There's been a lot of discussion regarding diesel engine remapping affecting engine and longevity, but very little regarding their affect on emissions. Now I understand that most aspiring hotrodders aren't especially concerned with NOx and particulate eissions, but it is something to bear in mind if anyone wonders why factory settings appear 'conservative'. I'm not puritanical on this subject, but it does irritate me that backstreet 'tuners' (kid with a laptop) won't even acknowledge any of the potential downsides of their facile modifications. Diesels, unlike gassers are currently subject to ridiculously lax emissions testing during the MOT. Gassers are subject to rigorous exhaust gas composition analysis, whereas diesels simply have to comply with an opacity test at maximum governed speed. Surprise, surprise, an unloaded engine not smoking-who'd have thought?!

Tuning companies frequently extol the merits of 'optimising' the fuel settings to improve efficiency-free horsepower and better mileage! Fantastic! Er no. There are three things you can do with diesel fuelling; advance the injection, retard the injection timing, or increase the fuelling. Advancing the timing tends to increase peak cylinder pressures and temperatures. This tends to yield slightly better efficiency at the expense of a huge increase in NOx. Other side effects include an increase in bearing loadings and diesel clatter. Perhaps one would prefer to retard the injection settings. The effect is to reduce diesel knock and Nox, but a large increase in soot, CO, HC emissions. At high load, EGTs will increase substantially. Funnily enough, tuning companies never mention EGTs, yet every heavy duty engine operator is taught to keep a close eye on excessive EGTs.

The other spurious claim is that factory settings are 'conservative' because of low quality fuel sold in Turkey, the hot climate in Greece or something equally spurious.

Really? Modern diesel engines are invariably equipped with air intake temperature sensors, air mass flow sensors and coolant sensors. Unlike gas engines, diesel engines do not NEED any of these data to operate. These parameters are measured solely so that the ECU can optimise the fuelling rate and timing to real time local conditions. Fuel quality across Europe is regulated to a common standard. Fuel quality outside the EU is of no concern to OEMs selling cars in Europe.

'Smoothing out the torque curve'

Everyone knows that diesels have a horrible torque curve, particularly at the very bottom of the RPM range. OEMs work very hard at trying to improve this. That they haven't succeeded should indicate that the problem is far from trivial. The problem is usually (and entirely erroneously) blamed on the turbo's failure to provide boost at low RPM. A quick glace at a typical compressor map should indicate that a good turbo will quite happily supply boost across the entire RPM range of a diesel. In order to improve transient response, factory exhaust turbines are small; the failure to boost at low engine speeds is a consequence of restricted fuelling. (smoke is not a problem in this RPM range). The ECU purposely restricts fuelling at low engine speeds in order to protect bearings and transmission components. Low RPMS are far harder on bearings than high RPMs. The high compression of diesel engines means that, compared to a gas engine of equal output, cylinder pressures and hence bearing stresses are much higher. The only way to increase output without additional mechanical and thermal stress on the engine is to increase airflow and drop the compression ratio. This cannot be achieved by plugging into a laptop.Low compression brings its own problems.

Finally, it is worth looking at the factory output of car derived engines used in commercial vehicles like vans. They are ALWAYS rated at lower output than their passenger car equivalents. This is not because 'van operators are not interested in performance' or something equally trite. Quite the contrary. It is because one cannot reliably operate an engine at high load for extended periods.

It is entirely possible to increase the output of an engine and not experience problems. Just remember that OEMs know their engines better than anyone else, have more money, and brains than anyone else. There are no free lunches!