Which is faster horsepower or torque?

[Kozy]

nicfaz said:

It has been said to me that cars like mine (Stage 3 tuned Saab 95 ~295bhp / 325lb/ft) can accelerate as fast as a BMW M3 E46 (338bhp / 270lb/ft). This is by people who have done the 'drag strip' test from a rolling 30mph. In fact, the Saab was slightly ahead until 3 figure speeds. I wasn't sure that could be right because the M3's power is quite a lot more. However, this thread indicates that average power over the relevant gear ratio spread would be the real measure and on that, the Saab would be comparable (high torque throughout the rev range, peak power held back because the turbo can't shift the air). Interesting.

Thanks to all contributors for an interesting thread!

My gearing model will calculate acceleration for different ratios and torque curves, and if you input a torque curve that gives a near constant power output of say 200bhp, and another one with a flat torque curve and a peaky power output, the one with a flat torque curve would need about 20% more peak power to keep up.

Average power at work.

[mumfurio]

My car (Mountune focus ST) has around 300bhp and around 300lb/ft...it feels super torquey rather than super powerful, maybe thats because it's not very revvy, no point in taking it much over 5500rpm where the torque starts to drop off...in the band 1700-5500rpm, any gear it's an overtaking champion.

My last track car was a 180bhp MR2, there was very little torque, but with little weight and lots of revs it went ok....downhill & on the flat!!

I can't do the maths...but I think I can feel which is which!

[radio man]

tvrolet said:

XJ Flyer said:

It's going to be a st engine that can only make 400 hp from a torque peak of 600 lbs/ft.LOL

Do the question again this time which is faster a car with 600 lbs ft peak torque and 600 hp peak power or a car with 400 lbs ft and 600 hp.It's all about the amount of torque and how flat the torque curve is either side of peak torque from which we can deduce average power and hence acceleration.

Edited by XJ Flyer on Monday 13th January 20:41

Christ on a bike, does no-one actually understand this stuff?

Your 600hp/600ft/lb motor can ONLY make that at 5252RPM. Not higher revs, not lower revs, but 5252RPM.

The 600hp/400ft/lb motor HAS to be turning ar 7878RPM, thus is it effectively making 1.5 times the power at the end of the gearbox.

Shall we try this again kids. All together now...

TORQUE IN MEANINGLESS UNLESS YOU CONSIDER THE RPM
TORQUE IN MEANINGLESS UNLESS YOU CONSIDER THE RPM
TORQUE IN MEANINGLESS UNLESS YOU CONSIDER THE RPM
TORQUE IN MEANINGLESS UNLESS YOU CONSIDER THE RPM
TORQUE IN MEANINGLESS UNLESS YOU CONSIDER THE RPM

Got it. Now try this one...

FOR A GIVEN TORQUE AND RPM THE POWER IS ALWAYS THE SAME (TOURQUE x RPM / 5252)
FOR A GIVEN TORQUE AND RPM THE POWER IS ALWAYS THE SAME (TOURQUE x RPM / 5252)
FOR A GIVEN TORQUE AND RPM THE POWER IS ALWAYS THE SAME (TOURQUE x RPM / 5252)
FOR A GIVEN TORQUE AND RPM THE POWER IS ALWAYS THE SAME (TOURQUE x RPM / 5252)
FOR A GIVEN TORQUE AND RPM THE POWER IS ALWAYS THE SAME (TOURQUE x RPM / 5252)

Lets consider a motor with the flattest torque curve in the world and it makes 10,000 Ft/lb

But it makes it at 10 RPM, so the power is under 20hp. So your 10,000ft/lb flat-curve motor will be accelerating slower than a 2CV.
So much for torque being the thing that makes cars go.

Jeez

Ah'm oot.

I don't know about Christ on a bike, I could do with him to explain the maths, I was lost when I read the initial question let alone your explanation! :-(

[tvrwedgehead]

Neither car is faster... for the simple reason everyone fell asleep through boredom and the old guy on the pushbike beat them all to the finish line...

[Hubris]

(Brake)Horsepower is a product of Torque and RPM.

bhp = torque * rpm/5252

They are inextricably linked.

[Hubris]

Err, as has been mentioned previously.

[anonymous-user]

Neither are faster, because both are stuck behind the same caravan......... ;-)

[McWigglebum4th]

Sod it

Instead of asking people why scaffolding pole can't out accelerate a ferrari maybe i should just hit them with it until they shut up

[andywaterfall]

V8LM said:

Point about units above is a good one, and explains much:

Torque is in [lb ft] which is a force x distance. This is equal to work.

Power is work per time. Power = [lb ft] / [s]

Power = [lb ft] / [s] = [lb] [ft] / [s] (which is why power is proportional to torque x rpm]

Power is therefore a force x velocity

Force is therefore power / velocity

So at any velocity the greatest driving force comes by maximising power.

Nice one, Some magazines talk about power to weight ratios, some talk about torque to weight ratios too. I've often wondered, can you divide your units when you're taking ratios too then (serious question)? Eg I've got 250lbft torque in a 3000lb car, so dividing numbers & units, my car's got a torque to weight ratio of one inch. Seems odd, but logical, no? <prepares to be told I'm a complete imbecile>

[hairykrishna]

andywaterfall said:

Nice one, Some magazines talk about power to weight ratios, some talk about torque to weight ratios too. I've often wondered, can you divide your units when you're taking ratios too then (serious question)? Eg I've got 250lbft torque in a 3000lb car, so dividing numbers & units, my car's got a torque to weight ratio of one inch. Seems odd, but logical, no? <prepares to be told I'm a complete imbecile>

Easier to see with the derivation from SI units;

Torque is in Newton meters. 1 Newton is the amount needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared ie force is [Mass][Length][Time]^-2 so Torque is [M][L]^2[T]^-2. Divide by mass and you get torque to weights units of [Length]^2[Time]^-2. Unless I've buggered it up.

