How NOT to performance tune a Diesel engine

This day and age tuning the diesel engine for increase power, torque and fuel economy is all too common. And like all things there is a right way to perform this and a wrong way. Let’s have a look at some of the ways to NOT modify your diesel vehicle.

In essence, all diesel tuning to increase engine torque (and therefore Power) is achieved by getting more fuel into the engine or altering the point at which the fuel is injected in the combustion stroke (Injection Timing). The methods of achieving more injected fuel in a modern common rail engine without modifying the hardware components is to either a, Increase the pressure acting on the injectors (Rail Pressure), so that for a given time constant more fuel will be injected and b, Increase the time (pulse width) that the injector is open for.

eBay Diesel tuning chips: These cheap electronic devices are normally extremely basic and commonly piggyback the rail pressure sensor and modify its signal output in order to fool the factory control unit to target a higher rail pressure, therefore increasing the injected fuel mass and hence producing extra torque.

The biggest issue here is that it is only effecting the fuelling to the engine and that this can lead to the engine running dangerously Rich air/fuel ratios and having excessive Exhaust Gas Temperatures (EGT’s). On top of this all common rail fuel injection system utilise a fuel rail pressure relief valve in order to safe guard the injection system in the event of a malfunction, and when one gets too greedy with rail pressure it is not uncommon at all to have cars going into “Limp Mode” when this valve cracks open.

The Bad Remap: Just like the cheap chip a remap too has to increase the power of the engine by getting more fuel into the engine or altering the point at which the injection happens (Timing). It is well known that in all industries you get what you pay for and a cheap remap often has had very limited time and resources invested into it.

The very essence of tuning is that we are trying to improve on the calibration that the factory engineers already spent months creating. It is very common for the cheaper remap to produce good power gains on the dyno but all too often it is at the cost of efficiency of the engine. Pouring black smoke out the exhaust is a clear sign of an inefficient tune. That black smoke that you are seeing is fuel that did not burn completely in the combustion chamber, to contribute into effectively and efficiently producing torque for the engine.

Often they will increase the boost pressure produced by the turbo to try to burn this extra fuel that is being injected and this indeed does need to occur. However, the method of which they achieve this is all too often butchered, commonly raising target boost indirectly by offsetting VGT vane position as a quick and dirty way of reaching higher boost pressure commonly results in strange part load drivability issues as EMP (Exhaust Manifold Pressure – also known as Drive Pressure) is also inadvertently raised, this can cause a variety of issues ranging from incorrect EGR flow, to the boost control system oscillating out of control as it tries to achieve a target boost pressure.

Another common pitfall is the severe reduction or complete elimination of maps in the ECU that are devised for engine and/or driveline protection.

Poorly calibrated tunes commonly disable protection features by ‘zeroing’ maps that offset injected fuel quantity based off sensor or powertrain inputs.

Injected fuel quantity should not always be constant, and is trimmed by parameters like Inlet Air Temperature, Coolant temperature, Transmission Torque reduction requests, Fuel Temperature, Mass Airflow, Boost pressure and many others.

To correctly tune a common rail diesel engine it is necessary to modify these tables. This must be done in a way that allows for increased torque production, but by no means should these tables be totally eliminated as they are an invaluable tool for accurate ECU calibration.

Article written by Kye Knight of AHT Performance

AHT Performance is a division of Andrews High Tech Automotive.