WHAT’S IMPORTANT

A summary of the important points before you have your diesel tuned:

•    Check your car is healthy before thinking of tuning – engine, tyres, suspension etc.
•    Think ahead and research tuners and tuning, with our advice and your own work.
•    Talk to tuning companies and judge them on their answers to your questions and openness; compare what they offer, check websites for customer feedback.
•    Eliminate any making questionable claims, or whose origin may appear unreliable.
•    Shop local, if you can, to ensure solid after-sales service and avoid long return trips.
•    Consider ‘try before you buy’ and ‘satisfaction guaranteed’ tuners. Quiz others on what they’ll do if you’re not happy with the results.
•    Do your homework on insurance availability and costs before paying out any money.

THE INDUSTRY

The principles of electronic tuning of diesel engines have been established for well over a decade.

Before that, when most diesel engines were naturally aspirated, there was limited scope for performance tuning, and manufacturers had often squeezed as much power and torque from engines as they reliably could.

Companies like Jeremy Fearn would tweak the wastegate of your engine, or play with the injector pump, but the gains back then were pretty modest.

So the arrival of turbocharging, direct injection, and a degree of electronic engine management really changed the game. Tuners discovered that there was scope to build on the excess air that most diesels ingested through the air filter to squirt in a bit more fuel, albeit within fairly modest limits, beyond which they soon ran into black smoke problems.

In the late 1990s, a whole new generation of direct injection, computer-controlled engines enabled manufacturers to get more power out of their engines, and offered the tuners much greater scope.

With variable geometry turbochargers that then followed, they could also modify turbocharger settings, along with fuel injection pressure and injection timing, and make radical improvements to power and torque output, yet the engines sipped less fuel, as a result of improved engine efficiency.

So the term chipping was born, as early tuners removed the microchip that controlled the turbocharger and fuel injection system and replaced it with a modified or replacement one that delivered markedly better performance.

In parallel with this, a simpler option of intercepting the signals from the engine management system and modifying them to similarly improve power and torque was achieved by introducing an add-on electronic box.

Such boxes are still popular, some offering scope for owner adjustment, and many are quite effective. Total replacement of chips has now been superseded by re-writing the software on the original chip, either through the on-board diagnostics port (OBD) or (if manufacturers have deliberately made things difficult!) by chip removal and performing the rewrite on the bench, so to speak.

More recently, DIY tuning has been facilitated on some engines with hand-held units that plug into the OBD port, uploading the modified software to the car, whilst storing the standard software in the unit, allowing owners to simply transfer back and forth between tuned and standard settings. This is particularly useful during servicing, when franchised garages sometimes install manufacturer software updates – a process that could wipe clean your tuning software.

At present, the legal limitations on electronic engine tuning are apparently minimal, bar the necessity of informing your insurance company, and ensuring that it will cover your modified car. The various legal implications of UK Constructions and Use, EC Type Approval, and EC Emissions legislation suggest that engine modification is probably illegal, but enforcement is at present not a serious threat.

The same applies to removal of catalyst boxes and particulate filters, and any changes to carbon dioxide emissions after tuning that arguably require reclassification of Vehicle Excise Duty grouping.

Tuning companies should ensure that the results of their tuning are environmentally acceptable and, whilst many now state that DPF and catalyst box removal will not be liable to any legal action, or failure on MOT testing, this is not the same as being legal, and the situation could easily change if regulation were to be strictly enforced.

Would insurers then offer cover on clearly illegal cars? The answer would of course be no.

WHY TUNE?

Just as petrol engines have been tuned by enthusiasts for years, the potential for extracting some extra performance is equally attractive to keen diesel drivers.

Diesel electronic tuning aims to improve engine efficiency and, in normal driving conditions, when use of the extra performance is modest, owners very frequently discover the bonus of better fuel economy, of around five to fifteen per cent.

So, with the cost of fuel, there’s keen interest in tuning for economy. It all depends on how often you use the extra performance, but the gains come from the improved engine torque that allows you to pull a higher, and more economical, gear at any given speed.

A tuned engine is also protected from abuse by a number of manufacturer limits placed on key criteria, like turbo boost pressure, or fuel pressure, that will prevent the engine exceeding.

On hitting such limits the engine may throw up an engine management light, requiring ignition on/off sequences to cancel, a fault code reset before re-starting, or at worst drop into ‘limp home’ mode, requiring dealer correction.

However, a good tuning company will keep their tuning safely below such limit values.

THE NEGATIVES?

Today’s diesel engines are more robust than ever, and are built with considerable safety margins that allow them to run safely at high power for extended periods.

Tuned diesels driven reasonably responsibly will rarely draw on their full potential power and torque outputs, but there is inevitably some erosion of those safety margins.

Use of the full performance will also put some added stress on gearboxes and clutches, along with running gear like tyres and brakes, and this may add to running costs due to more regular replacement.

Any engine problems with a tuned car (although such problems are relatively rare) will most probably not be claimable under any manufacturer warranty.

Owners will also need to ensure that their tuned car is declared to their insurer, with an increase in premium highly likely.

BEFORE YOU TUNE

Plenty of cheap tuning devices are offered on online auction sites like eBay, but the pedigree of the sellers may be questionable, and you’ll have little recompense if the product is rubbish.

