Tuning: Intake & Intercooler - Optimization Guide (Articles)

TDI engines that have been modified for performance are usually tuned electronically: This typically involves increasing the fuel injection amount through methods like 10c or piggyback tuning, or chip tuning, often accompanied by increased turbocharger boost or advanced fuel injection timing.

The side effects, which are often kept secret by professional providers, include:
Increase in fuel injection volume -> higher combustion and exhaust gas temperatures (EGT) -> higher thermal loads for the engine and turbocharger.
Boost pressure increase -> higher rotational speed loads for the turbocharger, higher charge air temperature (CAT) -> higher combustion and exhaust temperatures -> even higher thermal loads for the engine and turbocharger; additionally, higher combustion pressures -> higher mechanical loads for the engine and especially the cylinder head gasket (Gasket).
Advancing the injection start -> Lower exhaust temperatures, but higher combustion pressure peaks -> higher mechanical loads on the engine, especially the cylinder head gasket.

To ensure the durability of tuned engines, it is particularly important to keep increases in boost pressure as low as possible and to counteract the aforementioned side effects.
In the event of pressure spikes (if a delayed injection start is not an option), the only solution is to reduce the compression ratio, for example, by using special pistons or a thicker head gasket, which means the engine must be disassembled.
Fortunately, the increased turbocharger speeds and exhaust gas temperatures can be managed more easily within certain limits by addressing two common weaknesses found in many TDIs.



1. Intake paths under the control of developers

The 1.9 TDI engines consume at least 80 liters of air per second within their maximum power range.
This volume – a cube with sides of approximately 43 cm – must flow almost immediately at the beginning of the suction channels through a "suction nozzle" cross-section of about 13 to 20 cm², which, in relation, is almost like a 400m runner breathing through a straw.

The narrowest part of the proboscis is usually its outer opening.
Flow velocities of 50 m/s or more at Pmax are common in those applications, and the air must first be accelerated to that speed.
The energy for this is provided by the suction motor or blower, which creates a vacuum relative to the surroundings, drawing air through the suction nozzle.
The smaller the intake cross-section, the higher the flow velocity and friction losses, and consequently, the higher the required vacuum pressure.
Depending on the shape of the suction nozzle, some of the kinetic energy behind it can be partially converted back into pressure (diffusion principle). Therefore, the actual vacuum pressure inside the air filter housing is difficult to calculate.

Measurements taken on various TDIs indicate that negative pressure, potentially reaching up to approximately 50 mbar (without increased boost pressure), is likely to occur in most air filter housings – even with a clean air filter. This effect would be further amplified with a dirty filter.

Since the engine is (under load) supercharged with excessive pressure, the turbocharger must overcome the entire difference between the intake vacuum and the boost pressure.
Pressure-flow characteristic curves indicate that the speed increases with the ratio of intake pressure to outlet pressure. In this process, the vacuum pressure at the intake (which is speed-dependent) is not simply added to the boost pressure, but rather multiplied by the boost pressure.
To compensate for a vacuum pressure of 50 mbar, for example, in a 66 or 81 kW engine with a boost pressure of 1.0 bar, the turbocharger must generate a pressure ratio of 2 bar / 0.95 bar (absolute) = 2.1.
Its speed is so high that, with naturally aspirated intake and the same mass flow rate, a boost pressure of 2.1 bar absolute (1.1 bar overpressure) would be required, which is 10% more boost pressure than currently achieved.

The vacuum created by the intake manifold at maximum pressure (Pmax) already pushes the limits of today's small turbochargers (which often struggle to produce high boost at low RPM and suffer from turbo lag), meaning that there is usually very little remaining headroom for increasing boost through tuning.
If the boost pressure is increased, the turbocharger quickly enters an operating range where the compressor impeller efficiency drops and the rotational speed increases significantly, meaning that it operates more or less within the red zone of the manufacturer's specifications.
To achieve the very high charging speeds, a correspondingly higher exhaust back pressure is required, which in turn reduces engine power and hinders the gas exchange process.
As a (partial) solution without reducing boost pressure or changing the intercooler, practically the only option is to remove the restriction in the intake duct located before the air filter housing.

As shown above, the Pmax charging speed at a 66 or 81 kW motor, with a charging pressure of 1.0 bar and the original intake manifold, is approximately equivalent to a charging pressure of 1.1 bar without any intake throttling.
The usual 0.2 bar increase in boost pressure during chip tuning of 66 and 81 kW engines could, in a way, be achieved up to half of that by de-throttling the intake path; for other engines, it is likely to be similar in many cases.

