How much power do you need to tow?

And how much additional power is needed to tow your caravan out bush? Let’s talk about that…

Download the PODCAST for this report

---

Do you actually know how much power you really need to simply drive down the highway? What about towing your enormous caravan into the bush? I’ll bet you have no idea…

And yet, plenty of you hardcore off-roading bogans probably think you need the most powerful utes on the market in order to perform this task.

We will also talk about a term called ‘Brake Specific Fuel Consumption’, which might leave you bleeding from the ears.

Here’s the question I received regarding power outputs in relation to towing heavy trailers, when it comes to buying a big wagon:

As a thought, Genesis and Everest are interesting, but if you have a shortlist with these two vehicles on it, something has gone wrong with how you’re choosing.

Genesis is a luxury SUV, the Everest is a Ranger ute-based heavy towing work vehicle, essentially. They’re two very different animals.

Now, when you look at engine specs, you see X-many kilowatts, but that’s talking about their peak outputs. When the manufacturer sticks an engine on a dynamometer, they run it flat out, and they record the peak power and torque outputs, and at what revs that happens.

This is useful for measuring peak performance, as in, when you give it hell. This is useful for comparing, because an engine with 200kW and one at 300kW, at 6000 RPM, you can infer that in the mid-range, the higher output engine will be better. It’s not entirely inaccurate - it's not a bad way to compare engines, but it's not representative of how you drive.

Outside of a racing environment or a drag strip or an exceptionally steep hill with a really heavy caravan, you're not using full throttle all the time, against a balancing load - you just don't drive like that.

In fact, you use very little power to do mundane driving, often just two figures of kilowatts, around 30-60 kilowatts, even at highway speeds.

The peak power figures are only there for comparison purposes.

Carmakers, generally, won’t tell you what the ultimate weakest link is in their towing platforms.

Next, towing limitations are something nobody ever tells you are the weakest link; carmakers certainly do not tell you what the weakest link in their vehicle is. But these peak performance figures, be it power or towing capacities et al., they are based on a weakest link scenario.

The weakest link is unlikely to be the power or torque production of the engine. But it might be endurance related, like the heat capacity of the automatic transmission, because in their R&D torture testing they might tow a dynamometer and an electric load on a trailer to simulate a big caravan.

In this test they might notice that above a particular simulated weight equivalent of, say, 3.1 tonnes, hypothetically, at highway speeds, the transmission might start to overheat.

So, the engineers might suggest that in 10 or 15 years of service - worst case scenario -  they'll all be suffering failures of some kind, so they limit the rating to save on the life of major driveline components.

Another alternative is tyres can be limiting, because when you look at the specs of a vehicle like the Genesis GV80, it has 265/40/R22 profile rubber, which is essentially a high performance tyre, designed for cornering and braking under different conditions to that of the Everest’s 265/50/R20 profile. That’s actually 25 per cent more sidewall, at the same width. 

Even a 50 series profile on the Everest is still a pretty low profile tyre, but it's got that 25 percent more sidewall compared with the limitation on the Genesis, which might be something to do with the suspension setup. They've got luxury in mind, so putting springs in the Genesis at the rear to make it tow a little bit better, say, 3.1 tonnes, they would have to sacrifice too much ride quality, I’d suggest, and then knowing how many people are actually going to tow at the maximum capacity of a GV80, that increased towing prowess wouldn’t stack up.

Imagine turning up to Dingo Piss Creek World Heritage Area in your Genesis. In an Everest , it’s acceptable, but it's not a Toyota.

However, it's interesting that the Genesis has a payload of 738 kilograms whereas the Everest Titanium only has 623kg. I'd love to know exactly what does limit the tow capacity of both vehicles, but carmakers just never tell you this stuff.

My conservative advice on this is try to give yourself a 10-20kg safety margin when packing, at least, but also try to keep the caravan you're towing weighing less than the loaded mass of the towing vehicle. 

Payload includes everything you put in or on that vehicle - that's you, your lovely wife, the children, all of their crap, and every single ARB accessory you bolt to it.

Whatever that loaded weight is, your caravan should be equal to or less than that - which you can verify by stopping at a weighbridge before heading on your roadtrip.

Every kilogram that caravan weighs more than the prime mover (your tow vehicle) increases the capacity for it to nudge you off the road.

---

Is outright power all that matters when towing?

If you want to know exactly how much power it takes to do this kind of heavy caravan or trailer towing, just to drive your vehicle down the highway at a respectable highway speed, you need to learn about brake specific fuel consumption. 

If you wrap your brain around BSFC, you can figure it out. Let’s make a few conservative assumptions and this becomes dead easy.

The obvious point to be made here is brake specific fuel consumption is a kind of applied science. Science is like a club, and if you don't learn to speak the language, they don't let you in.

