Discussion
A up,
I like to think I know a little about how the engine works (not specifics, just the basics) but something has always puzzled me. What is Brake Horse Power?
I know how it was/is measured and I know it suggests engine power but what does it really represent?
Correct me if I'm wrong but Torque is the turning power of the crankshaft/flywheel/bottom end of the engine? So what is BHP, the speed at which it turns (surely thats the revs)... I just can't understand what BHP represents.
Someone please help me!
I like to think I know a little about how the engine works (not specifics, just the basics) but something has always puzzled me. What is Brake Horse Power?
I know how it was/is measured and I know it suggests engine power but what does it really represent?
Correct me if I'm wrong but Torque is the turning power of the crankshaft/flywheel/bottom end of the engine? So what is BHP, the speed at which it turns (surely thats the revs)... I just can't understand what BHP represents.
Someone please help me!
ah, so BHP/HP/PS what ever you call it doesn't actually relate to anything engine specific but the power output. It doesn't really make sense to me - Torque is sheer turning power, the power at which the wheels are turned, Power is the rate at which it works.
Oops, just remembered my Physics lessons
Still, rate at which it works, doesn't have a tangible value in my book 
Cheers
Oops, just remembered my Physics lessons
Still, rate at which it works, doesn't have a tangible value in my book Cheers
Torque isn't power. Torque is a twisting Force. Power is "rate of conversion of energy". Power and torque (most uselfully measured at the rear wheels) are two sides of the same coin. For example, when you consider what is happening when the car is going at its maximum speed, you can express it two ways:-
1. From the point of view of Torque. "The torque on the rear wheels gives rise to a force pushing the car along the road that exactly balances the drag forces that tend to oppose the cars motion (e.g air resistance and friction between the road and tyre, and within the wall of the tyre). The sum of all the forces on the car is zero, i.e. it is in equilibrium and does not acclelerate."
2. From the power point of view. "The power output at the rear wheels supplies energy at exactly the same rate as the car is loosing energy to its environment (e.g. kinetic energy of air that is getting blown away by the car, heat into the air and road due to friction) The flow of energy from the engine into the kinetic energy of the car is exactly equal to the flow of kinetic energy out of the car into its environment, therefore the car is in equilibrium, its kinetic energy remains constant, and thus its speed does not change."
These two statements are saying exactly the same thing.
1. From the point of view of Torque. "The torque on the rear wheels gives rise to a force pushing the car along the road that exactly balances the drag forces that tend to oppose the cars motion (e.g air resistance and friction between the road and tyre, and within the wall of the tyre). The sum of all the forces on the car is zero, i.e. it is in equilibrium and does not acclelerate."
2. From the power point of view. "The power output at the rear wheels supplies energy at exactly the same rate as the car is loosing energy to its environment (e.g. kinetic energy of air that is getting blown away by the car, heat into the air and road due to friction) The flow of energy from the engine into the kinetic energy of the car is exactly equal to the flow of kinetic energy out of the car into its environment, therefore the car is in equilibrium, its kinetic energy remains constant, and thus its speed does not change."
These two statements are saying exactly the same thing.
docevi1 said:
aha, thats what I thought. Torque is the true measure of an engine, BHP doesn't actually mean that much by itself!
Cheers matey!
No, this is incorrect. Torque means nothing by itself. You can have a static torque applied to something, but no power will be expended and thus nothing will move.
BHP = (Torque(ft/lbs) * RPM) / 5252
You can see that you could have infinite torque, but with 0 RPM you are producing no power and hence you are getting nowhere.
When people say that torque is more important than BHP, what they really mean to say is that they feel it's desirable for a given power to be developed by high torque at low RPM (TVR style) rather than low torque at high RPM (bike engine style).
this might help:
www.seatcupra.net/forums/showthread.php?s=c69833067fd5ea40546fd063eb29102b&threadid=23146
>> Edited by edc on Monday 22 September 12:51
www.seatcupra.net/forums/showthread.php?s=c69833067fd5ea40546fd063eb29102b&threadid=23146
>> Edited by edc on Monday 22 September 12:51
docevi1 said:Not always true, if you have 2 engines, one making half as much torque at three times the RPM it'll be faster
Ho hum, more of both is better, I know that much.