With consistent performance optimizations, the intake side is often de-restricted from the factory. The Seat Ibiza Cupra TDI (6L) with the 1.9L, 118 kW (BPX) engine is a prime example of this.
While the 96 kW engine in the 6L model can operate with a naturally aspirated intake area of approximately 20 cm² at a boost pressure of 1.3 bar, the intake area for the BPX engine is doubled from the factory to around 40 cm² with only about 0.2 bar more boost pressure, without increasing the rated engine speed. This is likely to reduce the load on the (already larger) turbocharger by minimizing the amount of vacuum required.
This also incidentally documents that there is no compelling reason for the usual, tightly fitted intake manifolds used in TDI engines, such as the need to meet intake noise limits.

At lower engine speeds, a turbocharger also extracts less power from the exhaust gases. This results in a lower back pressure in the exhaust system, a further slight decrease in exhaust temperature (due to the faster expansion of the exhaust gases), and an increase in engine power, even without any actual engine tuning – although this increase is usually only noticeable to a small extent.

If a tuner dismisses a request to remove the throttle restriction on a naturally aspirated engine as unnecessary, citing "too much effort and it's not worth it because it works fine as is," then the knowledgeable enthusiast can easily handle this modification themselves as part of a small DIY project.

If possible, before making any modifications, it's advisable to measure the vacuum to determine whether removing any restrictions is actually worthwhile.
"This can be done with any LDA (Linear Differential Analyzer) that has a suction range and a 0.1 bar scale division. The LDA is connected from the intake manifold to the atmospheric pressure port of the engine's pneumatic system, located on the air filter housing; the intake manifold connection for the LDA must, of course, be sealed."
To avoid distortions of the measured value caused by the air consumption of the pneumatic components, their external air connection should be disconnected and a temporary filter should be installed to prevent dust from entering (e.g., a fuel line filter, or if necessary, a piece of cloth wrapped around the hose).
For the actual measurement, a full-throttle acceleration to approximately 4500 rpm is sufficient, of course, with a functioning turbocharger system = full boost pressure. In the range of the highest engine speed, the vacuum pressure is simply read from the LDA (Lambda Direct Analyzer).


The vacuum pressure can generally be reduced to a negligible (and practically unavoidable) residual level by redesigning the air passages so that the path between the intake manifold and the air filter box is continuously expanded to the size of the inlet opening on the air filter box.
For funnel-shaped intake nozzles, for example, the length can be shortened to increase the inlet opening, or additional holes can be added in areas with sufficient cross-section, as long as this does not result in the intake of warm air from the engine compartment or spray (during rainy drives).
Replacing the standard air intake system with other parts (e.g., drain pipes with a diameter of 50 or 75 mm from a hardware store, possibly with angle pieces, etc.) is also possible. However, significant changes to the airflow path can cause the mass airflow sensor readings to drop if the flow profile shifts such that the sensor is located in an area of slower airflow – which often occurs when installing open "sport" air filters.
Therefore, the shape of the intake passages should be altered as little as possible, especially in the area where they enter the air filter housing.

Furthermore, attention should be paid to any possible changes in the intake noise - for example, due to pipe resonances - as this may, in some cases, cause the vehicle's operating permit to expire!

Certain vehicles (e.g., Ibiza 6L, Polo 9N) are equipped from the factory with intake manifolds and connecting parts of varying opening diameters, depending on the power level, and these parts are interchangeable. In such cases, you only need to purchase the parts from a more powerful version, and you can achieve a clean solution without any modifications.



2. Intercooler cooling under scrutiny by engineers and tuners alike

TDIs use a turbocharger to increase the density of the intake air and improve cylinder filling.
The increase in air temperature by approximately 90 K per bar of pressure increase (depending on the efficiency of the compressor) means that even at a boost pressure of 1 bar, without any intercooling, about 40% of the potential increase in air mass flow is lost; at higher boost pressures, this loss increases further.

To reduce these losses in cylinder filling and to provide thermal relief for the engine, intercoolers are used.
While conventional air-to-air intercoolers cannot completely cool the intake air down to the ambient temperature (which would require infinitely large intercoolers), especially in engines with high displacement and boost pressures, manufacturers try to get as close to this ideal as possible by using large intercoolers with good airflow.

When considering the effect of an intercooler, it is generally advisable to use the ambient temperature as a reference point, or to refer to the temperature difference between the intake air and the ambient temperature. In the following, this difference will be referred to as "LLT" (intercooler temperature) relative to "AT" (ambient temperature), with the unit being Kelvin (K).

