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Difference Between HP and BHP

horse-powerHP vs BHP

You are bound to have stumbled upon the term Horsepower, whether you own a vehicle or not. HP is a measurement which is used during ads for cars, and it’s also something that you will often hear being used by those who work in machine shops, or even men who are car enthusiasts, but what about the term Brake Horsepower, or BHP? The difference between the two is what we will now uncover.

First, let’s start with what HP is all about. Invented by James Watt, horsepower originally measured the amount of work, that a horse lifting coal out of a coal mine, could do in a minute. Back then, one HP equated to 33,000 foot-pounds. Today, you can easily convert HP into different units, like 1 HP that equates to 746 Watts. It can also be converted into British Thermal Units, or BTU, joules and calories.

However, the most common use of HP, as a unit, is to measure the power of an engine ‘“ which you can determine by hooking it to a dynamometer. What HP actually measures, is the maximum rate of acceleration and the top speed of the car.

On the other hand, Brake Horsepower measures the HP of an engine without considering the loss in power that is caused by some parts of the engine, like the generator, gearbox, water pump and other auxiliary parts.

There are actually no other key differences between BHP and HP, other than the fact that when BHP is measured, the engine torque is determined by applying a break to the flywheel ‘“ as opposed to using a torque converter, like in the case of HP.

To summerize, HP is measured with all the accoutrements attached to the engine, to determine its maximum rate and speed. BHP, on the other hand, is more of a theoretical calculation, which is made under lab-controlled conditions, and without having anything attached to the engine.

Summary:

1. HP is the output horsepower rating of an engine, while BHP is the input brake horsepower of an engine.

2. B HP is the measurement of an engine’s power without any power losses, while HP is BHP less the power losses.

3. HP is measured by hooking up the engine to a dynamometer, while BHP is measured in a controlled environment without anything attached to the engine.


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29 Comments

  1. I’d disagree with the descriptions used – much of it is closer in truth to the difference between gross power output and net power output.

    While the use of BHP to denote measurement at a load, or brake (not break, as spelled in the article) is correct, it does not necessarily mean that this must be at the engine flywheel – power as measured at the wheels by a dynomometer is also BHP, as it is still being measured at a brake.

    The article also omits certain erroneous uses of the term horsepower, such as the old British vehicle taxation system that rationalised HP as being related largely to cylinder bore in order to assign a levy – hence car models being known by numbers such as “seven” or “twelve”, denoting their taxable classification. Interestingly, this also had the side effect of encouraging manufacturers to build small-bore, long-stroke engines for a given capacity, to minimise the duty rate attracted.

  2. Dave is an idiot. Thanks for the article

  3. I agree with both Dave and Chris….both valid. I didn’t know about the taxation on the cylinder bores though. Ok so if I understand this correctly…BHP is in a controlled evnirnment with nothing attached to the engine except something such as a brake from an engine dyno, and the same can be said about “RWHP” rear wheel horse power……which is also considered a “break” or Brake lol ok now im confused again. All I do know is in the “real world” you loose power through the drive train= transmission, drive shafts, rear diffs, axles etc. these all play parts on “robbing” horse power or should I be more acurate and say Torque. Because the horse power is just the speed at which is turns…..not the force behind it kinda like volts vs amprage…….I do hear the term BHP and I have been in the auto industry for MANY years….in service depts and as a gear head growing up sooooo i need clarity here! lol

    • h.p. is the short version of h.p. It can be measured at flywheel, or rear wheels. It is torque in ft./lbs. times rpm divided by ~(if foggy memory hasn’t let me down) 5250. That is why the hp and torque curves always cross at that rpm, no matter the ultimate power produced or the rpm where peak occurs. There is another kind of h.p. that was used in detroit that is an unrelated figure derived by some geeky calculations that is highly inaccurate, but was used to understate the power produced for some tax avoidance or regulatory advantage when the d.e.q. or e.p.a. was going apesh*t in the gas-ripoff days of the 70’s and 80’s. An example would be a 1984 mustang h.o. 302 that was rated at 175 sae net and produced 205 on an engine dyno. Those net ratings were continued for many years and have no basis in real output.

