Bhp vs Hp?

[Daza]

Okay, i have a question(s).

This one i've always wanted to know.

What is the difference between bhp and hp? What does the term brake-horsepower actually mean? :sherlock:

Thanks

Daz

[Motu]

They mean the same thing - in the olden days,before computers....horsepower was calculated by running the engine on a brake dyno.An external contracting brake was attached to a spring scale one foot radius from the crank....the spring scale read in pounds.This gave a foot/pounds torque result,and by using a formula such as this....

BHP = rpm x load/4500.

There was no way to read BHP direct.

Modern dynos are computer controled,they can calculate HP directly.Brake dynos gave way to water dynos,eddy current dynos,inertia dynos etc.BRAKE horespower is an old way to measure power.Not many people would of seen an actual brake dyno.We had one at ATI when I served my apprenticeship,we used to run and engine up,apply the brake and take a reading,then calculate the bhp.....without a calculator!

[WarlockNZ]

ohhh ... good question ...

Let me see if i understand it .. and i'm quoting here

In order to make a Horsepower rating crystal clear as you are riding your bike..., the definition of one horespower unit is the amount of energy expended by a horse while lifting 33,000 lbs. one foot, in one minute.

HP is the "general" term. INDICATED horse power is the theoretical horse power that an engine can produce. BRAKE horse power equals the Theoretical HP minus the internal friction (power loss).

So.... BHP is really the HP available at the crank. For all intents and purposes, HP and BHP mean the same thing when talking about it or reading it in mags.

Rear Wheel Horse Power (RWHP) is, as the name suggests, how much HP actually gets to the tarmac after power is lost through the gears,chain, etc.

You can lose around 10 to 15% of the power from the engine once it actually hits the rear wheel .. or so i understand it ..

I'm sure i'll be called to task if i'm incorrect :dodge:

[WarlockNZ]

without a calculator!

Damn!! ... i have enough trouble working out my monthly pay check ... you a math wizz or something ??? LOL

[BarBender]

WMOMTMMMA (what my old man told me many moons ago...)

HP = pure power output i.e. power generated without any frictional loss
BHP= measured power at @ the crank.

[Ixion]

The distinction actually predates the internal combustion engine.

It is quite easy to calculate a theoretical horsepower figure for a steam engine, based on steam pressure and rpm. And steam engines were often advertised and sold on the basis of such calculated horsepower ratings.

In practice the actual horsepower, as measured on a brake (as Mr Motu said) was always rather less than the calculated figure.

With an IC engine, it is almost impossible to calculate a meaningful theoretical horsepower figure (though of course an experienced engineer can take a fairly good guess based on the design). So the only way to get a figure is to measure it on a brake or dynometer.

[merv]

then calculate the bhp.....without a calculator!

LOL ... because you couldn't buy a calculator even for another 20 years huh!

[Motu]

I wouldn't of believed it if someone told me I could do those bullshit calculations by pushing buttons on something I could put in my pocket.

There is also corrected BHP which the old guys had to work out to correct for temperature and barometric pressure.Here is a formula from Phill Irving's book Tuning for Speed.Never far from my hand for 35 years.....

observed bhp x 29.92/barometer reading x 400 + air temp/415

You young guys aren't picking up on the important word clue - Brake.

[Rhino]

There is also corrected BHP which the old guys had to work out to correct for temperature and barometric pressure.Here is a formula from Phill Irving's book Tuning for Speed.Never far from my hand for 35 years.....

observed bhp x 29.92/barometer reading x 400 + air temp/415
I remember buying that book as a teenager. I don't have it now, as my ex would have sold it along with 30+ years collection of books and tools :(

[merv]

In this age of electronics I take it all for granted now and I'm a bit of a techno freak I guess I like my gizmos, but in thinking about Motu and the calculator I still think about the fact it wasn't until my third year at Uni that I was allowed to stop using the slide rule in exams and take along a calculator and back then we thought the slide rule was a miracle compared to using the log tables book.

[Motu]

I think that's what this thread is all about to me Merv - with calculators people have forgotten the actual workings of maths,they just get results not how to get there.They don't have to know percentages,they just use the % button.