The compressor delivers the most power within the engine's maximum power range, and also produces the highest heating output (depending on the performance level, this can be approximately 12 to over 20 kW in 1.9L TDIs!), which in turn means the most work for the intercooler.
The cooling capacity, which refers to the heat transfer from the charge air to the cooling air, for typical side-mounted intercoolers (SMIC) in 1.9-liter engines, under normal installation conditions, at full boost pressure and the vehicle's maximum speed, is typically in the range of approximately 10 to 13 kW.

The actual heat exchangers (cooling circuits) of these SMICs are very compact in terms of their cooling performance, with dimensions up to approximately 20 x 20 x 6 cm. Therefore, they require a strong airflow to achieve the specified cooling capacities.
Its densely packed cooling fins disrupt the airflow, thereby promoting intense heat exchange between the metal surface and the air molecules.
This swirling motion slows down the airflow, creating a pressure buildup = overpressure in the air duct leading to the LLK (likely referring to a specific component or system).
This pressure buildup, in turn, utilizes all the gaps and spaces between the spoiler grid and the lower cooling module to easily escape, rather than forcing its way through the lower cooling module. At the same time, the spoiler grid (which is usually kept quite small for aesthetic reasons) forms the first restriction for the airflow on its way to the lower cooling module.
"If there are any leaks between the spoiler grid and the lower linkage component (LLK), the grid cannot supply enough airflow to fully compensate for the air lost through the holes behind it. Consequently, the airflow through the LLK is reduced (i.e., the LLK temperature increases) to a greater extent, depending on the total area of the leaks between the spoiler grid and the LLK."

Therefore, the cooling air ducts between the point where air enters the vehicle and the SMIC (Secondary Modular Integrated Cooler) should ideally be designed as follows:
a) The open area of the spoiler grid and the subsequent airflow path to the liquid cooling system should have a minimum flow cross-section corresponding to the area ratio of the cooling fin fields in the front of the liquid cooling system (rule of thumb: 50% of the total front area of the cooling system).
b) A as straight as possible airflow path between the spoiler grid and the SMIC (Secondary Air Mass Interface Component), allowing the oncoming air to almost unimpededly reach the liquid coolant and enabling the maximum cooling air flow rate.
c) No leaks between the entry point of the fuel line and the low-pressure fuel circuit.
d) The warm exhaust air should be able to flow freely away from behind the liquid cooling device, and the available cross-sectional area should again correspond to the fin surface area in the direction of flow.

In this ideal state, the SMIC (Supercharged Medium-pressure Intercooler) can be more or less "oversaturated" with cooling air, depending on the vehicle model and the maximum load. This means that slight deteriorations in the cooling air flow may not yet result in a clearly measurable increase in the LLT (Low-pressure Turbine temperature).

However, the reality for series-production TDIs with SMIC is usually different:
Small grille openings, which are often only partially open, restrict the airflow to the SMIC (Secondary Modular Intake Manifold) from the beginning. Depending on the model, even fog lights can act as a significant obstruction to the airflow towards the SMIC (examples: Polo 9N, Ibiza 6L, Skoda Fabia RS).
The laterally offset position of the spoiler grilles relative to the SMIC (Secondary Air Mass Interface Cooler) causes bends and turbulent edges in the airflow, which extract energy from the oncoming air and further reduce the cooling airflow.
Columns and holes in the airflow path between the spoiler grid and the intercooler cause further losses of cooling air on its way to the intercooler.
Followed by: The LLT (Low-Level Torque) increases significantly at high engine load (approximately 15-20 K, depending on the vehicle model), which is higher than what is unavoidable due to the size of the SMIC (Smart Motor Interface Controller)!

