP zone in a hurry is important for cars like this, so they have short first gears. In this race, Car1 may lose in some perspecives, but it accelerated twice as hard as Car2 ever did, and the driver felt it.

There is lots more to consider in real life though, like weight and gearing. On a race track, HP is better cause you have torque at higher speeds and you can accelerate better at these fluctuating high speeds. On a dragstrip, torque means more cause it gets you going. So, in real life, when dominance is determined in 0~40 mph stoplight races, torque dominates. The perfect engine would produce constant high torque from 1 rpm to the redline - this way acceleration would always be that huge except when shifting gears. HP would mean nothing.

well explained. :smile:

Yes, very good. I got that for sure. Now how about a car like that Saleen S7 (550bhp, 525lbs.-ft), where it has a flat-torque curve. Is that what you're talking about with the any-RPM acceleration?

Yes, very good. I got that for sure. Now how about a car like that Saleen S7 (550bhp, 525lbs.-ft), where it has a flat-torque curve. Is that what you're talking about with the any-RPM acceleration?
I think that is what he was talking about.

It's not even really a curve! Just torque everywhere. Can you imagine? Maybe someone should go test drive one, and let us know...:D

would love to test drive one but I can't afford the suit it would take to fool the dealer into thinking I can afford the car!!!

Yeah, like Nicholas Cage in Gone In 60 Seconds. I assume Ford dealers might have this vehicle? I'm in a city with only 200,000 people, so I doubt I will see one...