And so with dyno's.There is no measurable horsepower figure,it is calculated....it is a rate of work.The original pony (actually Prony,the guy who invented it) brake gave a weight reading,and from this was calculated HP.

This is from my book ''Modern Motorcycle Mechanics'',7th 1974 edition,the book apprentice motorcycle mechanics were issued with....Horse power;a unit of work.Power required to lift 33,000lbs one foot in one minute,or 330lbs 100 ft in one minute.

But the modern dyno can give a HP reading because such a messy calculation is nothing to a simple computer.No one knows what BHP really is any more....so we have urban myths to explain away the ''B''.

[slowpoke]

If "hp" is the abbreviation for "horse power" then "ps" must be the abbreviation for "ponies-shetland" because they are bloody close to horses only just a lil' smaller.....

Actually the ps unit ( Pferdestärke, translation = horse strength) is a pretty useful one because it's virtually hp but measured at the wheels, which is a more real world figure for most of us.

1 PS = 75 kp·m/s = 0.73549875 kW = 0.9863201652997627 hp (SAE)

Just when the Europeans give us a real world figure we can use (ps) they take it away by directing it be replaced with the kW, which once again can be measured any-bloody-where. Great for engineers but lousy for us real world (olde world?) types.

As per usual there are two things you can never trust: engineers and O-rings, the two greatest causes for failure in the modern world.

[Motu]

The kw is the perfect solution,it is a measurement of power - all power.

But some of us old farts like to have HP,mpg,weigh things in lbs and have 12 months in a year.

[scumdog]

The kw is the perfect solution,it is a measurement of power - all power.

But some of us old farts like to have HP,mpg,weigh things in lbs and have 12 months in a year.

And besides, 100hp sounds way more powerful than 75kw (or whatever the EXACT amount is).

Then there is the gross vs nett breakdown too - but that, as they say, is another story.:shutup:

[quickbuck]

As per usual there are two things you can never trust: engineers and O-rings, the two greatest causes for failure in the modern world.

I would have said "Operators and o-rings" but you are correct.

If you want something to be reliable "Don't let humans touch it". And to go along with that, "Don't seal it with things made by humans".

[Edbear]

and back then we thought the slide rule was a miracle compared to using the log tables book.


Ouch! I can remember Log Tables and how the Slide rule was so much harder to pick up on. Then maybe I'm just a simple bloke... Trouble with the calculator at first, was the teachers still wanted you to understand the Maths behind it! So you wound up explaining it anyway. (Not that I'd remember much these days...!)

[avgas]

Stop reading fricken UK Autocar mags....theyre BS, may as well read road and sport if you gonna believe that hype about BHP

[terbang]

Have to dust off me books (when I was an apprentice) for tha one. Just to add to the confusion the output of a turboprop (gas turbine engine) is measured in SHP Shaft Horse Power. Gotta get more books out to revise that one too (drive jet nowadays so I forgot it all). I'll be back..

[Jamezo]

Oh yay, more physics misinformation.

Lets get one thing cleared up: One does not directly measure the power output of a rotating wheel. To do that would require measurement of the actual energy output over time (theoretically possible with a water brake dyno or similar, where the mechanical energy can be converted into thermal energy in a fluid with a known specific heat, the change in temperature measured, and thus the input of energy obtained).

No, what can be measured to a very high precision is torque; the rotational force. By acting against the rotation of the wheel with a known torque, tuned so that it exactly balances the drive from the engine - and thus keeping the rate of rotation constant - you can measure the output torque at a particular rate of rotation.

Measuring the rate of rotation of the wheel is trivial, and can be done by a multitude of methods, the most sensible and accurate being a digital counter on the counter-torque provider.

With those two pieces of information, the power can be calculated, which is the torque multiplied by the rate of rotation. This is the rotational counterpart to the linear concept of power being force multiplied by linear velocity.

These measurements are taken all along the output range of the engine. For automotive purposes, a power plot will be constructed with the engine RPM being a ratio of the wheel RPM, based on the known gear ratios.

[V4ME]

For my money - horsepower is often in the wrist - ask Bayliss