Generally, the highest standard LLT (Low Temperature Intake Air Cooling) values are found in the 1.9L / 96kW TDI engine configuration, particularly in models with short intake paths between the turbocharger and the engine, such as transverse-engine vehicles with a compact intercooler (SMIC) like the Golf 4, Polo 9N, and platform-related models such as the Octavia, Leon, Ibiza, and Fabia.
In vehicles with longer tubes between the compressor, intercooler (SMIC), and engine, significantly lower Pmax-LLT values can be measured, depending on the type. However, this is less due to the additional cooling effect of the long tubes, but mainly due to the location of the LLT sensor.
If it's integrated into the SMIC, it's usually located on the back of the outlet air box. The charge air flowing past there is no longer cooled optimally in the cooling mesh because the wind has already absorbed heat by the time it reaches the back of the mesh. Therefore, the charge air in the outlet air box of the SMIC exhibits a temperature stratification (well-cooled in the front, poorly cooled in the back), and the LLT sensor is located in the warmest area.
If, for example, the LD/LLT sensor is moved about 50 cm towards the engine into the intake manifold in a 96 kW Polo 9N, the warm and cool portions of the intake air mix together by that point, and the maximum LLT (charge air temperature) readings are approximately 10K lower than in the SMIC (standard intercooler).
In vehicles with long pipes between the compressor and the engine, the LD/LLT sensor is often already installed in the intake manifold from the factory, which results in lower readings compared to models with LLT measurement in the SMIC.
In vehicles equipped with factory-installed front mount intercoolers (FMICs, the LD/LLT sensor is also located in the intake manifold. Due to the higher cooling performance of the FMICs, the LLT (lambda lean/rich) values are the lowest here, but the actual difference compared to models with a sensor in the standard intercooler (SMIC) is not as dramatic as the simple comparison of the log values might suggest.

As I mentioned, the 1.9-liter TDI engines with 96 kW generally handle the highest levels of exhaust gas recirculation (EGR) because they are only equipped with a small intercooler (SMIC) from the factory, while the more powerful 1.9-liter engines with 110 and 118 kW have large front-mounted intercoolers (FMIC).
In the 96kW TDI engines in their standard configuration, the intake air temperature (IAT) at the intercooler outlet reaches approximately 55-70 Kelvin above ambient temperature (AT) at maximum speed, even without any tuning, depending on the vehicle model and equipment (fog lights in front of the intercooler).
During the peak of summer in Central Europe, the maximum permissible turbocharger outlet temperature (Vmax-LLT) at the turbocharger outlet could easily exceed 100°C under prolonged full acceleration, even without any modifications. However, depending on the software, the engine control unit (MSG) will engage the emergency brake and reduce the turbocharger boost pressure and fuel injection volume from approximately 80°C LLT onwards, in order to, among other things, protect the variable geometry turbocharger (VTG) from excessively high exhaust temperatures.
Therefore, many 96kW TDIs of the Polo/Ibiza/Fabia platform (with particularly poor airflow to the intercooler) already operate near the threshold of such power reduction at higher highway speeds in the summer, even without tuning. For example, if the intercooler is no longer fully effective due to sprayed-on road dirt, the driver will experience a loss of power, which gradually diminishes the driving pleasure of the TDI (with increasing clogging of the intercooler fins), see, for example, https://community.dieselschrauber.org/en/viewtopic.php?t=12605.
Since this operating condition is not a true emergency mode, there are no error codes stored, and workshops are often at a loss because the entire engine hardware is completely intact.
Tips for testing the SMIC:
1. When driving at full throttle on the highway (in 4th gear or higher), continuously log the boost pressure and intake air temperature using VAGCOM, specifically using measuring blocks 11 and 7.
If the LLT (lowest liquid temperature) rises significantly above 80°C, the SMIC (system maintenance and inspection component) should be checked for contamination and thoroughly cleaned, even if it doesn't appear to be severely affected externally.

In vehicles with such weak intercooling, increases in boost pressure within the Pmax range are only partially converted into increased air mass, because the intercooler temperature continues to rise, reaching levels that are generally considered unacceptable with regard to the durability of the engine and turbocharger, especially in summer.
The aforementioned protective functions of quantity and boost pressure reduction in the event of excessive LLT (Load Limit Torque) do not operate with absolute limits, but as a percentage reduction of the normal maximum values, which are increased during chip tuning. Therefore, even with active quantity and boost pressure reduction, tuned engines operate with higher quantities and pressures than normal, meaning that the factory-specified limits of exhaust gas temperatures are inevitably exceeded.
To ensure the engine's thermal stability, the tuner would need to program not only the classic software modifications for increased performance, but also ensure that the low-temperature protection functions activate at lower low-temperature levels. This would allow the fuel injection volume and turbocharger pressure to be reduced to approximately the level of a standard engine at high low-temperature levels.

For comparison: In the Ibiza Cupra TDI with 118 kW, the exhaust gas temperature (EGT) at maximum speed is only about 20 K above ambient temperature (AT) thanks to the front-mounted intercooler (FMIC), despite a boost pressure of 1.5 bar. This is approximately 45 K lower than in a tuned 96 kW Polo with a non-functional EGR valve (NSW)!
This not only provides significant thermal relief for the engine and turbocharger compared to a tuned 96 kW engine, but a 10K LLT reduction also increases air density by approximately 3% and thus reduces the tendency for soot formation.

