We’ll not go into detail here about precisely how diesel engine tuning is done, but the end results very much depend on the skills of the tuner in designing the electronics of an add-on box, or rewriting the ECU software, whilst staying within manufacturer design limits to ensure the strength of the engine remains uncompromised.

The basic objective of tuning is normally increased performance, in terms of more power and torque. But those who lean towards economy motoring should not scorn the practice of engine tuning, because it offers the prospect of significant potential to go the extra mile whilst offering the enjoyment of greater engine flexibility and more power, if and when you need to call on it. Can tuning a diesel engine hold out the attractive prospect of better fuel economy, along with improved performance, and what’s the evidence to support the claims? Brim-to-brim refuelling checks by owners over a mixture of motoring are the only accurate way of evaluating the fuel economy spin-off of engine tuning, and there’s a mass of accumulated evidence that electronic conversions, both add-on boxes and software re-maps, can cut your fuel bill by around five per cent, or sometimes even by more. The gains will very much depend on your motoring mix, and stop-go urban motorists, in particular, may struggle to see much improvement.

The potential economy gains are fundamentally derived from improvements in the torque and power curves, and those gains are freely available in mixed motoring, where the tuned engine can quite often pull a higher gear, overtake more effortlessly, and climb gradients more efficiently, whilst saving fuel in the process. With good engine tuning, we’re looking at gains of probably 15 to 20 per cent in maximum torque, but somewhat greater improvements and enhanced accelerator response at the lower engine speeds, which makes a big difference in how you can treat the engine. It means you can often overtake or accelerate without needing to drop a gear, and when you’re cruising along in traffic, going with the flow at maybe 45 to 55mph, you’ll again be able to hold a higher and more economical gear.

So how do the engine tuners come up with the economy tune that some offer, or may produce on request? They will generally modify a standard performance tuning conversion for a given engine to specifically reduce the power and torque gains above a certain engine speed, perhaps somewhere beyond 3,000 to 3,500rpm, to dissuade drivers from needlessly using higher engine speeds. The power output and engine response will feel very strong within the maximum torque and economy engine speed band, but drivers will realise that there’s no gain in hanging onto the lower gears too long, and a strong incentive to change up early and stay in the purple patch where everything feels so good. That’s how an economy tune should work, and you really want to find a tuning company that will talk to you about the principles they employ. Maybe they will even show you a power and torque curve for a tried and tested economy tuning package for your engine, or a similar one, that clearly shows strong gains in low engine speed torque, but relatively modest increases in maximum power. Then you are definitely on the right track.

With some add-on boxes that offer variable tune level settings, you will probably find that the lower level settings will give you a very useful performance boost, but a greater economy potential than higher settings. The gains from such tuning are almost psychological though, as the improved characteristics of a well-tuned engine encourage the driver to drive more economically, with little need to select lower gears for acceleration and holding steady speeds on rising gradients. Some might say it all feels like a bigger engine has been dropped under your bonnet, but one that doesn’t drink more fuel, and often less. Having said all of this regarding economy tuning, most of the firm evidence of better fuel economy from tuned engines comes from standard performance tunes and, if your car has such a tuned engine, you can still expect good economy gains, particularly if you don’t explore the higher engine speed range, and learn to ride the wave of torque that should come with a good package.

Victor Harman