[Mave]

hairykrishna said:

andywaterfall said:

Nice one, Some magazines talk about power to weight ratios, some talk about torque to weight ratios too. I've often wondered, can you divide your units when you're taking ratios too then (serious question)? Eg I've got 250lbft torque in a 3000lb car, so dividing numbers & units, my car's got a torque to weight ratio of one inch. Seems odd, but logical, no? <prepares to be told I'm a complete imbecile>

Easier to see with the derivation from SI units;

Torque is in Newton meters. 1 Newton is the amount needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared ie force is [Mass][Length][Time]^-2 so Torque is [M][L]^2[T]^-2. Divide by mass and you get torque to weights units of [Length]^2[Time]^-2. Unless I've buggered it up.

Yep, that's right.
Andywaterfall, you missed that the "pounds" in lbft is pounds force, whereas the "pounds" in weight is mass. So you need to bung the missing "acceleration" units into the equation to go from mass to force. ie take your length unit (L) multiply by the missing acceleration (LT-2) to get hairykrishnas L2T-2.

[leon9191]

Andy M said:

Two identical cars in weight, layout, engine capacity and layout, body design etc with the same type of gearbox (but different ratios bespoke to their respective engines), which would be quicker over 1/4 mile:

A car with 600bhp and 400lb/ft?

Or a car with 400bhp and 600lb/ft?

Assuming a few things, 1) This is from a standing start 2) These are peak figures that occur at different rpm ie like a real car engine one being a height revving na the other a lower revving turbo for example. 3) all other things are equal with the exception of gearing which we could propose is geared for the same top speed accounting for varying rev limits.

Over the standing 1/4 the higher power car will be quicker in theory due to its ability to do more work, however, from the standing start and at lower engine speeds the car with more torque (and therefore generating more power at lower engine speeds) will likely be the quickest off the line but as the engine speeds increase a tipping point will be reached when the higher power car will be generating more power, therefore will be doing more work and as such will have a higher rate of acceleration. At which point it will likely pass the lower power car and be quicker over the 1/4.

[Rockstar]

McWigglebum4th said:

Sod it

Instead of asking people why scaffolding pole can't out accelerate a ferrari maybe i should just hit them with it until they shut up

[blahblop123]

to me, the actual torque figure doesn't say much, i think it is about the HP figure, and where maximum torque is in relation to the redline.

say..

in one gear, car 1 will hit 60mph at redline. it makes X amount of hp (6000 redline) and Y amount of max torque @ 4000rpm. this means below 4000rpm (40mph) it won't accelerate that fast.

car 2 in the other hand will also hit 60mph at redline, has the same hp (same redline) and the same amount of max torque, HOWEVER maximum torque is @ 1500rpm-4000rpm (15mph-40mph).

unlike car 1, car 2 will pull nicely from 15mph (that torque feeling) and would leave car 2 for dead until it reaches 40mph, from which it will accelerate the same. car 2 will feel as if it has more torque, despite having the same amount of torque.

i say max torque in relation to redline because there is also a car 3, which is a diesel. car 3 has the same HP (redline 4000) HOWEVER the max torque is DOUBLE of car 1 and 2 @ 2000rpm. if this diesel hits 60mph at redline like car 1 and 2, it will only pull from 30mph. see how the car with half the max torque (car 2) will feel torquier than a diesel with double the torque.

[lozzzzzz]

Why is this still being discussed? This is not that complicated.

Read a basic engineering text book. As many a frustrated person has already stated. Torque is meaningless without RPM.

[Mave]

doogz said:

andywaterfall said:

Nice one, Some magazines talk about power to weight ratios, some talk about torque to weight ratios too. I've often wondered, can you divide your units when you're taking ratios too then (serious question)? Eg I've got 250lbft torque in a 3000lb car, so dividing numbers & units, my car's got a torque to weight ratio of one inch. Seems odd, but logical, no? <prepares to be told I'm a complete imbecile>

Torque is actually measured in pounds-force per foot. So if the comparison was remotely valid, which it's not, you'd have a 1 inch*g car. Whatever that'd mean.

The force part of the torque is just that, a force, not a mass, and it's nto comparable with the mass of the car.

Nope, torque is measured in pound force foot, not pound force per foot. It may seem pedantic, but its really important when somoene is trying to understand how the units divide!

[jamieduff1981]

andywaterfall said:

Nice one, Some magazines talk about power to weight ratios, some talk about torque to weight ratios too. I've often wondered, can you divide your units when you're taking ratios too then (serious question)? Eg I've got 250lbft torque in a 3000lb car, so dividing numbers & units, my car's got a torque to weight ratio of one inch. Seems odd, but logical, no? <prepares to be told I'm a complete imbecile>

It's all a bit of a waste of time really because you can play whatever tunes you like with torque using gearing. You could fit a gearbox that would allow 100mph in 1st gear, or you could fit a gearbox that would let a Nissan Micra engine tow a jumbo jet.

You'll never multiply the power output of an engine with gears though because power already takes care of any torque/rpm trading and gets the matter down to brass tacks.

[AW111]

doogz said:

Durrr....

I calculate moments all day long, I've got it right in this thread every time when we've just been casually talking about it.

Then I try explain it to someone, and call it a stiffness. Dick.

So pounds/ft is the measure of a hard-on?

Nm is the only sane unit for torque, anyway.
Or ftlb for aged aussies.

[AC43]

Anyway, I'll have the petrol one because it sounds much better. And I don't care about the range.

[VBRJ]

Torque because diesel mpg.