      • First I want to point out to Will that torque is not measured in ft-lbs, it is measured in lb-ft. While these two units seem to be the same, they are, in fact, not, however, they are dimensionally equivalent Both of these units are what is known as vector quantities . This means they are directional, and that must be factored in. In the English Imperial system, force and distance (actually displacement) are at a maximum when they are in the same direction. Mathematically this known as dot product. This is expressed as work=force * distance X the sine of the angle between their directions. If the force and distance are in the same direction, the angle is 0 degrees, which has a sine of one. So to calculate work, just multiple the quantities. Torque is calculated using what is known as a cross product. Thus torque = force X distance X cosine of the angle between their directions. The sine of zero degrees is zero, so torque would always be zero if a dot product would be used. torque is maximized when the quantities are perpendicular because the cosine of 90 degrees is 1.00 To distinguish the difference in calculating work and torque, work is always shown as ft-lb and torque is shown as lb-ft. If this sounds like you’d need a degree in physics to understand, you’re probably correct. Lots of people make the same mistake. Maybe this is why the imperial system has all but disappeared. The metric system has no such distinction although in my opinion, it should.

        ,

        ;

        • Most of that was good, but a dot product is calculated by multiplying the cosine of the angle between vectors by their magnitude, not sine. That should clear up any confusion because of you saying that sin(0)=1 at one point and sin(0)=0 [which is correct] at another. You also said cos(π/2)=1, but it equals 0. Sin(π/2)=1. I don’t know if you just remembered wrong, or if you were trying to explain after barely understanding someone else’s explanation…

  4. When car manufactures advertise engine power these days, it isn’t BHP but HP. Or in other words net horsepower not gross horsepower.

  5. Whilst I applaud everyone for taking the trouble to contribute, the above exchanges are littered with techno-rubbish, and it is reasonably clear that none of the contributors have ever been directly involved in engine or vehicle performance measurements. I have, and rather than try to put everything straight here, I would recommend, to anyone interested, the Wikipedia article on Horsepower. Some may find the more technical content a little daunting, but I strongly recommend reading all the way through as there are answers there for almost everyone. (I confess I have not been through the Wikipedia article with a fine toothcomb, but on a speed-read it is dramatically more accurate than the stuff above.)
    BTW, Dave’s comments are basically correct. And Chris, that is NOT a maybe.

  6. Pls edit fine toothcomb to fine-tooth comb – I definitely do not comb my teeth.

  7. is bhp of a engine is greater than hp pls explian

  8. HP is LESS than BHP.

    Take the Bugatti Veyron, a much disputed example.

    Its BHP figure, before any losses, is 1001.

    Its HP figure, taking into account that the power must go through the gearbox etc is 987HP; If you were to apply the breaks at its top speed, they would only need to stop the power of 987BHP, because that is all that is being transferred to the road.
    That means 14BHP has been lost during the process of transferring power to the ground, meaning a 98.6% efficiency rating.

    Any questions? I hope not.

  9. Example of car: Bhp can be measured at the engine output and hp can be measured at wheel out put. (bhp-transmission loss =Hp).

  10. HP is called as Indicated Horse Power i.e.IHP means the Horsepower actually deveploped at the engine shaft. BHP means Brake horse power i.e. power actully used by the engine to perform the work. BHP is always less than IHP. IHP.-BHP.=FHP.FHP means power wasted in Engine friction.,it is also called as Frictional Horsepower.

  11. This article confuses more than it explains. A much simpler and arguably more accurate explanation would be:

    – HP = measured on the wheel, i.e. the power a vehicle will transfer to the road.

    – BHP = measured on the crankshaft, i.e. the power of the engine alone without any other components of a vehicle that “rob” the engine of power.

    One is a measure with everyday practicality. The other makes sense to engineers only, but is often used as a marketing gimmick to confuse the general public because it’s always higher.

    Neither measure the maximum rate of acceleration and are very different from torque, a measure that is thrown into this article for no good reason.

  12. “It can also be converted into British Thermal Units, or BTU, joules and calories.”

    uhhhhh, no.