Stick to companies with established reputations, whose customers offer good positive feedback, and whose websites offer a comprehensive Q&A page.

Many tuners offer DIY packages with add-on boxes, but check carefully with them how easy installation is for your car. It’s far better to pay a little bit more for an expert fitter to do the job on your driveway.

For chipping, when you’ll usually take your car to the tuner, a trip to see them face-to-face is wise, and you can grill them with a few appropriate questions.

If you have engine problems, get them sorted before you even think of tuning – any good tuning company will decline to work on an obviously sick engine.

If you’re intending to use the extra performance regularly, and fully, then also consider the costs of added maintenance, tyres, brake pads and discs etc.

Finally, get that insurance quote before you tune your engine!

WHAT CAN YOU EXPECT FROM YOUR TUNED ENGINE?

We’re talking here about electronic tuning, not more extreme tuning involving components like intercoolers, exhausts, and even turbochargers.

An electronic tune should usually offer (some cars are less responsive than others) around ten to 20 per cent added power and torque.

Some of the manufacturers publish quite modest output claims for standard engines, and Volkswagen Group 2.0 TDI 140 engines often deliver over 150bhp as standard. So a typical tune will likely take this up to around 170bhp, and the maximum torque from 240lb ft to 270 or 280lb ft.

There’s much to be said for knowing exactly what you got for your money though, so tuners offering an optional before and after rolling road dynamometer test at reasonable cost (under £100) are backing up verbal claims with solid facts, and you will find out what power and torque your standard engine was delivering.

Any disappointments you might have during a post-tuning road test, like flat spots, should be clearly identified on a dynamometer graph, and corrected. Usually you will be very pleased with the extra performance and engine response.

WHICH TYPE OF TUNING IS BEST?

The essential part of all diesel electronic tuning is the modification of the electronic instructions that are sent to the key engine components, like turbochargers and fuel injection systems.

Chipping and software rewrites actually change the software written to the ECU, substituting the manufacturer’s software with carefully developed modified programs that alter the engine management instructions at source.

In contrast, plug and play tuning packages involve intercepting the standard ECU signals by placing a small box of electronics between the ECU and the key engine components that modifies the signals, typically by changing voltages of signals that feed key components like injection pumps. The effectiveness varies, possibly according to cost, and is somewhat less sophisticated than the full software rewrite.

But the end results are often more than satisfactory for many owners, particularly with the better conversions; like many things in life, you generally get what you pay for.

The advantages of the box are that it is usually easily removable, when required, whilst a big advantage of the rewrite is that it’s invisible, and usually not detected during servicing, if that is what the owner desires, with the reservations mentioned above regarding the possibility of software wipe-out.

COSTS AND REWARDS

The cost range of electronic engine tuning lies somewhere between £250 and £500, which is not a lot compared with the price of a modern car.

If you achieve economy gains of only five per cent (and you may well do better), such as a typical 40mpg improving by 2mpg to 42mpg, at 10,000 miles a year, your £400 conversion will pay for itself in five years, which is not a bad return, if you keep your car that long.

Some companies like Tunit and Chip Express will give you a part-exchange allowance on an existing tuning module of theirs against a new one, or reprogram the tuning module for your new car for around £100 plus VAT, if the module type is transferable.

The payback mileage may be extended by increased insurance costs though. It all depends on what value you place on driving enjoyment, because the popularity of electronic tuning suggests that this is significant for many enthusiastic drivers, and the vast majority are very happy with the results.

MAINTENANCE

Manufacturers tend to prescribe servicing intervals for fleet cars, to achieve minimum running costs.

If you choose to tune your engine, we would advise you to consider increasing the frequency of your oil services.

With many computer-generated service intervals of around 15 to 18,000 miles, an extra oil and filter change at 8 to 10,000 miles is advisable for engine longevity.

If you’re using the added performance, you should consider choosing top quality tyres, check their pressures regularly, and keep a close eye on brake pads and disc wear.

Otherwise, standard servicing schedules are probably quite adequate.

If you are working your tuned engine fairly hard with reasonable frequency, you should be free of the diesel particulate filter (DPF) problems that often plague engines that are under-stressed and rarely get properly warmed up to working temperature.

We might add that, with more torque to play with, there’s an even stronger argument to consider switching to full winter tyres for the colder months, as the extra torque will be more likely to spin standard all-year tyres and there’s a possibility that you could spin into a ditch or, worse still, another car.

EXTRA TWEAKS

You might want to think of helping your car breathe more freely, with a free-flow air filter, like one of the renowned K&N filters, with its 100,000 mile cleanable cotton filter element or one of their comprehensive high performance air intake kits.

You should possibly consider up-rating your brake pads, with something like EBC Red Stuff front pads, but be sure to take expert advice to maintain a proper balance in the braking system.

Milltek has been long-established, offering a range of well-respected and reasonably priced performance exhaust systems, in standard (quiet) or noisier (non-resonated) sporting form, that should add another eight to ten per cent to your power output.

Think about using only top quality branded fuel in your tuned car, to prevent build-up of deposits that might stifle its boosted performance, and regularly use fuel additives, like Millers Eco Power Max, to keep your engine in tip-top condition.