Power to move

Posted on March 24th, 2007 in Opinion,Power by Julian Edgar

108228_2mg.jpgThe other day when I had Frank the EF Falcon on the ChipTorque dyno, I did something pretty interesting. But first, some background.

I’ve installed in the Falcon the trip computer that’s normally fitted to higher trim models. It displays all the usual trip computer stuff – average fuel economy, average speed, and so on. It also displays instantaneous fuel consumption.

From watching this display a lot, I know that at an indicated 110 km/h on a level freeway (actually 105 km/h when a speedo correction is applied), the instantaneous fuel consumption figure fluctuates between 7 and 8 litres/100 km. (Unfortunately, the instantaneous display doesn’t have any decimal places.) Over a long distance in these conditions, the average is 7.5 litres/100 – so that instantaneous number makes sense.

On the dyno it was easy to dial up an indicated 110 km/h and then increase the load until the instantaneous consumption figure was fluctuating between 7-8 kilometres/100. Then it was just a case of reading off the dyno screen how much power was being absorbed at the wheels. The answer was 13kW.

So, in the Falcon, it takes 13kW to propel the car along level ground at an actual speed of 105 km/h.

Now the interesting thing (apart from the fact that it shows how little power you actually need on level ground without acceleration!) is that most of that power is being used to push air out of the way. And if we know that the Falcon takes 13kW to push it along at 105 km/h, and we know the frontal area of the car (basically height x width), we can calculate the drag coefficient (Cd).

The equation is:

power at the wheels Cd = —————————————— 0.6 x area x speed x speed x speed…where power is in watts, area is in square metres and speed is in metres/secondSo with the Falcon, power at the wheels is 13,000W, frontal area is 2.7 metres and speed is 29.2 metres per second.

Plug the figures in and you get a calculated Cd of 0.32. At the time of release, the claimed Cd of the EF Falcon was 0.31.

If you know the Cd and frontal area, you can also work out approximately how much power a car needs to travel at a certain speed. A Honda Insight needs only about 9kW to travel at 105 km/h – 30 per cent less power than the Falcon (although in fact with a more efficient engine and battery assist, it actually uses about 50 per cent less fuel in the same conditions).

Top speed can also be easily calculated. The Falcon has (at the time of writing) 145kW at the wheels. Using the 0.32 drag coefficient, the max speed of the Falcon calculates out to 236 km/h – probably a bit optimistic, but not hugely so.

Of course, these figures are all a bit rubbery – the frontal area is not usually just calculated from height x width (it should subtract the frontal area between the wheels, etc) and even minor changes in speed make a big difference to the final result. However, the numbers are certainly indicative of what is happening – and so are interesting none the less.

Incidentally, another interesting fact comes from Frank’s figures.If you are tuning a car on a dyno and you want to gain good cruise fuel economy figures, you really need to know how much power is needed to obtain your cruise speed. Otherwise, how does the dyno operator know where to do the lean tuning?

If you don’t have a trip computer with an instantaneous consumption readout, another way of gaining the on-road information is to measure airflow meter or MAP sensor output. Gain the same number on the dyno and then have the tuning done around those load sites.

The dyno run was courtesy ChipTorque.

3 Responses to 'Power to move'

Subscribe to comments with RSS

  1. Ben said,

    on August 5th, 2007 at 9:30 pm

    Another interesting thing that can be derived from this is the fuel economy potential of your car.

    If we need 13kw to drive this car @ 105km/hr, we need 44397 BTU/hr. Using wikepedia’s figure (regular US unleaded) of 34.8MJ/L, or 32,984 BTU/L, we then figure that we only need to burn 1.35L/hr to make our 13kw. And so only need 1.35L to do 100km, or 1.29L for 100km.

    Or we can travel 77.5km on 1L of fuel. Which beats the 14.3km/l (7 L/100km) that the car is currently getting. For those imperially minded that means that the best possible economy is 181.8 MPG, as opposed to 33.6.

    Of course this is never going to happen, as it assumes that we pointed fuel at the wheels and they converted all to movement with no losses whatsoever. But we are still only getting use of roughly 18% of the energy in the fuel. What happened to the 30% or so our engines are supposed to get? And what are they actually capable of?

    Just a (long, complicated) thought.


  2. Sami said,

    on September 12th, 2007 at 9:49 pm


    Fair comment. Although it needs to be kept in mind that at 13kW, the engine is essentially at part load. Spark Ignition engines aren’t efficient at part load, thus your efficiency figure of 18% is possible.

    One can use a “hot air intake” to increase throttle openings at part load, to reduce pumping losses, or you could use a smaller turbocharged engine which will produce the 13kW more efficiently.


  3. Ben said,

    on January 16th, 2008 at 8:54 pm

    Yes, and you could use a fuel system that actually vaporises all the fuel, perhaps sacrifice some of the falcon’s (needlessly high) horsepower, give it an atkinson cycle valve timing and higher static compression. That would lift it a bit.

    Hmm… I also have an EF 5spd. And enough tools to make a (very crude) vaporising fuel system. And enough long pointless highways to pull some good numbers out of.

    Place bets on what I achieve anyone? (how about whether or not I actually do it?)