[Lawrothegreat]

Hi,

I'm interested in upgrading to the works kit but I am unsure about the torque figures. The official figures are:

Production R56 Cooper S - 240Nm (1600-5000rpm) overboosting to 260Nm (1700-4500rpm)

JCW (kit) R56 Cooper S - 250Nm (1750-5000rpm) overboosting to 270Nm (1750-4500rpm)

These official figures state that torque is higher throughout the rev range. However a few on here have stated that the torque is reduced at lower engine speeds to reduce torque steer. To add further confusion I attach the official power/torque curves for the production and kit car (from the John Cooper website).

The curves show that the torque is higher for the JCW car across the rev range, but strangely that power is higher for the production car up to around 3000rpm. This makes no sense? It was always my understanding that power at a particular engine speed is a function of that engine speed and the torque developed at that engine speed.

Can anyone shed some light?

[kevkbuk]

The graph on the website isn't correct, if you get a brochure from your dealer it has a different graph in it.

Anyways I wouldn't worry about it, the JCW is better than the figures suggest and if you want to retain the warranty its the only way to go. Demo a car with it on and take it for a long test drive to see if you feel its worth the expense.

[Fin]

You are correct Lawro.

To understand the performance of an engine, you have to understand the relationships between force, work done, power and time. Also a little bit of history is thrown in to demonstrate where the units of measurement have come from, and I will use the older (empirical) measurements first.

Force is where we start. You exert a force (measured in pounds, lb) in order to carry out an action, ie moving an object. If the force you exert is enough to move the object over a distance (in feet), this is measured as work done:

Work Done = Force(lb) x Distance(ft).

The units of Work Done are therefore: (lb) x (ft) = lb ft (pound feet).

Power is a measurement of the rate of work done, ie the work done per unit of time (in s):

Power = Work Done
Time

The units of Power are therefore lb ft/s.

Now, back in the day James Watt was making steam engines. The story goes that he wanted to market these engines to mine owners so that they would use his engines to lift coal out of the pits. At the time ponys/horses were used.

He started to observe horses and noted that, the average horse of the day could lift a 550lb weight a distance of 1ft in 1 second. From our definitions above, we can see that the horse was carrying out 550lb ft of work done per second. This equates to 33,000lb ft per minute. He then published his findings and concluded that 33,000lb ft of work per minute was equivalent to 1 horsepower (hp). From here, he could then market his engines as 3 or 4hp etc.

Ok, so now we know about force, work done and power and how the measurement of horsepower came about. While we are planning on talking about the internal combustion engine we need to think about a twisting force. This is called Torque. The units of Torque are in fact the same as those for Work Done:

Torque = Force(lb) x Distance(ft)

where the distance is measured between the point at which the Force is applied and the fulcrum (the pivot or the axis of rotation).

So, 1ft lb of Torque can be defined as the twisting force required to support a 1lb weight on a weightless horizontal bar at a distance of 1 foot from the fulcrum.

Now when you take your car to a rolling road, it is the Torque that is being measured, the actual twisting force being exerted by the engine on the wheels (or the flywheel). When you put your foot down and you feel youself being pushed back in your seat, it is not the Power you are feeling (ala Clarkson: 'You can feel the power!), it is the Torque. We use the measured Torque to calculate the Power.

So lets take the 1lb weight on the weightless bar, 1ft from the fulcrum that I described above and rotate it around 1 revolution against a 1lb resistance. We have moved it a distance of 6.283 feet (circumference of the circle = diameter x pi) and therefore have done 6.283lb ft of work.

Now, back to Watt's definition of 1 horsepower being 33,000lb ft of work done per minute. If we divide 33,000lb ft of work done by 6.283 lb ft of work done, that will tell us how many revolutions per minute we would have to rotate our 1lb weight to carry out that work at a rate of 1 horsepower.

So:

33,000lb ft = 5252

6.283lb ft

If we were to rotate the 1lb weight at a rate of 2626 rpm, then we would be doing work at a rate of 0.5 horsepower. Therefore our equation to calculate the Horsepower from Torque is:

Horsepower = Torque x rpm
5252

Here you can see as Lawro stated, the power is a function of the Torque at a particular engine speed. Also, from this equation, when the rpm is 5252, the horsepower and torque are equal .

Now most of the time you will see the power of a car engine given in horsepower (hp). The standard index measurement for power is the Watt (W) and so often you will see engine power in (kW). The other commonly used measurement is the Pferdstarke (PS). Conversions are:

1 PS = 0.735 kW = 0.986 hp

Force is more often quoted in Newtons (N), with Work Done/Torque being measured in Newton metres (Nm).

[Lawrothegreat]

Wow that is quite some explanation...... thanks.

I'd love to compute some kind of comparison between cars on average power output during a 0-60mph or 0-100mph run, assuming they change gear at a fixed point (somewhere between maximum power and the rev limiter), and then perhaps as a power to weight ratio. I guess the MCS turbo would compare favourably with other hot hatches despite its lowly 175 hp.