Torque vs bhp

[leosayer]

I've read loads of posts on the subject but I still can't quite say I understand the difference.

Let me give you a hypothetical scenario to help me illustrate:

Assume a car has a flat torque throughout the rev range (unlikely I know).

If this is the case then should acceleration times for a sprint (say 30-70mph) should be the same in any gear, regardless of rpm (assuming no rev limit etc)??

So therefore, 2nd gear time for the 30-70mph sprint will be the same in 3rd gear, 4th gear etc.
Am I on the right lines?

[ultimasimon]

BHP is for pub conversations - torque wins races

[veewhy]

BHP is an illusion, Torque is a reality. think of it like arm-wrestling, it's the torque that gets you past your opponents resistance threshold, and it's the illusionary BHP, that slams his arm on the table...

[matt_t16]

BHP is how fast you hit the wall
Torque is how far you take it with you

In simple terms BHP is the "work" the engine can do, Torque is the actual turning force is can apply to the wheels.

[lunarscope]

BHP ccan not really be measured as it is simply a number derived from the torque value.

[bridgdav]

Torque = how it gets there.
BHP = what it does when it gets there.

[greg_D]

lunarscope said:

BHP ccan not really be measured as it is simply a number derived from the torque value.

correct, that is why the torque curves and BHP curves on a printout always intersect at 5,252 rpm, go and check. BHP is a function of Torque

greg

[leosayer]

OK thanks guys, but can anyone answer my question???

[mike s]

ultimasimon said:

BHP is for pub conversations - torque wins races

Think you'll find it's, "BHP sells cars, Torque wins races"

[RobDickinson]

Torque is a force.

BHP is a measure of work done - but is calculated from torque mathermaticaly.

Its actualy the area under the torque curve which is important, no good having gobs of torque but over 5rpm.

With bhp and rpms you can take more use of gearing than you can with lots of torque over a few rpm.


Whatever its better to make torque at higher rpms which gives more power than torque at lower rpm. More bangs = more power.

[RobDickinson]

As answered.

if you have the same power/torque but different gear ratio's you'll take differing times to do the work.

i.e.
2nd will be 0 - 70mph
3rd will be 0 - 100mph

So to do 30-70 your using a much more 'concentrated' gearing with 2nd than 3rd.

You make more power further up the rpm range with a flat torque curve.

[uriel]

The reason it won't accelerate at the same speed in all gears in your example is that the car is only producing the same torque at the crank. The gears then modify this figure. So even though the torque at the crank is the same in 2nd as it is in 5th, the torque at the wheels is much higher, accelerating the car more quickly.

I've heard engine builders quote a saying 'build for horsepower, gear for torque'. This seems to be backed up by F1 cars. IIRC the figures are 800+ HP, but only ~250 for peak torque. They make up for it with crazy high revs and gearing.

>> Edited by uriel on Thursday 20th May 14:43

[ultimasimon]

mike s said:

ultimasimon said: BHP is for pub conversations - torque wins races Think you'll find it's, "BHP sells cars, Torque wins races"

I stand corrected

[Munter]

Check out this guys page.

www.pumaracing.co.uk/

Scroll down a bit and theres a few articles about power and torque. Now as he builds race engines he should know his stuff.

Also he has a very funny page about customer types!

(And he's rebuilding the head for my MR2....so dont give him no grief or i'll get charged double!)

[HiRich]

Leosayer,

As a couple of people have correctly spotted, your car will still accelerate at different speeds in the different gears.

What really matters most of all is the tractive effort (force) delivered where the driven wheels touch the ground. Your gearbox, diff and wheel diameter change the alter the rotational speed and torque. rotational speed is reduced by a total ratio factor, and torque (and therefore force) is increased by the same ratio (let's ignore frictional losses in the drivetrain for now). Using a shorter ratio (lower gear) increases this ratio compared to a higher gear. Therefore there is more tractive effort in the lower gear. Therefore it will accelerate quicker. If the ratio for first is half (or twice, depending on how you look at it) that of 4th, then acceleration in 1st would be twice that in 4th. As:
Final speed = Initial speed + (acceleration x time)
your 30-70 time would halve.

Now, since power is torque x rotational speed, you will quickly spot that Power In = Power Out. If you had an engine that produced constant power through the rev range, then you WOULD see identical acceleration figures.

So I suspect you might have your assumptions the wrong way around. A few things to remember:
- Power means nothing without knowing weight (anyone who disagrees with this can try to explain why my 140bhp car does 60-100 in 2.5 secs)
- Torque means nothing without knowing gearing (anyone who disagrees with this can try to explain why my 70Nm car can....etc.)
- Power in = power out (so power at the engine = power at the wheels). Not strictly true because of losses, but a good mnemonic when you remember...
- Torque in does not equal torque out. It is always factored by the gearing.
- so, correct gearing is crucial.
- Where power and torque appear is almost as important as what figures you actually get.
- For foot-to-the floor motoring, power tells you more than torque.
- Torque gives you more flexibility, whether that is when you are not using full throttle, running outside the normal rev range (like stump pulling), or in transient variations (whether that means kickdown, or running over a cat's eye)
- If all you know is the max power and max torque, max power tells you more. However, it's like trying to estimate the contents insurance of a house by looking through the letterbox or keyhole (ie not very)

Now to prove that you've understood it all, another practical question:
You have four identical swimming pools that need draining. You have four similar pumps:
Pump A gives 1hp power, 1lbft torque
Pump B gives 1hp power, 2lbft torque
Pump C gives 2hp power, 1lbft torque
Pump D gives 2hp power, 2lbft torque

Pump A takes exactly 12 hours to drain its pool
Estimate how long each of the other pumps take, and justify your estimates.

Your homework is due by Friday afternoon.

[GavinPearson]

HiRich said:

Now to prove that you've understood it all, another practical question: You have four identical swimming pools that need draining. You have four similar pumps: Pump A gives 1hp power, 1lbft torque Pump B gives 1hp power, 2lbft torque Pump C gives 2hp power, 1lbft torque Pump D gives 2hp power, 2lbft torque Pump A takes exactly 12 hours to drain its pool Estimate how long each of the other pumps take, and justify your estimates. Your homework is due by Friday afternoon.

A = 12 hrs (given)
B = 11.5 hrs - fewer internal gearing losses
C = 6 hrs (double the power of A)
D = 5.75 hrs - (fewer losses from gearing than C)

[V8 Archie]

My old granny/fishwife says:
Torque is cheap
Power is money

Please don't flame me. It sounded good and I'm pd

[ATG]

Gazboy said:

Torque is power, BHP is how quickly it makes it.

<patronise>No ... bhp is power (hence the "p" at the end.<patronise/>

[Marki]

matt_t16 said:

BHP is how fast you hit the wall Torque is how far you take it with you

Now that has got to be the clearest description ever
thank you

[fish]

All the pumps take 12hours as the BHP/torque relate to the motor. All the motors drive the same pump which was only designed fro a 1/4bhp motor and thus runs to full capacity with all motors. Therefore they all take the same time.

Alternately...