Improving airflow: often pointless with TDI engines (Articles)

First of all:
Thank you to the forum members Julian, Ernst S, garth.brooks, Ben1972, CoBrAtH, maniac, and Uli S for their contributions in clarifying the physical background, especially regarding LLK tuning !

To ensure sufficient air for combustion in the cylinders and to allow for a significant increase in fuel injection compared to a naturally aspirated engine, TDIs are equipped with a turbocharger and an intercooler.

Based on experience with gasoline engines, where the general rule "more air equals more power" usually applies (as long as the fuel injection system has enough capacity to increase the fuel supply accordingly and the engine hardware can handle it), it is also often attempted with TDIs to increase the turbocharger boost pressure and/or improve the intercooling.
For example, this can be achieved through water sprays for the air conditioning system, or by installing a larger air conditioning unit.

However, the expected increase in performance is often not observed in TDIs, because TDIs react differently to the air filling in the cylinders compared to gasoline engines.

The reasons for this lie in the diesel principle and the operating strategy of the TDIs.

Therefore, increasing boost pressure or performing LLK (Ladeluftkühler) tuning alone cannot provide a noticeable increase in performance, but only results in an "useless" increased excess air in the cylinder.

In the event of increased boost pressure, the additional power required for the turbocharger is drawn from the exhaust gases, which, due to the increased back pressure, can even reduce engine power – although usually only to a negligible extent.

That is, simply increasing the boost pressure on a healthy TDI results in higher turbocharger speeds, more thermal stress for the engine and turbocharger due to increased exhaust gas temperature (EGT) and intake air temperature (IAT), and a slight power loss, and can therefore be considered technically unsound - unless the fuel injection amount is increased as part of a proper tuning process to the point where a boost pressure increase becomes necessary.

The situation is somewhat different when it comes to LLK (charge air cooler) tuning. In addition to the thermal relief for the engine and turbocharger provided by cooler charge air, the engine's efficiency can slightly increase even with the same fuel injection amount, which can generally increase performance, although in rather academic terms: After a relevant discussion in the forum, the gain will hardly exceed 1%.

However, an LLK (luftkühlter Ladeluftkühler) modification also shifts other parameters, because it now allows "more air mass" (with unchanged boost pressure) to enter the engine.
For the turbocharger, it appears as if it is suddenly being supplied with a larger displacement engine, meaning the airflow increases, and the turbocharger has to work harder to maintain the boost pressure at the usual level.
And for the usual level, the turbocharger control system is responsible, which, as you know, doesn't know anything about the LLK tuning and now controls the turbocharger in a "sharper" way. Again, the turbocharger extracts more power from the exhaust gases, which reduces the usable engine power (see above). If the turbocharger is already overloaded to some extent by chip tuning or a fake pressure sensor signal, its shortened lifespan will be further reduced by a significant reduction in LLT without a corresponding adjustment (= reduction) of the boost pressure. It becomes particularly problematic when a modification to a larger turbocharger results in higher pressure losses between the compressor and the low-pressure sensor, which the turbocharger then has to compensate for by operating at higher speeds.


A turbocharger modification (without simultaneously increasing the fuel injection volume) only makes sense if the boost pressure is reduced to a level that provides approximately the same cylinder filling as before.
Then the LLT (Low Load Temperature), EGT (Exhaust Gas Temperature), and the turbocharger are relieved, resulting in an overall benefit for the entire engine.

With inherently unfavorable airflow for cooling, even a slight improvement in the airflow management can significantly reduce the Low Temperature Operation (LTO): see Part 2.


A noticeable increase in performance achieved solely through increasing boost pressure or intercooler tuning is possible, for example, when there are issues related to boost pressure/mass airflow sensor/a dirty intercooler, if the soot limit falls below the torque and driver-demand limits, meaning the engine is losing power.
Then, increasing the boost pressure / LLK-tuning will increase the performance to the point where the torque limit reaches the value of the next higher limit, which simply means that the engine is only producing its normal power again.
The supposed tuning only masks the symptoms, while the actual problem persists .

