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Horsepower


My92TaurusP.O.S
10-30-2004, 02:41 AM
Can anyone tell me what exactly horse power means, what it does, and if its a major factor when picking out a car?

curtis73
10-30-2004, 03:16 AM
It means very little actually. Its very hard to describe (especially for me since I tend to get wordy).

Engines produce torque which is a measurement of twisting force. Torque is technically static, meaning that it is usually measured while not moving. Like opening a jar. The amount of twisting force it takes before the lid moves is torque. Engines produce torque while they're moving its just measured differently.

Engines also make different torque depending on where it is in the RPM range due to several factors. For example, after an engine reaches a certain RPM, the intake or exhuast become restrictive. After that RPM they start losing torque. This is why aftermarket performance parts help power production; by freeing up this flow of air.

Horsepower is mathematically derived from torque. Think of torque as a lead weight on a string. If you swing it around and hit something it exerts a force. To get a force of 100 lbs on what you hit, you can use a little weight and spin it fast, or you can use a heavy weight and swing it slow. Engines work the same way. You can make lots of horsepower out of a little 4-cylinder by spinning it fast, or you can do the same thing by taking a V8 and spinning it slow.

A stark comparison for you. Certain Honda V6s have 215 hp and 195 lb-ft of torque. I had a Ford diesel V8 with the same 215 hp, but 550 lb-ft of torque. The reason is that the honda takes a little weight and spins it to 6000 rpms. The diesel takes a really heavy weight and spins it to 3000 rpms.

Ask the salesman or do the research on torque as well. To boil it all down, if you had a choice between a 4-cylinder engine with 200 hp and a V8 with 200 hp, chances are the V8 would feel stronger.

I don't know if you're a math person, but the equation looks like this. HP = TQ x RPM/5250. All power graphs cross at 5250 rpms since the two would cancel out.

It also depends on the car you're seeking. A light car can benefit from a high-revving little engine (the light weight spinning fast on a string) A big heavy car can benefit from a low-revving torquey engine (the heavy weight swinging slow)

I read back over this and I know its not quite clear, but I hope it helps. If I can I'll post two dyno graphs and maybe the picture will be more descriptive

MagicRat
10-30-2004, 09:57 AM
That is an excellent explanation from curtis.

It is astonishing how many people, even experienced mechanics who do not understand the difference between torque and hp. Even my college auto technician instructor did not know the difference.
Torque is a force, with no movement. Hp is work, that is, force applied over a distance.

Therefore, theoretically, if an engine can produce the SAME amount of torque at 2000 rpm and 4000 rpm, the hp at 4000 rpm is TWICE the hp at 2000 rpm.
As curtis says, in a practical sense, the peak hp number is less important than the power characteristics throughout the powerband.
Small displacement, high revving engines may produce big hp numbers at high revs, but they feel as soggy as day-old cornflakes at low revs, compared to a larger displacement engine.

KC Ron Carter
10-30-2004, 10:21 AM
Horsepower sells cars.

Torque wins races.

Here is a good Dyno run on a Mustang.

http://www.kcrealtime.com/images/John_Dyno.bmp

Later,

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