Think about driving a car with a large engine. For many people, a major attraction is how you can effortlessly loaf along, the driver never needing to wring the engine’s neck or to have the throttle mashed to the floor. So what is it that gives this result – and how can it be achieved without having to also confront the high fuel bills associated with big engines?
This article is about an arcane, obscure characteristic of car engines – and yet it dramatically influences real-world performance, fuel consumption, throttle response and driving enjoyment. It’s also, I think, the most important criterion to keep in mind when modifying an engine... and yet it’s something rarely - if ever - discussed.
So, instead of tackling the idea head-on, let’s approach it with some circumlocution.
Commodores and Falcons
Here is Australia there still remain tens of thousands of new car buyers wedded to local production cars – big engine’d machines called Commodores and Falcons. In terms of model names, these cars stretch back over 30 years... in terms of maker philosophy, even further.
These cars use relatively large engines, usually tied to automatic transmissions, and with the engines tuned to give lots of low-rpm torque. The result is that off the line, in contemporaneous terms, Commodores and Falcons have always been damn’ fast.
You put your foot down and they simply launch with neck-snapping response.
Owners, especially older ones who have shown their brand loyalty to these machines over decades, revel in this low-down grunt. But the interesting thing is what you find when you go for a drive with these very same people. Then you’ll see that while they occasionally give the throttle a spurt (especially away from traffic lights and when changing lanes), they usually drive in such a way that the engine revs mostly remain low. (In other words, the engine is never wound out - in auto trans cars by keeping the right foot planted, and in manual trans cars, by changing-up at high revs.)
Now since engine power is torque multiplied by rpm (all then divided by a constant), the fact that revs are low means that the maximum power that these drivers actually use is also low. In fact, if revs never exceed half of peak power, that 190kW Commodore is actually more like a 95kW Commodore. That’s fine – the driver is happy.
But what it indicates is very important: what feels – and is – fast in normal traffic (and in many other uses) actually requires only little power. What it does need is power developed at low rpm – what is normally referred to as ‘lots of torque’.
The idea that a car can be punchy, throttle-responsive and fast in most real-world driving situations – and yet have little peak power – is beyond many people. So conditioned are they to accepting that the max power developed by the engine is the only number that matters, it is to them ludicrous that any other paradigm might apply.
But, unless you drive around all the time in a very low gear with the engine revving hard (whereupon you need top-end power, not bottom-end power), it’s true.
Of course, some driving situations require just raw power – and in big numbers, too. Overtaking on the open road is one, and another is top speed. (But, at least here in Australia, top speed is now completely irrelevant.) Outright, sustained acceleration – eg 0-100 km/h or 160 km/h – is another car application for which big peak power is needed, and of course the standing quarter mile also needs lots of power to gain good times.
But tell me, how often on the way to work today did you run a full-throttle 0-100 or a standing quarter? Or to put this another way, what was the longest you were able to hold full throttle?
OK, so if it’s the power available in the bottom part of the rev range that is important for real world driving performance, what are the implications?
As already indicated, because most large engines develop relatively lots of power at low rpm, a big engine will deliver this sort of performance. And an auto trans using a torque converter will also certainly help, because at low engine rpm the torque getting to the rear wheels (and so the rear wheel power at that road speed) can be short-term multiplied – even doubled – by the action of the torque converter slip.
But large engines and slipping torque converters are ruinous for fuel consumption, and so this approach is rapidly losing favour in all but luxury cars.
Hmmm, so a small engine is needed – but one that delivers large amounts of power at low revs? That’s the case – and this can be achieved in two ways.
The first method is to use forced aspiration – either a turbo or supercharger.
By pushing in more air than the engine would naturally breathe, it behaves like a larger engine and so can develop the lots-of-power-at-low-revs that we’re seeking. But a supercharger has to draw its power from the engine crankshaft, so unless it switches off at low loads (eg by a magnetic clutch on its drive pulley) the fuel consumption will be relatively poor. And, even if it uses a clutch, when the supercharger is running, it is still drawing its power from the engine.
However, a turbo is different in that the energy to drive it is largely free – cars without turbos normally waste this energy as heat out of the exhaust. (However, and this is an enormous ‘however’, for a turbo to be effective in the way we’re describing, the turbo must be matched to the engine so that in fact it blows hard at low engine revs – more on this in a moment.)
The other way of gaining lots-of-power-at-low-revs is by the use of an electric motor. The motor can be either in a hybrid driveline (think Toyota Prius) or it can be in a pure electric car (think the pictured Tesla Roadster). In either case, the fact that the electric motor can develop such relatively high powers at low revs means that, either working alone or with the petrol motor, real-world response and performance can be excellent.
While electric and hybrid cars will increasingly be the fun driving machines of the future, right now it’s the turbo that has the best promise for achieving what we’re describing.
