This issue focuses on performance, and here we tackle the subject of whether, and how, electronic tuning of diesel engines for added performance delivers a fuel economy bonus, as many tuning companies claim. You may well wonder how you can get more power without burning more fuel, particularly if the whole objective is to go faster, and there are some bold claims of fuel economy gains, both from tuning companies and owners. But we’re well aware that some tuning conversions can change key calibrations and generate falsely optimistic on-board computer mpg readings. So long-term, brim-to-brim, refuelling checks are the only way of substantiating true fuel economy improvements; but there’s no doubt in our minds that many electronic conversions, both add-on boxes and software re-maps, can deliver real life economy gains of the order of around five per cent, or sometimes even better.

The gains are very much dependent on your motoring mix, and city dwellers who seldom see the open road will possibly struggle to see much improvement, as will many motorway drivers. Why is this? Well, the gains are mostly derived from taking advantage of improvements in the torque and power curves, not from any stunning gains in overall engine efficiency, because no such huge gains are available from standard engines that are already highly efficient. What good electronic tuning offers is an improved delivery of torque, generally over a wider engine speed range, allowing you to use the higher gears, which are simply more fuel-efficient, more often. Some of the gains are arguably almost psychological, as the improved characteristics of a well-tuned engine almost lead the driver into driving more economically, with the need to drop down a gear for acceleration much reduced. But in stop-start low speed driving, and high-speed cruising, there’s less scope to benefit from such potential fuel saving and the advantage of any increased performance potential is also limited by road conditions. Even so, a skilfully accomplished overtaking manoeuvre can often be performed more swiftly and safely in a suitable gear with a well-tuned engine, and yet use less fuel for the operation; it’s something that can be very helpful if you’re following a “stop-go” driver who is causing you to waste fuel.

Much of the time though, when you may feel that your tuned engine is “raring to go”, you’ll be trickling along with the traffic flow, often in a higher gear than you might have previously selected, and simply using less fuel. But not every driver actually knows how best to drive a diesel car, and this is where the economy tuning offered by some tuners can be really effective. They generally tweak a standard performance tuning conversion to intentionally reduce the power and torque gains above a certain engine speed, maybe somewhere around the 3,000rpm plus region; this way it suppresses the instinct of some drivers – maybe those brought up on petrol power – to over-rev the engine to the detriment of fuel economy. The power output and engine response will feel very strong, within the maximum torque band, such that there’s no gain in hanging onto gears too long, and a real incentive then to stay in the purple patch where everything feels so good. That’s how an economy tune should work and, with a relatively modest increase in maximum power, it’s a great way of tuning an engine like a 2.0-litre common rail unit with 150bhp, or even more, as standard, where more significant increases in power are not that necessary.

Space doesn’t allow us to go into detail here about how the tuning is done – that’ll be covered in our special tuning issue later in the year – but much depends on the skills of the tuner, either in designing the electronics of an add-on box, or rewriting the ECU software, whilst staying within the original manufacturer’s design limits that ensure that the basic strength of the engine remains uncompromised. Those who lean towards the economy side of motoring should not scorn the practice of engine tuning, because it can offer significant potential to go the extra mile whilst offering the enjoyment of greater engine flexibility, along with modestly improved outright performance on offer, when it’s appropriate and safe to use it.

Victor Harman