Dingdongler

Sorry about asking a question that has probably been done to death. I've read various explanations about the difference between BHP and Torque over the years and if I'm honest I still don't really understand.

Can somebody explain it to me using plain English?

Thanks

banny sti

iTrader: (68)

Torque gets your going but bhp keeps you there

MMT WRX

Go ask on PHeads, you'll be even more confused then.

john banks

"Understeer is when you hit the wall with the front of the car and oversteer is when you hit the wall with the rear of the car.
Horsepower is how fast you hit the wall, torque is how far you take the wall with you."

MMT WRX

Dougmy06wrx

iTrader: (4)

Hahaha that's the best explanation I've ever heard.

Setright

Torque is strength, Horsepower is stamina.

Or...torque is how heavy a weight you can lift....horsepower is how fast you can lift it.

Steve vRS

Power is what you talk about, torque is what you feel.

LG John

I'll give it a go

Firstly torque is a measure of force. If you push against a wall you are exerting a force on that wall. If you twist a screwdriver you are exerting a force. If an engine turns a driveshaft then it is exerting a force.

You must also understand that you can multiply a force through gearing. For example, your best effort at peddling on a mountain bike has a rather different outcome on the road depending on which gear you are in.

BHP is a measure of power and, in the case of engines, indicates it's maximum energy output.

What you need to understand is that they are both related and that you must consider the job at hand to appreciate which is more useful.

For example, suppose you have to push an object off someone to save their life and you must move it 1m to free them. Well, you are buggered if you can't get it moving at all. You can huff and puff and do lots of 'work' but your patient still dies. In this case you lack torque. However, you are equally buggered if you can get it going with ease but are out on your **** after 10cm of moment - your patient still dies because you the power to get it off them in good time.

A craft with very high torque and relatively low power can excel at moving massive weight. An example of this might be a super-tanker which I recently learned can have as little as 1hp per ton of weight. Imagine trying to move a Ford Puma with a 1hp engine! This isn't a problem with a supertanker because it needs monster force to even get the thing moving at all - thereafter it doesn't need to go particularly fast.

On the other hand, by applying a small amount of torque in a very short space of time you can achieve a lot of power. A good example of this is the small engines you get in r/c cars. The little pico engine I have is good for something like 1.5hp at a dizzy 20,000rpm (or thereabouts). That is a heck of a lot of power for a device weighing just a few kg and that is why it can accelerate so fast and achieve such high speeds (relatively speaking).

However, compare that 1.5hp engine from an r/c car to a 1.5hp outboard for small boat. Do you think for a second that the r/c car engine would get a prop turning and a 2-3 up dingy moving? No. Similarly, would an 1.5hp outboard motor suitably attached to a chassis and wheels rocket from 0-45mph in 2 seconds....no.

So, long story short. You need both and they are ultimately related. Which way you want to swing depends on what your priorities are. Diesels make great load luggers (and that fits with the above) and high revving petrol cars make great race cars (and that also fits with the above).

To save yourself the heartache, just go for overkill in both areas (i.e. Vette ZR1; power, torque, displacement, FI....the lot)

hodgy0_2

torque is "work" (or a measure thereof), BHP is the "rate" at which the "work" is done

SRSport

Ok but in the real world when we're applying what does it mean for the average car?

This is only my basic understanding and am in noway saying that I am 100% correct so please feel free to re-educate me if I need it but I believe BHP is what will ultimately make you go fast, e.g. Honda S2000, low torque but high(ish) power. It will go very quickly, but only during the upper most part of the rev range. If you were to compare it to a car that has 50bhp less but an extra 100lbft, more power would be accessible earlier in the rev range than compared to the S2000 even though it has lower overall power rating/output.

e.g. A Ford Mondeo ST TDCi would feel quicker than an S2000 most of the time but when the S2000 wants to get going and is willing to go past 5000-6000rpm it will blow the ST out of the water all day long. Its just we dont all go round driving like that which is why I would imagine the ST to feel quicker.

LG John

100% right, ST TDCI's used to pull away effortlessly from my S2000 at 60-70mph in top gear. However, if we both adjusted our gearing for optimal acceleration (i.e. dropped down (in his case to 4th probably and 3rd in my case) then I'd go flying past effortlessly.

I might be wrong on this, but I'm pretty sure the reason for this is that the S2000's torque at the wheels would be better than the Mondeo's at that point do to the gearing?

Ultimately to move something from point a to point b you need energy. The more outright energy that is available the more likely it is you can do that quickly. For this reason I'm an very concerned with the bhp of the cars I buy and less concerned with the torque.

FWIW, the reason this is always so complicated is because engines can achieve different speeds, they can vary that speed, and you can apply gearing to them.

Jimpreza

Power = torque x revs

hodgy0_2

so the S2000 "when pushed" has a higher work rate, and is ultimately quicker

(they are related btw - you can't have one without the other)

SRSport

If this was true in the real world then the ST ford would be a lot faster than the Honda when it reaches higher revs and beat it at equal revs throughout the range.

SRSport

Yes but the value of BHP is higher than the value of lbft when they are both accessed at their peaks, which is why I made the above point.

Devildog

From basic physics

Power = force x distance / time

So think of torque as the turning force being applied to the wheels and power as the speed (distance/time) at which the wheels can be turned whilst that force is applied.

For internal combustion engines,

BHP = torque x rpm / 5252.

