Article: BHP vs HP vs kW and other power conversions

[stradlater]

This article arose out of my own curiosity with respect to the difference between Brake Horse Power (BHP) and Horse Power (HP). I figured I'd research it myself and write a little article on the differences.

It then grew a bit as I decided to whack in the other Power Conversion Factors.

Firstly, the difference between BHP and HP.

Technically there is NO difference short of the fact that you won't find power ratings for BHP at the wheels. The reason for this is the concept of BHP was born out of the old way of measuring engine power output.

A resistance (brake) was applied to the end in test to determine how much power it was putting out, as opposed to today's methods where an electronic Dynometer or Torque Converter is put on the engine. Hence BRAKE Horse Power. A 'brake' is applied to the engine to provide the resistance to measure the power output. The output is still measured in Horse Power, so there is hence no difference between BHP and HP.

BHP is an old unit of measurement and only get's used in places where 'old' forms of measurement are common place (read UK & US).

One other common mis-conception is the PS. It has been listed in a few supposedly reputable magazines as "Power Stroke".... Clearly they were "Power Stroking" something because it DOESN'T stand for power stroke.

PS stands for PferdeStarke which is the German word for Horsepower. It's also commonly know in France as the CV or Cheval Vapeur. Both words are the translation for Horsepower. These are used across Europe because they provide a reasonable metric conversion for a Horse Power.

Where a HorsePower is measured in Imperial Units (50 foot pounds per second), the PS is measured in comparable metric units (75 kilogramme metres per second). Clearly the numbers don't exactly match up, so there's a conversion factor of PS to HP. One PS is approximately 98.6% of on HP.

Of course we all know the KiloWatt. This is purely a measure of energy transfer (a Watt being Joules transfered Per Second) and doesn't have to have any relation to an engines power output, it ofcourse can be used for any appliance or device that consumes or creates energy. There is however an application for it in the automotive sense, and it does help to 'standardize' the automotive power quotation convention.

Now while the kW doesn't have roots leading to a torque based calculation, the other two do. I have the information on where torque fits into all of this but I don't have time to explain it all now, so I'll leave it for next time..

I will however leave you with a neat little conversion table that lays out the differences between the ratings.

Source -> To -> Conversion Factor

kW -> PS -> 1.360

kW -> BHP -> 1.341

PS -> Kg M/second -> 75

PS -> Ft Lbs/second -> 542.476

PS -> Watts -> 735.5

BHP -> PS -> 1.01387

BHP -> Ft Lbs/second -> 550

BHP -> Watts -> 745.7

Lb Ft -> Nm -> 1.356


PS: Because this won't let me post HTML, I can't do a table to make it look nice, sorry..
Damn hard to read, I know.

[SL666]

PS stands for PferdeStarke which is the German word for Horsepower

i thought it meant 'power stroke'

[Norbie]

Good info stradlater, +rep.

i thought it meant 'power stroke'

Go and stand in the corner.

[RONA]

Great thread any chance of a sticky for it, i'm always looking for the lb ft/Nm conversion

[stradlater]

Hehe. No, it doesn't mean Power Stroke... I found it interesting that the French call it CV...

[YLD-16L]

Good work Strad +rep.

I will add a few of these conversion factors to the converter in my phone It's a killer at parties and for picking up chicks. Also reasonably handy at dyno days

[stradlater]

Killer for picking up chicks? I thought you were a chick>?

[SL666]

Norbs: believe it or not.. im 100% sure thats what it says in this book i have at home, a book on rotaries... i agree the literal translation is horse-strong through

strad: the bloody french.... does explain the citroen 2CV though..

[stradlater]

2CV.. Hehe. I like that...
that's one powerful car.

[o_man_ra23]

Good work Strad +rep.

I will add a few of these conversion factors to the converter in my phone It's a killer at parties and for picking up chicks. Also reasonably handy at dyno days

I have NFI how you would pick up chicks with a power conversion table...

On topic tho, Power is derived by multiplying torque with rotational velocity in RPM and then multiplying by a constant. Ill see if i can dig all the formulae out of my old lecture notes. If i remember correctly, Horsepower is gained by multiplying Foot Pounds directly by RPM... ie conversion factor of 1... but i would have to confirm that. Ill see if i can find these old notes and let everyone know.

Props to Stradlater for putting this stuff up, I was always wondering WTF the difference was between BHP, HP and PS... Oh, and if anybody wants to believe that PS stands for Power Stroke, they are more than welcome to continue Power Stroking as much as they like... just please dont do it anywhere I may witness.

Cheers, Owen

[stradlater]

Horsepower = Torque x 2 pi x rpm / 33000

So sort of what your talking about, yes.

[SL666]

Owen: I think you are power-stroking right now power is a measure of work done, torque is a measure of force. and i think the multiplier is something like 5454 or something.. i suppose i could ACTUALLY look it up.

[mr2gze]

Google has a fairly good translater. You simply type -
"200 foot pounds in newton meters" in the search bar and enter, and it outputs -
"200 foot pounds = 271.16359 newton meters"
or basically any conversion you can think of
if you didnt already know.....

and yeah, good info stradlater

[Roundy]

5252 is the multiplier

hp = torque x rpm / 5252

[YLD-16L]

Google has a fairly good translater. You simply type -
"200 foot pounds in newton meters" in the search bar and enter, and it outputs -
"200 foot pounds = 271.16359 newton meters"
or basically any conversion you can think of
if you didnt already know.....

Thanks, didn't know about that one.

I have NFI how you would pick up chicks with a power conversion table...

.....and that is why you will be power stroking yourself to sleep each night