    That’s like saying you could measure a cup of water in units of Niagara falls.

    No conversion can take away the time portion of a unit of power without defining time.

    • Yes you can, you just get both the time it takes to do the job, and the energy needed, so if you have a timeframe, you can tell how much energy you need in cal. or Joules to get there, and since all work takes time, it’s the same as to say that the work you do and the sallery you get has nothing to do with eachother, since it is relative to the time you use doing it, you can still measure your work in it, you just have to include the factor of time.

  13. Ok Mr Bugatti, knowing that no engine runs at 100% efficiency and considering that no self respecting company would sell a mode of transport that would have 1/2 hp to every bhp. My question is this, what is a reasonable % efficiency rating that I could use as a rule of thumb for figuring the hp or bhp when the other is given?
    And is there a slope involved that I could factor size of engine or other notable variables?
    Bottom line question- what is the formula for the hp and bhp?
    I’ll try google bhp-hp to see if I get anywhere.
    Ps: for those who want to correct me, my autocorrect already beat me up

  14. One very very important thing that has not been mentioned in this article is that horsepower has a different definition depending on where you are. A metric horsepower is 98.6% of the Imperial mechanical horsepower. I’m only mentioning this because the difference of bhp and hp is in the same category and sometimes car manufacturers use different definition of hp depending on where the engine came from.

  15. Everyone, including the original writer of the article, who has passed comments on this page (except for Billy) is an idiot!
    Yes, do try the Wikipedia article…
    Gerhard

  16. I’m really confused.

    I thought BHP was power calculated at the wheel (after loss of drive train) Every Dyno and vehicle spec sheet reflects that.

    Here it states BHP does not consider the loss through drive train.

    Ehhh ?

  17. OK, so some of you are ignoring my earlier suggestion to have a look at Wikipedia. So try this …
    Power is the rate of doing work. Work is done when a force moves its point of application. (So to move the point of application faster requires more power.)
    Power can be measured in various units, but most commonly kilowatts and horsepower. As technical units of measure, these are exact and unequivocal. One kilowatt is 1000 watts where 1 watt is 1 Nm/s, for example a force of 1 Newton moving at 1 meter per second, or a force of half a Newton moving at 2 meters per second – you get the idea. Similarly, 1 horsepower is 550 ft-lbf/sec, for example a force of 550 lbf moving at 1 foot per second.
    So what about bhp? The problem here is that, as some above have said, the term horsepower has been abused. For example, the RAC in the UK (for Americans and others, the Royal Automobile Club in the United Kingdom) conjured up the approximate power rating of a car engine by using some function of bore, stroke and number of cylinders (and NOT the engine capacity in litres or in cu ins) – but a more-or-less meaningless number – and called it horsepower (usually RAC horsepower).
    Well, the b in bhp stands for brake, and is intended to show that the value represents ‘real’ horsepower, usually obtained by measurement on a dynamometer, commonly called a brake.
    In a vehicle system, starting in the cylinders where the power is generated, there are losses through the engine itself, through the transmission, wheel bearings and tyres all the way to generating the force at the road surface which propels the vehicle. (Let’s forget gas turbines and rockets for now.)
    Two common types of dynamometer are engine dynamometer and chassis dynamometer (rolling road).
    The engine dynamometer typically measures power delivery at the flywheel and, after correction for STP (standard temperature and pressure), would then be quoted as engine bhp. That number has an exact equivalent in kilowatts. It is the power at the flywheel which is normally quoted by vehicle manufacturers.
    The rolling road dynamometer measures the power delivery after all the transmission and tyre losses etc. This would generally be used for development, quality control or other purposes and would usually not be published. (As a possibly interesting aside, a measure called indicated horsepower attempts to estimate the power generated in the combustion process before any engine losses.)
    For anyone wondering about torque versus power, power is generated by a torque rotating at some angular velocity (rotational speed) and is equivalent to a force moving at some linear speed. (A static torque is generating exactly no power.)
    So the power at the flywheel minus transmission losses gives the power at the wheels. This could be given in bhp (or just ‘real’ hp), kW or any other ‘proper’ power unit.
    I hope this helps somebody.

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