Devices of this type include: Superchips, Eurochips

Dozens of companies now offer "performance upgrades" to vehicle ECUs. The usual claim for these is increased performance, but interestingly there is also a fuel consumption effect as well.

Adding 10 or 20% more torque / power to a turbocharged engine (petrol or diesel) is relatively simple, and there is no question that these upgrades do approximately what they say. As a result, the engine may be able to cope with a particular driving situation in a gear one higher than is normal (say fourth rather than third). This means the engine runs more slowly, and so there is less power wasted in friction. The principle of using very "tall" gearing for better economy is well understood and indeed can be seen in many of today's 6-speed vehicles.

Generally the engine's combustion efficiency will be worse at full power, since both petrol and diesel engines "over-fuel" to some extent at full load and so some fuel is wasted. But the friction reduction effect will normally more than compensate for this and so overall the economy will be better.

Performance "chipping" is the one "bolt-on" device that really can improve fuel consumption, which is ironic since it is almost never marketed in this way! Having said that, I would not recommend anybody to go down this route - certainly not if your aim is to save fuel, and not generally either, for a number of reasons.

Firstly, safety. The companies making these upgrade chips are undoubtedly clever and resourceful, and in some ways I admire them. But they don't have full knowledge of the detailed operation of the software in the ECU, so there is a risk that the changes they make may cause unexpected behaviour in certain rare fault conditions. With a modern "drive-by-wire" engine, there is then a chance of uncontrolled acceleration! Having said that, the risk is extremely small, is theoretical rather than actually having been observed, and any reasonably alert driver could easily take "avoiding" action such as pressing the clutch.

Secondly, emissions. Smoke (diesel) and CO (petrol) emissions inevitably increase when an engine is worked harder. Admittedly this performance will only rarely be used, but if environment-friendliness is your main concern then performance chips should be avoided.

Thirdly, engine durability. The car maker will have set the engine power at the maximum level the engine can safely produce. Increasing the power risks damage to the engine, particularly if the extra torque is used extensively (eg towing a caravan up hills) rather than just for short overtaking bursts. Of course the makers will have built in a certain safety factor, but it inevitably increases the risk.

Many engine "families" are produced in several different power ratings, for example VW's 1.8 turbo petrol (150 / 180 / 225 bhp) and 1.9 PD diesel (100 / 130 / 150 bhp). If you have one of the lower-rated engines you may think you can safely "chip" it to the higher level. Unfortunately the higher-rated engines often have mechanical changes as well (piston cooling oil jets, modified crankshafts, etc).

Finally, economics. Although using a higher gear does save fuel, a lot of the time the engine will be in the same gear as normal (for example, a motorway cruise would be in top gear either with or without the upgrade). As a result the overall "real-world" saving will be quite small, and will probably not pay for the cost of the upgrade. Also note that the trip computer's fuel consumption reading may be very different to the real value, because of the modifications to the ECU.

Of course, the great majority of "chipped" engines do deliver many years of trouble-free service. But I, personally, wouldn't take the chance.

Special mention should be made here of the devices that are connected to the engine ECU's air temperature sensor plug. These are sometimes referred to as "performance chips" by the people who sell them, though in fact they are nothing of the sort. These devices are just ordinary resistors, worth about 2p, which "work" by fooling the ECU into thinking that the intake air is much colder than it really is. These devices have two main effects:

1) Richer fuelling. On engines that use a manifold pressure sensor (not an air meter) to determine the air flow into the engine, air density is a critical parameter. Colder air is more dense, so at a particular manifold pressure there is more mass of air flowing into the engine. If the ECU thinks the air is cold, it will inject more fuel to cope and so the mixture will be richer. Unfortunately, while it is quite true that a rich mixture (around lambda 0.9) gives more power than a stoichiometric mixture (lambda 1), all engines are set to run pretty close to this optimum mixture at full load anyway. Hence adding more fuel is unlikely to be of benefit - what is likely to happen is that your fuel consumption will be terrible since the excess fuel is simply wasted.

If your engine uses an air flow meter, then air temperature has very little effect on the injected fuel quantity so the device will do almost nothing to the mixture. Even on systems with manifold pressure sensors, the lambda sensor will try and correct back to lambda = 1 and so the effect on fuelling may be small.

2) More advanced ignition timing. When the intake air is cold, the engine is less likely to "knock" . Therefore you can use more ignition advance, which would normally provide more power. (As a general rule, on most engines, the ignition timing is set somewhat retarded from optimum because of the danger of knocking). So, if you fit a device like this that fools the engine into thinking the air is very cold, it will (in most cases) apply more ignition advance, which will probably produce more power. But, it gives a very much greater risk of catastrophic knock, which can effectively destroy the engine! Normally there is a certain amount of safety reserve built in and so generally you would get away with it, but the risk is very high.

So one of these "chips" may give you a slight increase in performance, but may also seriously damage your engine. And on any modern vehicle you are quite likely to trigger the Check Engine Light / Malfunction Indicator Lamp, as the ECU will realise that the air temperature signal is implausible. Avoid!