Only under extreme heat, very low ambient pressure (resulting in reduced boost pressure, e.g., for turbocharger overload protection in high altitudes), and/or low vehicle speed (when the intercooler is not adequately supplied with air, e.g., driving uphill with a trailer) can even perfectly functioning TDI engines experience a loss of power due to exceeding the soot limit.
Then, of course, increasing the turbocharger boost or turbocharger tuning would provide more power - as long as the car is operated under normal conditions and the modification can be reversed without any loss of power .


Chip tuning (also known as OBD tuning) typically involves increasing the turbocharger boost pressure to the extent that the desired performance is achieved without requiring a larger intercooler or excessive soot production.
The increased boost pressure provided by the LLT alone will result in a higher exhaust gas temperature (EGT).
In addition, the extended burn time due to the longer injection times further significantly increases the exhaust gas temperature (EGT).

Result for the loader: It has to work harder and, at the same time, is facing double the (exhaust) emissions regulations. In particular, the delicate VTG (Variable Turbine Geometry) mechanisms can be permanently damaged by excessively high exhaust gas temperatures (EGT), which often manifests as a loss of power and/or abnormal operation – requiring a replacement of the turbine.
However, the actual tuning is initially cheaper than selling a larger cooling system along with its components, air hoses, etc. to the performance-hungry customer – which would further increase the overall price due to material and installation costs.
The high art of chip tuning, therefore, involves, among other things, maximizing the performance of the turbocharger without causing it to fail prematurely.

"In plain language:"
To reduce the additional strain on the engine and turbocharger caused by chip tuning with increased boost pressure, it is always advisable to perform a corresponding intercooler tuning; however, this usually does not result in additional performance gains.
Unless there is a defect or extreme environmental conditions, the soot limit will again come into effect – see above.
The same applies if tuning software is designed in such a way that the engine (under normal ambient temperatures) "runs rough" at full throttle even without a defect. Then, the increase in air mass achieved through an LLK (low-temperature coolant) tuning also results in more power, just as one would expect from gasoline engines.


To estimate the performance impact of a planned boost pressure increase or turbocharger tuning, for example, one can analyze the behavior during a full-throttle acceleration in 3rd gear. Determine the fuel injection quantity limits at an engine speed of approximately 1,500 to 4,500 rpm (logging MWB 8 with VAGCOM is required; if unavailable, a diagnostic tester such as model 1551 may suffice).
If the air mass limit is at its lowest under normal environmental conditions, the engine should first be checked for issues related to boost pressure, mass airflow sensor (MAF), or a clogged intercooler, and any corresponding problems or defects should be addressed.
Subsequently, a second measurement usually reveals that increasing the boost pressure/turbocharger tuning would be pointless in terms of performance gain (see above), unless a chip-tuned engine is deliberately running in a state of partial load at full throttle, as mentioned above.
The potential performance gain, at best, is determined by the difference between the (lower) torque threshold and the (higher) torque limit.

Otherwise, the maximum possible increase in output from a boost pressure increase or turbocharger tuning can be estimated based on the difference between the air mass and the next higher limit: for every mg of difference, there is roughly 2 to 3% more torque in the relevant speed range, or. Performance to be expected - when the intervention causes the air mass limit to increase to at least the value of the next higher limit.

Exception 1:
The more a "tuned" engine smokes , the more unused energy is blown out through the exhaust. Here, increasing the boost pressure / intercooler tuning can make the combustion process "more complete" by increasing the air mass in the cylinder, and thus release weather-dependent additional power (even without increasing the fuel injection amount), regardless of the values read for air mass and other limitations.

Exception 2:
More recent TDIs (especially the PD engines) have software-based overheat protection features that are dependent on the coolant temperature (LLT). When the LLT is too high, these features noticeably reduce the turbocharger boost pressure and fuel injection volume, even before the engine starts to show signs of overheating.
These protective functions are most likely to be activated during prolonged high-speed driving on the highway and at high ambient temperatures. However, a more effective cooling system allows the activation threshold of these protective functions to be raised, and it can help maintain full performance for a longer time under unfavorable environmental conditions. However, achieving a performance gain beyond the motor's rated specifications in this way is not possible!


Furthermore, any (or every) reduction in intake air temperature, regardless of the impact on performance, inherently leads to a fundamental reduction in NOx emissions.
Unfortunately, that is not enough to replace the EGR system while maintaining the same emission class. . . .