Educating Sarnie

[Sarnie]

What does BHP mean? Obviously its Brake Horse Power, what what does it actually mean in real terms? I'm sure it has some sort of scientific equation to derive it but what is it?

 

Also, along the same lines, what actually is torque? How is it measured and what does it represent? How is it generated and what do you need to modify to improve torque?

 

How doe the two relate to each other? Which is better to have more of?

 

[ir_fuel]

Horse power at a certain RPM = Torque at that RPM times the RPM (and then scaled by some constant, not gonna explain how to calculate that )

 

horsepower is the amount of work done by the engine, torque shows the force that the engine delivers on a lever. 1Nm = 1 Newton of force given on a lever at a distance of 1 meter. (This is because engines tend to make things turn, hence the fact they use torque and not simply force ).

 

So basically: the more torque, the harder it will pull on your wheels, for the same car ofcourse (gearboxes and other stuff can influence the final amount of torque on the wheels ).

 

As you can see, one is related to the other (from one you can calculate the other and vice versa). Now one can say more torque is always better, and it is, unless you can start playing with the rev limiter. For instance it might be better to have 40Nm less torque, but because the rev limiter has been upped 2000 rpm more horsepower (if the torque were to stay constant), coz one can stay longer in the same gear (the Honda V-Tec principle lets say ) , than to have 100Nm more, but 1500 rpm less (your average diesel engine).

 

Now ofcourse if the both of you are driving at 2500 rpm and floor it, the one with more torque will be quicker intially, but when you keep going the one with much more rpm before the limiter will be able to go faster in the end because the guy with lots o torque but no revs already has to change a gear while the other dude stays in the same gear. (As you might have noticed, changing gear up reduces the "pulling-power" at the wheels ).

 

This all under the assumption that the high-rpm low-torque car does not decrease its torque when getting to the rev limit.

 

Hope this makes sense?

 

If you have any more questions, just shoot, i ll try explaning

[Sarnie]

So does one relate to another or are they separate entities?

[Guest prescience]

They are related as noted above.

 

If you know the BHP at a certain rpm, you can calculate the torque and vice versa.

 

Rolling roads and engine dynos ALWAYS measure torque and rpm and calculate BHp from that

[ir_fuel]

for you weird-unit-using ppl:

 

Torque in lb-ft = HP * 5252 / RPM

 

So a car producing 200 HP at 6000 rpm gives:

 

200 * 5252 / 6000 = 175 lb-ft of torque at 6000 rpm

[Sarnie]

Ahhhhh.

 

Starting to make a little sense now

 

Where does the 5252 come from?

[Liam's Z]

Ahhhhh.

 

Starting to make a little sense now

 

Where does the 5252 come from?

 

Dividing factor for calculating power against bhp

[ir_fuel]

Ahhhhh.

 

Starting to make a little sense now

 

Where does the 5252 come from?

 

http://auto.howstuffworks.com/question622.htm

 

and

 

http://auto.howstuffworks.com/horsepower.htm

 

Have fun

[H5]

Power being the ability to maintain a top speed, torque being the ability to get there.

 

Equation goes out of the window when FI comes into it IIRC.....??

[ir_fuel]

Power being the ability to maintain a top speed, torque being the ability to get there.

 

Eh no?

 

Equation goes out of the window when FI comes into it IIRC.....??

 

Eh no? What does FI change to this matter?

 

[Sarnie]

Power being the ability to maintain a top speed, torque being the ability to get there.

 

 

That just sounds wrong to me

[Jackal]

Torque :

 

You wake up in the morning and part of your body is more 'awake' than the rest - seems more alert and ready to go...but before you can enjoy it, you have to pee.

 

So you run off the to the bathroom and stand there at the toilet. But your Old Man, your Manhood, 'Junior' is pointing at the ceiling! That will never do.

 

So what do you do?....you put both hands on your Big End and push down.

 

And as you are pushing down on your Gear Knob, your heels lift off the floor - that's torque!

 

[H5]

Power being the ability to maintain a top speed, torque being the ability to get there.

 

Eh no?

 

Equation goes out of the window when FI comes into it IIRC.....??

 

Eh no? What does FI change to this matter?

 

 

Why no?

 

OK, I didn't remember correctly! It's when diesel + FI are involved it goes out of the window!

[Sarnie]

Torque :

 

You wake up in the morning and part of your body is more 'awake' than the rest - seems more alert and ready to go...but before you can enjoy it, you have to pee.

 

So you run off the to the bathroom and stand there at the toilet. But your Old Man, your Manhood, 'Junior' is pointing at the ceiling! That will never do.

 

So what do you do?....you put both hands on your Big End and push down.

 

And as you are pushing down on your Gear Knob, your heels lift off the floor - that's torque!

 

 

That clears that up then

[mugwump]

and because that relationship is a constant between HP and torque a correctly drawn plot of the two values should *always* cross at 5252 rpm.