Where torque is measured in ft-lbs (as 1 hp = 33,000 foot-pounds per minute)

rb5_336

In real life terms, torque determines acceleration (hoe hard you get pushed into the back of the seat) and bhp determines top speed. It's only race cars that crave high bhp, day to day card need good mid range torque for overtaking. A fast car will have 3-4k revs worth of 80% peak torque available.

Sean

gallois

although torque is often talked about, it is often very misunderstood, (usually by diesel drivers) afterall, what is the point in knowing what the peak torque of an engine is and at what rpm unless you know what the torque curve looks like.

in theory and for example, the S2000 has a very flat torque curve, and relatively low at 153ft/lbs, and that sounds ****.

But it is around 150ft/lbs from 3500rpm up to 8500rpm, this doesn't mean that the acceleration will be flat, because the torque is multiplied by the rpm, so as the revs rise, the acceleration rate increases too. it would accelerate like an upward curve, even beyond it's peak torque.

Compared to a high powered diesel car, the 335d, peak around 400ft/lbs at 2000rpm, and gradually dips to 300ft/lbs at about 4500rpm. when in fact, it is not a decrease, but an increase, otherwise the acceleration would feel like it is dying off, (it also will accelerate like an upward curve)even though it's peak is 400ft/lbs at 2000rpm, 300 ft/lbs at 4500rpm is significantly more (work done)....if a variable ratio gearbox were fitted, the optimal rate of acceleration would be at a constant 4500rpm, not the expected 2000rpm.

similarly, for the S2000, using a CVT gearbox, it's optimal acceleration would be at 8500rpm at around 145ft/lbs, not it's peak of 153 at 7000rpm. The differnce between the two ENGINES in a 'work done'/acceleration test (hypothetically speaking and disregarding weight difference etc) would not be that great.

Of course this is simplifying things, as does not take into account weight, drag etc, but to give it some perspective, my own car, scoob produces 295ft/lbs at 3000rpm, peaks at 321ft/lbs at 3800rpm, down to 295ft/lbs at 5000rpm, down to 235ft/lbs at it's redline. So it's optimum RPM is pretty much anywhere between 4k and it's redline, it would (and does) feel very linear accelerating once on boost (3k-). So although the beemer offers a better peak torque, it's optimal torque is less than my car's optimal torque, and like for like (disregarding drag, using a CVT etc etc blah blah) my little scoob with less peak torque would get more 'work done'/accelerate quicker (although use twice as much fuel)..............see, easy peasy...........

LG John

This is the traditionally held view but it is not the whole picture as the post by gallois explains. This is the argument that diesel fanboys use but it doesn't explain why an S2000 can easily out accelerate an ST TDCI. The simple formula's offered by devildog and gallois's post are the clues as to why.

Assuming I am using the formulas correctly:

If an engine had 500lbft of torque at 1200 rpm then it would have 500x1200/5252 = 114bhp. That's not a great deal of power so the engine cannot do massive amounts of work. But, **** me could 500lbft at such a low rpm shift some weight.


On the other hand, a motor with 100lbft at 12,000 rpm would have 100x12,000/5252 = 228bhp. It can't pull the skin off a rice pudding but it sure can do a lot of work when you get it spinning that fast. It is also the case that we have 5x less torque but have achieved double the power by producing that torque at a far greater engine speed.

^^^ think big truck vs sports bike here.

tony de wonderful

So does a dyno only directly measure HP then and torque is calculated given engine RPM which is known?

LG John

I'd imagine it would be the other way around, i.e. it'll measure the torque at the wheels (i.e. the force the car can apply to the rolling road) and then use that data to calculate then bhp at the wheels and then estimate it at the fly.

??

[-(o)-]

iTrader: (1)

Its been alluded to already, but to keep it simple diesels require very different gear/final drive ratios to petrols as they still need to provide the same range of wheel speeds with an engine that runs to 4.5k rpm. Less step up in torque is given by the transmission to match a petrol that may run to 8k rpm. A better comparison would be torque across all driven wheels at a range of wheel speeds.

Think of torque as twist - you don't have to be moving to exert a twisting force (think chinese burn!). Start moving and its becomes a component of power.

scoobyc

iTrader: (2)

BHP = torque x rpm / 5252.

why is it 5252?


I think i get the explainations so far. Does a rolling road measure the rate at which the rollers accellerate given they are of a known mass/rolling resistance to work out the torque provided.

are we saying that a engine which produces 100 torques at 100 rpm, produces the same bhp as an enginen that produces 50 torques at 200 rpm?

LG John

Yes it will and that is logical because the first engine is apply 2x the force of the 2nd engine. However, the 2nd engine is apply that force at 2x the speed of the first engine. Therefore, the energy output must be the same.

scoobyc

iTrader: (2)

So if they were both put in an identicle vehicle. with a cvt. would they accellerate at the same rate?

(theoretically speaking)

Setright

Read the post before replying guys !

LG John

Not sure, I don't really understand CVT's and their implications. My gut instinct is that they would if the figures you quoted are where they make peak power. This, I guess, would be because you could continuously vary the transmission to keep the engine in that sweet spot.

However, I defer to the more knowledgeable.....

scoobyc

iTrader: (2)

yeah cvt = continiously variable trasmission. i.e. you can use that peak rev.

Here we are talking super theoretically. i.e. no other rev speeds or loss n 100% efficient cvt etc etc.

adding curves of power rpm and all that logorhymic **** will make it harder to understand.

hodgy0_2

see post #10 - explains it all in one line