brake horsepower (bhp)


11 replies to this topic
  • frog13

Posted November 09, 2009 - 05:49 PM

#1

How do you know what a motorcycles bhp---brake horsepower is?.Isn't bhp the power from/off of the crank,not the rear wheel?.Whatever a 650L is rated at---what we read on the specs---what is the bhp?. TIA Frog13

  • bigredpig

Posted November 09, 2009 - 08:43 PM

#2

Brake horsepower (BHP) is the amount of work generated by a motor without taking into consideration any of the various auxiliary components that may slow down the actual speed of the motor. Sometimes referred to as pure horsepower, brake horsepower is measured within the engine’s output shaft. Depending on the configuration of the engine, the point on the output shaft that is the focus of the measurement is the engine dynamometer.

Edited by bigredpig, November 09, 2009 - 08:46 PM.
I was wrong


  • frog13

Posted November 09, 2009 - 09:25 PM

#3

Thank you bigredpig!.So,let's just say a 650L is listed at 35 h.p. stock;it's bhp could very well be 40 bhp---right.The dreamed up number of 35 h.p. would be the number they(manufacturer)give as a general figure?.

  • Manos

Posted November 10, 2009 - 03:29 AM

#4

Bigredpig, I don’t exactly know if we are saying the same thing here – I think we are not.

An engine generates work (or power, over time) in the combustion chamber of the cylinder. This is ultimately delivered to the rear wheel. From generation to delivery, there are many points where you can measure this power. There are losses along the way, which act cumulatively, meaning that at each later stage, you have to add the losses of all the previous ones. Therefore, the energy that finally arrives at the rear wheel is less than the one produced by the burned fuel in the combustion chamber. The reason is simple: the engine has to move all its moving parts, ancillaries, transmission, wheel etc and what’s left is what propels the bike.
Engine power can be measured in the cylinder head by measuring various parameters, such as the compression and combustion pressures, then drawing PV diagrams, measuring the mean indicated pressure and calculating the resulting power. This is standard procedure with ship engines (which of course are very big and very slow and a lot easier to work on). This power is the “indicated power”.
When you have an assembled engine (without clutch or transmission) you can attach a measuring device at the crank/flywheel. This is what is commonly known as a bench dynamometer. There are various kinds (inertial, mechanical, hydraulic, electric). On ships’ engines this is a hydraulic brake (and I think this is where the term was started). This device has a known behaviour over rpm, so when accelerating or decelerating at a measureable (and therefore known) pace it is also known what power was required to do it.
What is measured by this rig is termed “brake” power (or brake horse power, if you are measuring power in hp units).
Finally, when you have a complete bike, i.e. engine, transmission and a wheel, you can again use a similar device to measure what power arrives at the wheel. The most commonly used rigs are the rolling chassis dynamometers (dynos), which are inertial, but there are other variants. You can measure the power at the flywheel, or get a number very close to it, by accelerating and then shifting into neutral. Having disengaged the engine, the deceleration of the dynamometer will show you the power consumption of the transmission, i.e. the driven clutch plates, the gear shafts, the sprockets, the chain, the wheel, the tyre etc. It won’t show you the driving clutch plates, but you have no choice but to neglect this. If you deduct this last number from the figure you got for wheel-power, then you should get a figure very close to the brake power.

Having read my text, above, it seems I may have over-expanded the topic.
In short, “brake” power is a term used conventionally to describe the power output of an engine i.e. the engine alone, see bench dyno, above.
Technically, “brake” power is any power measured using a... brake.
And also, don’t confuse “power” with “horsepower”. It is the same thing as “weight” and “kilos” or “pounds” or “ounces” etc. Power measured in kilowatts (kW) is not horsepower, but it still is... power!

So, frog13, if the 650L is listed at 35 h.p. stock, this would probably mean that it develops 35hp at the crank, which means 35bhp. This is the number most manufacturers publish (it’s a marketing thing). Some publish wheel power or a number closer to wheel power (such as Ducati). In any case, it is very improbable that the listed hp is less than the bhp.