A longer throw crank will generally make more torque (just like a longer lever) but will not rev to the same redline
This is why you see (especially on older engine designs) that for a range of engines, the one that is best for tuning (race engine) won't always be the one with the highest capacity (I'm thinking specifically here of the Triumph straight 6 engine (you can get a 2.0 to be much faster than the 2.5))
Of course it blows up more often but that wasn't the point was it
It was explained to me as such:
A sack of sand weighing 50kg on a rope with a pulley. Imagine you are pulling on the rope/or your engine is spinning a pulley connected to the rope.
It takes 50kg of torque or weight just to lift the 50kg load off the ground. That'd be 50kg of torque then.
To keep lifting that load up requires quite a bit more energy, but theres a time factor & a distance included.
Say lifting it 10m up in 1 min
Therefore: 50kg up 10m in 1min: This is basically what BHP is.
The faster you lift the sack the more bhp you have.
So by doubling the torque you'll lift the sack twice as fast.
Or by doubling the revs/speed you'll lift it twice as fast if you can rev that fast.
A turbo by forcing more air in can double the torque of an engine whilst the revs can stay the same. Though in practice you'd accelerate unless there was hill.
The reason torque drops off at peak revs is usually as not enough air can be sucked into the engine at higher speeds and so the bang in the cylinder becomes less. The bang is converted torque in the first place.
BHP is an old measurement I think, it can also be Kg/M per min as per my description.
Fairgound bellringer game: same hammer but different effect by different people?, that'd be the speed the hammer hits the pin then, (a good technique is all thats needed by the way) same effect as the bang in your engine cylinder.
Hope that makes sense, I think its correct.
A sack of sand weighing 50kg on a rope with a pulley. Imagine you are pulling on the rope/or your engine is spinning a pulley connected to the rope.
It takes 50kg of torque or weight just to lift the 50kg load off the ground. That'd be 50kg of torque then.
To keep lifting that load up requires quite a bit more energy, but theres a time factor & a distance included.
Say lifting it 10m up in 1 min
Therefore: 50kg up 10m in 1min: This is basically what BHP is.
The faster you lift the sack the more bhp you have.
So by doubling the torque you'll lift the sack twice as fast.
Or by doubling the revs/speed you'll lift it twice as fast if you can rev that fast.
A turbo by forcing more air in can double the torque of an engine whilst the revs can stay the same. Though in practice you'd accelerate unless there was hill.
The reason torque drops off at peak revs is usually as not enough air can be sucked into the engine at higher speeds and so the bang in the cylinder becomes less. The bang is converted torque in the first place.
BHP is an old measurement I think, it can also be Kg/M per min as per my description.
Fairgound bellringer game: same hammer but different effect by different people?, that'd be the speed the hammer hits the pin then, (a good technique is all thats needed by the way) same effect as the bang in your engine cylinder.
Hope that makes sense, I think its correct.
cptsideways said:
BHP is an old measurement I think, it can also be Kg/M per min as per my description
It could be, but kg/m/minute is most certainly not a SI standard unit! The SI unit of power is the watt (1 joule per second) which is a little small for measuring the output of most engines so kilowatts are generaly used. 1bhp=0.746kW
It's been said here already but BHP is a calculation derived from Torque and revs.
One thing worth noting is that if you plot the two curves they will always intersect at 5252rpm ie torque(ft'lb)=Bhp.
If you can get a flat torque curve above this 5252rpm threshold the BHP will climb steeply..... hence why F1 engines rev to 19,000rpm.
For me I think Bhp is THE performance figure and torque is just a means to calculate it......
but I also believe Bhp figues should be quoted as an average across a % of the rev range rather than just peak.
One thing worth noting is that if you plot the two curves they will always intersect at 5252rpm ie torque(ft'lb)=Bhp.
If you can get a flat torque curve above this 5252rpm threshold the BHP will climb steeply..... hence why F1 engines rev to 19,000rpm.
For me I think Bhp is THE performance figure and torque is just a means to calculate it......
but I also believe Bhp figues should be quoted as an average across a % of the rev range rather than just peak.
edc said:
this might help:
www.seatcupra.net/forums/showthread.php?s=c69833067fd5ea40546fd063eb29102b&threadid=23146
EDC - that's excellent, there's a really good analagy in there.
Rightly or wrongly, I've always convinced myself that torque is how much "ooomph" your car has whereas power is how quickly that "ooomph" accelerates, ie. power is torque acceleration.
My big lumpy Land Rover won't accelerate particularly well, however, neither will it slow down when given a hill to climb, because the engine has loads of turning force. Whereas your little hot-hatches will beat me from the line, but need to change down to get up the hills.
The TVR on the other hand, is a very different matter...
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