Power versus torque

[VladSoilerOfCarpets]

This topic always fascinates me.

Take my previous car and my current car.

Previous: Civic Type S 2.0 - 165bhp - 132lb/ft - 1285kg.

Current: Octavia L&K 1.9TDi PD - 130bhp - 210lb/ft - 1455kg.

I don't know the specific aerodynamics for each car, but they both run the same 16" tyres, so road contact is very similar.

Now...

The Civic, when I first got it, didn't feel that quick. It started to sing at about 4,000rpm when it was on the way to peak torque and peak power. It used to top out around 135mph, 0-60 was delivered in around 8 seconds. In gear, it was difficult to judge. It was best going from around 4,000 to the redline at 6,800, and felt like it could easily go on to around 7,500.

The Skoda is totally different. The sheer amount of low-down torque means the car can literally drive itself- you stick it in 6th at 35mph and it'll happily pull itself along, over bridges, flat roads, etc. For reference, it's happy to top out about 130, although I've not had access to my private track to test it yet. It's undeniably best between 1,800 and 3,000, where I presume peak torque delivers.

Now, with the extra torque it's definitely quicker in-gear than the Civic. But- extra weight and less power notwithstanding- why is it so much slower to 60, at around 10 secs? When you punch from 1st to 5th using that 1,800-3,000 powerband, it feels so much stronger than the Civic ever did. Is it a placebo effect caused by shedloads of torque?

It's truly fascinating seeing how differently the cars handle their similar power, weights and gearing. The best way to describe the Civic was 'frenetic'. Lots of action, a real game to play to go fast. The Octavia is 'loping'. It gathers speed in the same way a curtain is drawn back, akin to a very well-built bouncer shouldering aside a crowd.

Can the way power is delivered and how gearing is arranged trick us into thinking something is faster than it is?

[Chris GB]

Can the way power is delivered and how gearing is arranged trick us into thinking something is faster than it is?

Most definitely. For example, you can make a change that drops out midrange without adding anything to the top end and as the torque rises faster as the revs rise (still to the same level) the car will feel like it gained top end power. The butt dyno is not a reliable device. This is why I never trust people who offer on road tuning instead of using a proper dyno.

Chris

[john_y]

VladSoilerOfCarpets, on 07 February 2012 - 21:17, said:

Can the way power is delivered and how gearing is arranged trick us into thinking something is faster than it is?

I don't want to be pedantic but by power do you mean torque or bhp

[VladSoilerOfCarpets]

Both, essentially.

Which one of the two actually gives us the illusion of speed? Or is it a combination of both?

[john_y]

lots of TORQUE is the main factor and it's not an illusion it's just in a diesel you get lots of torque low down then at about 3500rpm it's dropping off which is when the pistons are moving at there quickest speed producing maximum power/ bhp. So torque is the twisting force that accelerates the car overcoming weight of car and enviromental factors.

[Ben99Felicia]

To sum up previous posts speed and acceleration at a given speed is a product of torque at the current engine speed(revs) and gear ratio.

Hp is calculated by taking torque measured in pounds per foot (lb/ft), multipling it by engine speed measured in revolutions per minute (rpm) and dividing by 5252. Which is why all dyno graphs measuring torque in lb/ft and hp cross at 5252, or basically make the same value at that engine speed.

If an engine makes peak torque low in its revs it won't make as much hp as an engine that makes the same amount of torque higher in the revs.

Depending on how you drive torque figures can be more useful than hp figures as day to day most people find large torque figures more useful on the road, where as peak hp is more useful on a track. These are generalisation because as i mentioned before the gear ratio makes a massive difference on how the car accelerates and the cars top speed

[john_y]

To sum up previous posts speed and acceleration at a given speed is a product of torque at the current engine speed(revs) and gear ratio.

Hp is calculated by taking torque measured in pounds per foot (lb/ft), multipling it by engine speed measured in revolutions per minute (rpm) and dividing by 5252. Which is why all dyno graphs measuring torque in lb/ft and hp cross at 5252, or basically make the same value at that engine speed.

If an engine makes peak torque low in its revs it won't make as much hp as an engine that makes the same amount of torque higher in the revs.

Depending on how you drive torque figures can be more useful than hp figures as day to day most people find large torque figures more useful on the road, where as peak hp is more useful on a track. These are generalisation because as i mentioned before the gear ratio makes a massive difference on how the car accelerates and the cars top speed

Hi,

I have seen this formulae before and tried to calculate what it would predict for my old cars 300 lb ft at 2600 rpm:

(300*2600)/5252 = 148.5 bhp

It was dyno tested at over 200 bhp.

Does this formulae fit petrols better than diesels, or are my calculations incorrect?

Best wishes

John

[STSKODA]

torque is a turning force, it is forces that make cars accelerate - more torque more acceleration

power is related to torque by equation power = torque x engine speed

so if you can generate a high torque at a high engine speeds you have a powerful car, doesn't mean it will be any good though!

power on its own doesn't really mean much, neither do one off peak power or torque values - you need to know them throughout the full rev range to get an idea of a cars performance

[Chris GB]

Hi,

I have seen this formulae before and tried to calculate what it would predict for my old cars 300 lb ft at 2600 rpm:

(300*2600)/5252 = 148.5 bhp

It was dyno tested at over 200 bhp.

