BHP and Torque Explained

[Ian]

At least. I'd like to think so.

As this subject of figures (MPG included) has always interested me, I've become very curious lately in the whole BHP and Torque figures - Explanation.

However, before I start rambling, I'm sure a quick google search will quantify my answers. But what does that matter? This Forum is open for discussion, and I'd apprecate some input, or even question!

In theory, BHP is a made up figure. It comes from the Torque of an engine, multiplied by the speed at which the crankshaft turns (ie. Your RPM) divided by 5252.

I was thinking, why 5252? This is just a variable right? Any mathmetician / physicist will be able to explain what a variable is, but not neccersarily why that number precisely.

So, you can get the BHP at any RPM of your engine - assuming you know what the Torque figure is at that specific point - Get a Dyno and work the BHP out for yourself, if you really wanted to. OK, so that's a bit far fetched and nobody would really do that.

I've also learnt recently that if you do look at dyno grahps, then you'll realise that the Torque and BHP curves actually cross points at the 5252 variable. I did mathematics at A level, and I still don't understand why; at this point?

Here's an example; Take my Bora for example. Thr torque at 2,800rpm is around 300lb/ft. (Educated guess)

=SUM((300*2800)/5252) -For your excel users, copy and paste this into a cell and press Enter.

That tells me (and you) that my BHP at 2,800rpm is 160BHP. Give or take 0.1BHP.

But who cares about BHP? What really matters is that you get from A to B safely.

This still leaves a lot to be desired. I'm still curious as to why 5252 is the variable used for this formulae, and even so, if you look at a Dnyo Graph for a Bus or HGV, you'll know full well that they'll never reach 5252RPM so the torque and BHP figures would never actually cross. Does this mean that BHP is completely meaningless. Period? Probably.

 

[doost]

fascinating! no, really! i'm interested because i'm looking at a new car and don't want one that couldn't pull the skin off a custard but the only figures you get are bhp. but i know that this is fairly meaningless, especially looking at diesels, which i want. Is the 5252 'variable' just a 'random' constant, like pi? What are the axes on the dyno graph?

 

[Matt]

This site http://www.howstuffworks.com/ answers all that stuff and has fancy diagrams to boot.

 

[Kevo]

I dont know anything about torque really but reading that, the reason why they cross at 5252rpm is solely because that is the value used,

torque x rpm(5252)/5252 = torque

No?

 

[Ian]

Matt, thanks - I know about the site already

The axis on a Dnyo Graph are Y = Torque/BHP and X = RMP.

 

[Ian]

Kevo. No.

My Bora at 5252RPM produces only 90lb/ft torque.

that means at 5252RPM my Bora only produces 90BHP.

Compares to my 160BHP at 2,900RPM.

 

[Kevo]

Is that not somehow related to the turbo? Doesnt the power fall off with more revs? I know the main power band in my JTD is between 3 and 4000rpm maybe upto 4500.

Like I say I don't know anything about it, Im just guessing

 

[Joske]

My powerband in my JTD is around 2500 - 2900rpms.

 

[Greg]

oohh something useful from my degree,

5252 is not derived from anything, like many equations, especially in my field of aeronautical engineering its a conversion collection number, basically they have taken the constants and collected them together. The 5252 comes from the fact that 1 horsepower is equivalent to 550 foot pounds, however to get the torque we need to divide the force by the angle turned through, so rpm of the engine has to be put into radians. 1 rpm is (gets calc out..) = 0.1047rads.

Therefore 550/0.1047 will give you 5252 or near enough. Simple!

Greg
TD100sx

 

[Ian]

The Power falls off with more revs because a diesel engine has typically longer strokes than a petrol engine.

Most diesels don't even rev over 5,000rpm.

 

[Jay]

if i was feeling a bit better i would dive into this with my wealth of knowledge from my motorsport engineering course but i'm really not with it today and give you a less in depth view,

we were all taught that torque (measured turning force) multiplied by speed (rpm) = nominal power at those revs, power being a by product (also simplified as "work done") of what the engine is doing...for example a 14litre truck might have 2000nm of torque but only revs to 2500rpm and therefore only makes 420bhp, where an f1 engine being only 2.5litres revs to close to 20,000rpm and makes nearly 800bhp but nearly no torque. it depends on the valve timing, volumetric/compression ratio, stroke and the fine balance of internal components (blueprinting etc)

there's a lovely equasion that can work out the power of an engine given a couple of readings (i think it was volumetric ratio and something else) and it will work out the maximum power the engine will give under perfect conditions.

what you have to remember is all engines are very much limited by their (fixed) cam profiles, even supposed "variable" cam timing engines are basically limited to 2 or three cam timing (lift & duration) possibilities. unless you can get constantly variable valve timing (something lotus is working on with pneumatic valve actuators) there will always be the inefficiency of the torque/power crossover point.

erm... not sure what else i can say apart from all internal combustion engines are inherantly inefficient (everyone knows even power at the crank isnt what you get at the wheels, just imagine how much it loses before it even gets to the flywheel keeping valves moving and oil pumping etc..)

anyway, enough rambling im fairly sure i've answered no questions at all here,

i'm off to be ill..

Jay

 

[Nathan]

WTF - Ok who's been smoking pot?????? really, i have not a clue what you are saying!!! lol

In fact i have to zone out now after reading that as i am damaging my brain cells, and its making me thirsty

Nathan

 

[Ian]

Nevermind.. It was open for discussion. Let's not turn this thread into a drug-based one

 

[Anonymous]

I know what you are getting at, i was just joking, and plus i have always been interested in how its all worked out, but alass, im just a layman

Nathan

 

[rsmallwo]

bhp vs torque

Now then everyone

Heres my layman speak for difference in torque vs bhp.

torque = what pushes you into the back of the seat

bhp = a figure that is worked out by having torque multipled by how long that "back of your seat feeling" goes on for before you have to change gear.

So you might have loads of torque but if your engine can't rev much then this torque can only be felt for a short time then you have to change up a gear. However in 5th for example because the gearing is much longer than 2nd it takes much longer before your out of the "back of your seat feeling" and need to change up a gear.

if you have a car with a high bhp but low torque then this will result in a slight "back of your seat feeling" but it will last for a long time until you have to change up a gear.


depends what you want to do with your car, if you want to race round tracks then high bhp is good as you don't lose time changing gears. if you want to be able to race people from the lights you want a balance of good torque and good bhp. high speed accelartion (70 upwards you want lots of torque)

on www.parkers.co.uk you can see the bhp and torque figures for all makes of cars back to 1989 i'd bang these figures on a spreadsheet and then compare the cars likewise.


disclaimer: this is my generalisation, gearing of cars can make alot of difference i.e low bhp cars doing 0-60 really quick due to low gearing, high bhp cars 0-60 not that good but blistering accelaration at higher speeds.