Some factory turbo machines, especially European petrol and diesel turbo cars, are engineered to behave in just the way that’s been described. To achieve this, the turbo is on full boost at very early revs.
In this particular household, the petrol turbo car (a 1.8 litre turbo Volkswagen Polo GTi) develops peak torque of 220Nm at just 1950 rpm – therefore, at 1950 rpm, it has 45kW available.
Another car we own, a 1.9 litre turbo diesel Skoda Roomster, develops 240Nm at 1800 rpm – as it happens, that’s also 45kW.
For the Polo, that 45kW is 41 per cent of peak power (a peak power that’s achieved some 3850 rpm further up the rev range!) and for the Roomster, the 45kW is 58 per cent of peak power (that occurs at 4000 rpm).
So to put it in round figures, these two cars have around half of their maximum power only about 1000 rpm above idle!
The result is, like those big engine Commodores and Falcons, in normal driving these manual-transmission cars almost never need to be revved past about 2500 rpm. Together with their relatively light masses (the GTi is 1190kg and Roomster is 1260kg), the very low revs at which significant power outputs are gained results in excellent fuel economy – in our mix of rural and urban driving, about 6.5 litres/100km for the GTi and about 5.5 litres/100km for the Roomster. (And 5.0 litres/100km for the Roomster on an interstate trip!)
Now you can bring in all sorts of other factors to explain that good fuel consumption – aero drag, relatively low 0-100-type acceleration performance (in the case of the Roomster), that low mass and so on. But the reality is that the most significant ingredient in the exceptional fuel consumption is the use a small engine that delivers high power at low rpm, resulting in engine revs able to be kept low in normal traffic. To put that another way, nearly every up-change can be made early.
(And why is that important? Brake Specific Fuel Consumption, which is a measure of how much fuel is used to produce each horsepower, is reduced at low engine revs. See Brake Specific Fuel Consumption for more.)
But I started this article by saying that I was going to mention a bunch of factors, including driving enjoyment. So, in addition to achieving excellent fuel economy, why else is it good having a small engine developing lots of power at low revs?
Firstly, it makes for much more effortless driving – there’s less need to change gears (or have an auto trans change gears for you). In fact, often you don’t even need to use all the forward gears that are provided – instead of going 1-2-3-4-5, shifting 1 – 3 – 5.
In addition, engine revs (and so noise and vibration) are kept lower. That might not immediately sound such a plus – until you think of driving a small engine car that doesn’t have lots of power at low revs. Inevitably, the driver will complain about having to drive everywhere with the ‘engine screaming its head off’.
Another important advantage is much more immediate throttle response. In terms of cut-and-thrust through urban traffic, having immediate and strong throttle response is vital if a car is to be fast. Having to change down one, two or even three gears before the car goes hard at (say) 60 km/h will not result in a real-world fast car!
And, coming back full circle, if the engine develops lots of torque at low revs, the gearing can be kept taller, so improving top gear fuel consumption.
So what are the implications for modification? Lots!
Almost all traditional modification is about improving top-end power. In fact, when describing the power gains made from modifications, it is completely normal to describe only the increase in top-end power!
For example, cams, exhausts and headwork all result in a power increase at the top end of the rev range. Upsizing a turbo is another that results in the increase in power happening at the top end – and worse than that, usually a major decrease in power at low revs!
On the other hand, fitting a small turbo (perhaps one smaller than standard!), removing any turbo wastegate creep, or fitting a big exhaust and intake (allowing a quicker turbo spool) can work well. On an atmospherically inducted car, fitting tuned-length extractors can increase bottom-end power. A positive displacement supercharger will also increase low rpm power – but in most cases at the expense of fuel economy. Fitting a cam designed for lifting bottom-end power (a “towing” or “economy” cam) can also be done, as can retiming the cam.
None of the modifications described in the above paragraph will increase the peak power that you can boast about to your mates. Nor will they make much different to a 0-100 km/h time (they’ll make a bit of difference but not a helluva lot – after the first gear-change, you’ll be above the revs at which most improvement has occurred.) But what they will do is massively improve driveability, especially in urban use. They’ll give better fuel economy (particularly if overall gearing is raised to match), faster throttle response and result in the need for less gearchanges. Progress in nearly all driving situations will also feel much more effortless.
Especially in these times of strictly enforced speed limits and other public road driving constraints, a car that develops a lot of power at the bottom end of the rev range is very often a more enjoyable drive than one with a much higher peak power, but where that power is developed at high revs. In nearly every case, the car with more bottom end power – and the same size engine – will also be more economical.
So the next time you are selecting a car to buy, or thinking about modifications, consider whether you want a more enjoyable, relaxing, throttle-responsive and fuel-efficient car – or one that has just a bigger peak power number to boast about...
Drives like a big engine... but drinks like a little one...