Brake specific fuel consumption has a loosely related cousin 'brake mean effective pressure’. ‘Specific’ just means ‘in relation to’, and ‘brake’ means power. So it’s fuel consumption in relation to the power that's produced. It's about normalising the amount of power.

It’s the mass of fuel needed to burn in order to make one kilowatt in an hour.

The typical modern 4x4 diesel engine is roughly about 200 grams of fuel per hour for every kilowatt that comes out of the crankshaft.

The bigger engines get, the smaller the number.

Engineers use this language whenever they talk about BMEPs (break mean effective pressure) and BSFCs (brake specific fuel consumption), so we're going to help you make sense of all of this and use it in a meaningful way.

Let’s look at Marcus Craft's tow test. Crafty is a really good 4x4 journo, he works at CarsGuide. I don't often praise motoring journalists but I've got deep respect for Crafty and his work, so I trust his results here.

He did a tow test with a LandCruiser 200 and a Nissan Patrol Ti.

It was V8 diesel versus V8 petrol and they used a control caravan called the Jayco Silver Line, which weighed 2650kg tare weight; a nice conservative towing assignment for vehicles such as those.

What impressed me was the results: Crafty got a consistent and expected 23.5 litres per 100km for the LandCruiser, and 12.6L/100 unladen. This doubled fuel consumption makes perfect sense because with more wheels on the deck roughly double the amount of rolling resistance, caravan's substantially larger, so roughly double the amount of aerodynamic drag and roughly double the weight. The LandCruiser weighs in at about 2.7 tonnes, so roughly double everything and they got roughly double the fuel consumption.

It probably was a little bit less than double because you can't go quite as fast when you're towing a big heavy caravan and not be conservative.

Crafty drove a 140 kilometre loop on the highway and some good dirt roads, some unsealed roads and some B-roads as well. 

To make sense of this test, we're going to have to make a couple of assumptions, but we're using the data to make BSFC work for us. 

Essentially, we need to work out how many kilowatts it takes to do this towing job.

Fuel consumption is 23.5 (from Marcus's test), diesel is about 850 grams or 0.85 of a kilo for every litre, and let’s estimate Crafty managed 75km/h average speed for his test.

For every hour that he drives, the vehicle is going to use three quarters of the 23.5 amount of fuel, which is 17.6 litres of fuel, which weighs about 15 kilograms, so we've got 15 kilos of fuel to burn in an hour. 

Fuel consumption between a Patrol and LandCruiser 200 was line-ball either towing or not, according to the good test done by Marcus Craft.

So how many kilowatts come out the crankshaft? 

Easy: 15 divided by 0.2 equals 75 kilowatts.

The LandCruiser peaks at 200 so we're well under half of the peak power output.

Even down lower in the rev range we know that the LandCruiser makes peak torque of 650Nm between 1600-2600 RPM. There's a relationship between torque and power: power is strictly mathematically joined to torque, which is, kilowatts are the newton meters multiplied by RPM, divided by 9549. This comes from first principles, but you have to convert RPM into radians per second, and that's difficult, mathematically. 

Or you can just plug it into this formula.

So, 650Nm at 1600 RPM is 109 kilowatts. At 2600 it's 177 kilowatts and it just goes up on a straight line between 109kW and 177kW at 1600 RPM, and 2600 RPM. Then, when you get up to 3600 RPM, it's 200kW. 

Even at 1600 RPM there's still enough power available with a heavy throttle application, but we're not even using three quarters of the power that's hypothetically available at 1600 revs to do this average driving at 75lm/h. In practice, a vehicle’s transmission is really just an exchange system, exchanging torque and revs.

You can have more torque or you can have more revs, but you can't have both. At high speed, you've got lots of revs driving the wheels, but you don't have as much torque available or being used.

This exchange mechanism, in practice, means that at 1600 RPM going downhill with your caravan, you might be doing say 75-80km/h and you reach 90-95km/h on the way down - but then you get to the flat section before and you start going uphill, where you increase throttle, and transmission during this sequence has been responding to the amount of power and torque required by changing gears. It changes gears to exchange the right amount of torque for power needed to meet the demands of the throttle (you, the driver).

Going downhill, the transmission shifted up, doing quite low revs and then as the accelerator demand from the driver increases, the transmission delivers more torque by changing to lower gears to give more revs and therefore you've got more power available, hypothetically.

Essentially, you don't have to go flat-out with high revs because when you change back a couple of gears and go from 1600 to 2600 RPM for any throttle position, you're roughly 70 percent increase in power thanks to the revs increasing.

It really doesn't take that much power to drive a caravan anywhere, it's a small amount of power. This is why the power output of the engine is rarely the limiting factor when it comes to a particular vehicle's towing capability.

Unfortunately, we're never going to be able to drill down into the weakest link of every vehicle because manufacturers won't tell us enough information, but I'll guarantee with just about every vehicle capable of heavy towing, it's not power output that is holding you back.

Have your say