Does this formulae fit petrols better than diesels, or are my calculations incorrect?

Best wishes

John

What this means is that peak torque is prduced lower down than peak power. If, for example, you had a car that ran fourth gear at 2600rpm at 70mph and third at peak power at 70mph (say 4500 for example) the car would accelerate faster in third than in fourth at 70mph as that is where the torque coming out of the gearbox will be highest. Subjectively, the car may feel quicker at peak torque, and it will certainly have more reach in that gear, but at that exact moment, the car will be quickest at peak power.

Chris

[Ben99Felicia]

Hi,

I have seen this formulae before and tried to calculate what it would predict for my old cars 300 lb ft at 2600 rpm:

(300*2600)/5252 = 148.5 bhp

It was dyno tested at over 200 bhp.

Does this formulae fit petrols better than diesels, or are my calculations incorrect?

Best wishes

John

Peak hp normally happens further up the rev range than torque so your 200hp comes at say 5500rpm where the engine only makes 191lb/ft torque. You can see this happening on a graph readout from dyno. lb/ft * rpm / 5252 always works, hp is a result of torque multiplied by how often you can make it

[john_y]

Max bhp was made at 4242rpm so I don't understand the calc, can you help plse.

John

[Ben99Felicia]

yeah no problem, if it made 200hp at 4242rpm then (200/4242*5252) it makes 247lb/ft of torque at 4242rpm.

Cheers

[john_y]

Thanks, for some reason I thought peak bjp would predict peak torque and vice versa.

Could you use formulae to calc peak bhp from peak torque please

John

yeah no problem, if it made 200hp at 4242rpm then (200/4242*5252) it makes 247lb/ft of torque at 4242rpm.

Cheers

[Chris GB]

Thanks, for some reason I thought peak bjp would predict peak torque and vice versa.

Could you use formulae to calc peak bhp from peak torque please

John

Hi John

Power is the product of torque and rpm. Because different engines or engine maps have different shaped torque curves, the peak power is not in any way directly relateable to peak power.

Chris

[john_y]

Thanks Chris,

Ben, if you look at the rolling road print out it does make the formulae calculated torque at that rpm.

Best wishes

John

[andreroest]

Both torque and power tuning is possible.

I have tuned my skoda yeti 1,8TSI tot an amazing 216 HP and 354 Nm of torque which means that the torque line AS WELL as the power line is very flat.

It is an amazing machine now so I am fully convinced that a petrol engine which is tuned to produce much torque and a flat torque line is the way to go!

[john_y]

Both torque and power tuning is possible.

I have tuned my skoda yeti 1,8TSI tot an amazing 216 HP and 354 Nm of torque which means that the torque line AS WELL as the power line is very flat.

It is an amazing machine now so I am fully convinced that a petrol engine which is tuned to produce much torque and a flat torque line is the way to go!

What do you get litre per 100km fuel efficiency please.

John

[andreroest]

John,

I am using as an averge 12 tot 13 km per liter.

The benifit of much torque in a lower region is that you can do 2000 revs in six gear and then it uses 16 to 18 km per liter.

That is better then it used before I tuned it and the car was doing 160 hp and 230 Nm.

According the tuner the problem with todays car is the extreme environmental demands which results in a bad fuel economy.

[andreroest]

I am a kind of a nono with this medium.

Anyone knows how I can add a jpeg file of my dyno reading on this forum??

perhaps that is interesting for the readers.

[john_y]

I am a kind of a nono with this medium.

Anyone knows how I can add a jpeg file of my dyno reading on this forum??

perhaps that is interesting for the readers.

try advanced/ more reply options and look at the icons

[BonnevillevRS]

Hello guys, this topic is quite complex!

An articulated lorry only has 350bhp but has 900 lb/ft of torque, but an RX7 can have 750bhp but only 200lb/ft of torque.

In short only torque is measurable, power is calculated.

So your car goes on a dyno and that measures how much rotational force the car (or engine on an engine dyno) applies to the roller / drum.

This is calculated by T=F x D (T=torque, F=force, D=distance)

So the torque produced if you put 20lbs of force to a 3inch lever is 60lb/ft.

Horsepower is the mechanical power delivered to the load and is calculated by using the following formula:

horsepower = torque x rpm / 5252

So lets say the above Torque figure was captured when the engine was going at 1000 rpm, the calculation would be:

horsepower = 60lb/ft x 1000 / 5252

so hp = 11.42bhp

Then you have to work out whether the figure is at the wheels or at the flywheel as you have to accounbt for coatsdown losses.

The transmission, bearings, the rotating mass of the wheels and brake discs plus the friction of the tyres on the dyno rollers all sap away at your wheel horsepower figure.

Lets work it backwards, Vlad said above their 1.9 puts out 130hp and 210lb/ft torque

By backtracking the formula their peak torque was taken at 3,250 rpm!

If you want to increase your bhp you either increase the torque the engine produces by chipping or mechanical changes (including internal friction reduction) etc or you decrease the coastdown losses by superfinihsing the transmission, lighter brakes and wheels etc.

Sorry to go in depth (and thats only scratching surface etc) but I hope this helps!

TTFN

[john_y]

I think the above post summarises the discussion nicely