Horse power - why is it used in this age?

Trifid · 28 Aug 2006 at 22:18

I have never fully understood horse power and today I finally looked into it and I am even more confused as to why it is used today, mainly with this:

Code:

  HP =          Torque (lb. -in.) x RPM 
                 ___________________
                      63,025

To me that seems really inaccurate and begs the question why is it used today?

I can understand that Mr. Watt wanted to make a yard stick of the power that steam engines had and horses were a good measurement back then as every scientist would have rode to town on one. But, this serves no purpose today with our automobiles that we drive to work in.

The use of horsepower now is made even more absurd when comparing diesel to petrol engines when some petrol engines max revs are twice that of diesels.

What are your thoughts on this? Or am I alone in thinking this? Perhaps completely off track? :confused:

Trif.

Simon · 28 Aug 2006 at 22:21

Trifid said:
Code:

HP = Torque (lb. -in.) x RPM ___________________ 63,025

To me that seems really inaccurate and begs the question why is it used today?

Inaccurate? How?

It's just the standard unit of measurement that allows engines ot be easily compared. It is being replaced with PS now a days mind you.

Zip · 28 Aug 2006 at 22:22

Simon said:
Inaccurate? How?

It's just the standard unit of measurement that allows engines ot be easily compared. It is being replaced with PS now a days mind you.

We get Kilowats down here :mad:

God damm they are confusing :mad:

Clarkey · 28 Aug 2006 at 22:23

we use horsepower for the same reason we use miles per hour. Because it is what we know.

Also what difference does the rpm of a diesel compared to a petrol make?

Trifid · 28 Aug 2006 at 22:24

The RPM bit I am talking about. With diesel and petrol engines this stands to be a even more odd form of measurement.

I am prepared to be wrong though.

Clarkey said:
Also what difference does the rpm of a diesel compared to a petrol make?

The equation I got from google.

Simon · 28 Aug 2006 at 22:24

Trifid said:
The RPM bit I am talking about. With diesel and petrol engines this stands to be a even more odd form of measurement.

I am prepared to be wrong though.

Now I'm lost.

kaiowas · 28 Aug 2006 at 22:25

Don't go posting that formula around here, you'll scare all those diesel boys who seem to think that torque is somehow something completely unrelated to and superior to power

Jokester · 28 Aug 2006 at 22:25

Horse power is just a unit of power like KW or PS.

Jokester

Trifid · 28 Aug 2006 at 22:27

Jokester said:
Horse power is just a unit of power like KW or PS.

Jokester

Yes, but I don't think it is the best one for comparing an engines power output.

It is, if anything, more confusing than torque.

kaiowas · 28 Aug 2006 at 22:28

Trifid said:
Yes, but I don't think it is the best one for comparing an engines power output.

Why not? If you want to compare power output, surely a value which is a measure of power is the best way to do it. Or would you rather just say that the car with the most cupholders is the more powerful one?

DRZ · 28 Aug 2006 at 22:29

PS means Pferdestarke, which is horsepower in German.

The only "real" measurement is kW (thats the correct unit for power) but trying to get people to move to metric is easier said than done. PS is a halfway house in many ways.

PMKeates · 28 Aug 2006 at 22:29

Trifid said:
The equation I got from google.

The equation is why, all other factors the same, a 100HP diesel will have the same top speed as a 100HP petrol - because HP is a unit of power.

I cannot see why you think it's wrong? It's perfectly normal.

Jokester · 28 Aug 2006 at 22:29

Trifid said:
Yes, but I don't think it is the best one for comparing an engines power output.

It is, if anything, more confusing than torque.

The one with the highest number (no matter what unit) has the most power.

Jokester

Trifid · 28 Aug 2006 at 22:30

kaiowas said:
Why not? If you want to compare power output, surely a value which is a measure of power is the best way to do it. Or would you rather just say that the car with the most cupholders is the more powerful one?

Again, I am talking about the RPM part of that equation. And particually the difference in peak RPM of a diesel and petrol car.

DRZ · 28 Aug 2006 at 22:32

Trifid said:
Again, I am talking about the RPM part of that equation. And particually the difference in peak RPM of a diesel and petrol car.

Why does the RPM matter at all when comparing peak power (rather than operating characteristics like powerband, accessible power etc)??

Answer: It doesnt

PMKeates · 28 Aug 2006 at 22:32

Trifid said:
Again, I am talking about the RPM part of that equation. And particually the difference in peak RPM of a diesel and petrol car.

But HP (power) is how the output is measured? What is wrong with that, why is the RPM of a diesel of any importance? A 130HP diesel outputs the same amount of power as a 130HP petrol!

Simon · 28 Aug 2006 at 22:33

Trifid said:
Again, I am talking about the RPM part of that equation. And particually the difference in peak RPM of a diesel and petrol car.

Well petrols engines rev higher but make less torque.
Torque is the measure of the engine output if you were to measure each rotation and rpm is how often it happens.

If one engine pushs twice as hard but only pushs half the amount of times then it makes sense that they both do the same work overall (therefore are of the same power)

kaiowas · 28 Aug 2006 at 22:37

Trifid said:
Again, I am talking about the RPM part of that equation. And particually the difference in peak RPM of a diesel and petrol car.

This is where we come back my first post and diesel fanboys.

First of all to simplify matters ignore the dividing by 63,025. All this acheives is some correction for units (pi is in there too if you're interested)

Regardless of the units you use, Power produced at any given speed is the Torque produced at that speed multiplied by the rate at which the engine is spinning. This is fact. No matter what units you use this will be the case, if we wanted power in kW we would have exactly the same basic equation with the exception that the 63,025 number would be a different value.

This gives two different ways in which you can get a given power output from an engine, you can have a fast spinning engine which develops less torque (as a petrol engine typically does) or you can have a slower spinning engine which develops higher torque (as a diesel typically does)

Matt82 · 28 Aug 2006 at 22:38

bhp is useful if you know other characteristics of an engine, like you said, rpm and torque too.

bhp is only confusing and wierd when you think how different manufacurers seem to differ on the actual ability of their claimed figures and also different rolling roads too

penski · 28 Aug 2006 at 22:41

Trifid said:
Again, I am talking about the RPM part of that equation. And particually the difference in peak RPM of a diesel and petrol car.

Oh dear God, please let it be a wind up...

Right...bhp is a unit of measurement.

bhp is calculated by using a fixed equation with a single variable input.

The variable input is the amount of torque which the engine produces.

RPM DOES NOT MATTER IN ANY WAY. IT IS SIMPLY USED TO CALCULATE THE HORSEPOWER BY VIRTUE OF THE AMOUNT OF TORQUE PRODUCED AT THAT SPECIFIC ENGINE SPEED.

*n