Hope this helps
Manos

  • akarob

Posted November 10, 2009 - 07:09 AM

#5

For what it's worth, most people measure a bike's (or car's) HP on a dyno. You know, the machine that measures how much power is at the wheel.

Anyway, even that will vary depending what wheel you have on. A knobby will read less HP that the same bike running a smoother tire. As said above, it is dependent on how much resistance there is in the system.

Just FYI, I tested my bike with a knobby (thx lateapex). There are two runs. One with the stock silencer and one with the XRs only silencer and rejetting.

The stock HP reading was about 28 HP. The most interesting part is how much more low end torque there is with the new pipe.

Posted Image

  • HeadTrauma

Posted November 10, 2009 - 08:41 AM

#6

Thank you bigredpig!.So,let's just say a 650L is listed at 35 h.p. stock;it's bhp could very well be 40 bhp---right.


No, Honda(and just about all motor vehicle manufacturers) list brake horse power(bhp), which is power at the crankshaft. Actual output at the drive wheel(s) will be lower due to driveline losses. For example, the '01 CBR600 F4i was rated at 110bhp, but they routinely make around 95-97hp at the rear wheel. Manufacturers do that because the higher bhp figures look better in a sales brochure. The rest of us usually use a chassis dyno and measure output at the drive wheel for reasons of practicality.

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  • bigredpig

Posted November 10, 2009 - 08:43 AM

#7

dude, great point on horse power and power/watts. I got that info from here
http://www.wisegeek....-horsepower.htm

I thought bhp was the same as rwhp, but that is not right.

with a street bike's I am out to get the most hp for speed/high rpm.
with the dirt bikes the torque is were its at.
I think a engine with high hp will not last as long as the same engine with high torque.
so for me the torque is what I want. just my .02


Akarob good looking dyno run . is 29hp what most of the xr650l make?

now my bike have some mods but here are my #
1988 xr600r 37rwhp fcr carb and super trapp hot cam stage 1
2001 xr650r 49 rwhp 42mm hsr Mikuni fmf, hifo white bro pipe

  • HeadTrauma

Posted November 10, 2009 - 09:02 AM

#8

I think a engine with high hp will not last as long as the same engine with high torque.


That's because hp is a function of torque and rpm. For power to increase, at least one of the others has to as well. With a normally aspirated engine, it is easier to just spin it faster at the expense of longevity than increase cylinder pressure.

  • akarob

Posted November 10, 2009 - 09:07 AM

#9

Akarob good looking dyno run . is 29hp what most of the xr650l make?


My stock '08 L (with Dave's mods) did about 27 HP max.

If I switched over to street tires, it would probably be in the low 30s. The real value is comparing runs after making one change in an otherwise identical setup. Then you can see if what you did was better of worse.

In the graph, look at the beginning of the torque curve - that's a difference you can feel.

  • bigredpig

Posted November 10, 2009 - 01:57 PM

#10

My stock '08 L (with Dave's mods) did about 27 HP max.

If I switched over to street tires, it would probably be in the low 30s. The real value is comparing runs after making one change in an otherwise identical setup. Then you can see if what you did was better of worse.

In the graph, look at the beginning of the torque curve - that's a difference you can feel.

yea big jump in torque.
on the same dyno is best each run,one dyno will be different from the next. what is the Comparison ratio on the 650l stock?
I would think low so it will last a long time 9-1 ?



Originally Posted by bigredpig
I think a engine with high hp will not last as long as the same engine with high torque.
HeadTrauma Quote:
That's because hp is a function of torque and rpm. For power to increase, at least one of the others has to as well. With a normally aspirated engine, it is easier to just spin it faster at the expense of longevity than increase cylinder pressure.

I have found with changes to the jetting and the back presser on say a supertrapp the torque will go way up.

  • HeadTrauma

Posted November 10, 2009 - 04:32 PM

#11

what is the Comparison ratio on the 650l stock?
I would think low so it will last a long time 9-1 ?


Lower than that: 8.3:1.

  • frog13

Posted November 10, 2009 - 09:39 PM

#12

WOW---thanks a bunch guy's!.So basically a 650L makes moderate H.P.--descent torque for longevity sake---engineered that way by da little guy's!.





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