Therefore, a material-compatible tuning, especially for the 1.9L TDIs with 96 kW and poorly positioned intercooler, should not only involve removing restrictions in the intake system but also improve the intercooling system.
For this purpose, high-performance SMICs are occasionally offered at prices starting from around 500 euros (plus any necessary special hose fittings and installation costs), which can reduce the LLT (leakage limit) by as much as 10K.
However, many tuning customers are discouraged by these prices and the relatively small reduction in emissions, and would rather let their engines work harder and produce more soot.

To minimize the risk of thermal overload in the tuned 96kW TDI engine, it is essential to optimize the potential of the standard SMIC (Standard Series Intercooler). To achieve this, the standard air intake system leading to the SMIC must be checked and/or modified, taking into account the points a) to d) mentioned above.
"Projects like these can often be done at a very low cost, depending on the materials available. For example, old plastic containers can be used as raw material for air ducts, a heat gun can be used to shape the parts, and foam or tape can be used to seal gaps, etc."

The less the standard cooling air routing conforms to points a) through d), the greater the potential for improvement. An example for the Polo 9N1 is shown in the appendix.
The discerning reader will recognize that the lowest LLT (Low-Level Turbulence) values are only achieved when there are no NSW (Narrow Slats) in the cooling air stream. Therefore, removing them is at least recommended for the summer months! Those who are bothered by the appearance of the empty NSW slots in the plastic grilles can purchase "No-NSW" spoiler grilles as original parts for many vehicle models.
If something like that isn't available (e.g., for the Fabia RS), you can use "racing-look" aluminum grilles as an alternative. However, you should pay close attention to their airflow characteristics to ensure that the final result doesn't lead to a higher leakage rate than with the original parts! Therefore, the grilles should...
- exhibit a minimal surface area of the bridges relative to the area of the holes.
- have the shortest possible total length of fins, meaning fewer large openings rather than many small ones: because even the narrowest fin slows down the airflow near it (surface effect) and thus reduces the cooling airflow through the heat sink.
Ideally, something like a rabbit hutch grid would be .

Those seeking the optimal balance between design and low-temperature cooling (LTC) should include LTC log data for each version, as mentioned above. As a (theoretical) optimum, one should log LTC data once without any spoiler grilles. This allows for subsequent testing of each grille variant to determine the amount of low-temperature cooling that can be sacrificed.
If the SMIC (Surface Mount Cooling Interface) is already saturated with cooling air due to optimized conditions (see above, points a-d), it may even be possible to use a visually acceptable, unobtrusive grid without increasing the Pmax-LLT (maximum power - lowest layer temperature).

To obtain comparable data when logging LLT (likely referring to a specific engine parameter), it is essential to always use the same driving cycle and to run the engine at full throttle until it reaches approximately 300 rpm above its maximum power output.
According to the author's experience, the best comparability is achieved with the least effort by maintaining a constant high speed in 4th gear, from approximately 1800 to 4300 rpm. Since this results in speeds of up to over 150 km/h depending on the transmission, it is advisable to choose a section of unrestricted and lightly trafficked highway for this purpose.
In lower gears, the thermal buffering effect of the low-temperature coolant becomes noticeable compared to the faster increase in engine speed, which is achieved through lower coolant flow rates. Furthermore, the profile and final value of the coolant flow rate are significantly influenced by the driving profile of the last approximately 20 seconds before the measurement.

Assuming that the high LLT (low-level turbine) temperatures in the standard configuration of the vehicles do not exceed material-related limits, modifications to the cooling air flow apparently reveal hidden reserves that can either...
-> without modifications to increase stability.
-> These can be used in conjunction with tuning measures to reduce material stress (by using lower pressure increases compared to vehicles with no hardware modifications) or to increase tuning performance by utilizing a more powerful intercooler.

After a chip tuning with a significant increase in boost pressure, it's important to consider that the turbocharger speeds will increase even further due to the higher air mass flow, before improving the intercooler.
In this case, it is all the more advisable to remove any restrictions in the intake path to generally reduce the turbocharger's rotational speed.


Finally, the question remains: why do many/most TDIs leave the factory with restricted intake manifolds and more or less unfinished air ducts leading to the SMIC (Secondary Modular Intercooler)?
"Someone with malicious intent might believe that usable tuning options are being systematically blocked."
"This allows VAG to always claim that electronically tuned TDIs always exceed certain (deliberately built-in) stress limits, and that performance-oriented drivers should therefore buy the stronger factory-installed engine. This is unfortunate for those who already have the most powerful factory engine in their model..."