 

Examine the excellent example here

http://www.350z-tech.com/zwiki/Image:HKS_Rotrex_Supercharger_05_z.jpg

which shows two plots on the same chart, one standard engine, one with FI.

 

It makes no difference what the engine technology is the two lines should always cross (with power rising above torque) at that rpm no matter what the engine type or how flat the curve.

 

See a rotary example here

http://www.myrotarycar.com/portal/forum/uploads/RX8TX/files/2005-11-22_191417_rx8tx800.gif

 

You will of course find examples where the two lines do not cross at 5252rpm but these are plotted against axes with different scales on the left and right for bhp and torque, why people do this I dont know, perhaps its to mislead!

 

One further point (for you all to flame me about) peak torque occurs where the engine is breathing at its best, this is the speed at which the engine delivers its "peak power for a single stroke of the engine", the reason the total power continues to climb after this is because the torque has fallen away but because the engine is performing more "less optimal" strokes in a given second the power is greater.

 

and finally there is nothing magic about 5252rpm its just the speed at which power and torque are the same as a result of the physical constant.

[H5]

So is it turbo diesels this doesn't work for?

 

Example - Astra 1.8 petrol

Power - 125 @ 5600 rpm

Torque - 170 @ 3800 rpm

 

Astra 1.9 TD

Power - 120 @ 3500 rpm

Torque - 280 (!) @ 2750 rpm

 

The diesel one doesn't go into the equation? (using the other info on the net)

[mugwump]

Another way to put it,

 

below 5252 rpm torque will always be more than power *at that engine speed*

above 5252 rpm power will always be more than torque *at that engine speed*

 

but *peak torque* can easily be above *peak power* as your astra 1.9D shows but these peaks occur at different speeds

[H5]

Another way to put it,

 

below 5252 rpm torque will always be more than power *at that engine speed*

above 5252 rpm power will always be more than torque *at that engine speed*

 

but *peak torque* can easily be above *peak power* as your astra 1.9D shows but these peaks occur at different speeds

 

Gotcha!

 

So how can a car with a 5 bhp power difference have a 110 lb ft difference in torque???

[Rish]

New to this all, but copuld it be that turbos help with torque more than bhp?

[mugwump]

That does seem whacko I agree. I need to find a calculator and a fag packet........

[ir_fuel]

Another way to put it,

 

below 5252 rpm torque will always be more than power *at that engine speed*

above 5252 rpm power will always be more than torque *at that engine speed*

 

but *peak torque* can easily be above *peak power* as your astra 1.9D shows but these peaks occur at different speeds

 

Gotcha!

 

So how can a car with a 5 bhp power difference have a 110 lb ft difference in torque???

 

Coz the way the curves are? Look at the rpms associated!

 

The petrol one has max hp at 5600 rpm

the diesel one at 3500 rpm

[ir_fuel]

New to this all, but copuld it be that turbos help with torque more than bhp?

 

Easy, coz most turbos influence your torque curve in the lower and middle range of the rpm range of your car, but wont influence it at the top end, or at least not that much as in the middle. So the TOP rpm hp will not change that much, but if you were to look at the hp/torque somewhere in the middle of the rev range the difference will be much bigger.

 

The way the engine works doesnt matter, the calculation from torque to hp ALWAYS is the same, even if you were to put a guy on a bicycle on a dyno

 

[H5]

Another way to put it,

 

below 5252 rpm torque will always be more than power *at that engine speed*

above 5252 rpm power will always be more than torque *at that engine speed*

 

but *peak torque* can easily be above *peak power* as your astra 1.9D shows but these peaks occur at different speeds

 

Gotcha!

 

So how can a car with a 5 bhp power difference have a 110 lb ft difference in torque???

 

Coz the way the curves are? Look at the rpms associated!

 

The petrol one has max hp at 5600 rpm

the diesel one at 3500 rpm

 

Agree that the curves are like that, but if the turbo / & diesel bits doesn't do anything to the relationship, why is the peak torque so different?

[mugwump]

I know I am going to regret posting this!

 

============================================

 

Maybe the graph below will help explain it.

 

Yes I have made this up but the points of the torque curve are calculated from the equation in this thread (torque=power*5250/rpm) so the relationship between the power and torque is *calculated* and not massaged by me.

 

The pink/yellow lines are the diesel and the cyan/blue are the petrol and the peak power and torque are exactly as you have specified them in your earlier post.

 

The key thing to note is that the diesel makes all its torque and power very early on and then drops away rapidly, the lines are still convergent at 5252rpm but the diesel owner will have changed up to the next gear long before then and the manufacturer will have put a rev limiter on it!

 

It just shows something we all intuitively know from the way a diesel pulls at low revs.

 

Prize to the first person that can find a *real* dyno plot for these two engines!

 

I shall go away now and get a life ......I think its singles night at Tesco.

[H5]

lol - cheers mugwump!

 

Sorry if I'm being thick, how does that work with the diesel peak torque being so much higher than the petrol off the same equation??