Tuning: Pay attention to intake manifolds and intercooling (Articles)

TDI engines that have been modified for performance are usually tuned primarily through electronic adjustments: This typically involves increasing the fuel injection amount using a 10c module, a piggyback ECU, 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.
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 excessive pressure (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 400-meter runner breathing through a straw.

The narrowest part of a suction cup 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... A loader that creates a vacuum relative to its surroundings, which draws 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 design 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 suggest 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 verschandeln air filter. This effect would be further amplified with a dirty filter.

Since the engine is (under load) turbocharged with positive pressure, the turbocharger must overcome the entire difference between the intake vacuum and the boost pressure.
Pressure sensor data indicates that the rotational speed increases with the ratio of the intake pressure to the output pressure. In this process, the intake manifold vacuum (which is speed-dependent) is not only added to the boost pressure, but is essentially multiplied by the boost pressure.
To compensate for a vacuum 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 the standard boost level often leaves little room for further increases 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 far into 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 for 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 already de-restricted from the factory, as impressively demonstrated by the Seat Ibiza Cupra TDI (6L) with the 1.9L, 118 kW (BPX) engine:
While the 96 kW engine in the 6L model can manage with an intake area of approximately 20 cm² at a boost pressure of 1.3 bar using a naturally aspirated intake manifold, 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 in very small increments.

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

"Those who have the opportunity should consider..." Measure the vacuum to determine whether removing a restriction is worthwhile.
This can be achieved with any LDA (Laser Displacement Analyzer) that has a sensing range and a resolution of 0.1 bar. The LDA (Lambda Probe) is disconnected from the intake manifold and connected to the external pressure port of the engine's pneumatic system, located on the air filter housing. Of course, the intake manifold connection for the LDA must be sealed.
To eliminate any distortion of the measurement caused by the air consumption of the pneumatic components, their external air connection should be disconnected and a temporary filter (e.g.,...) should be installed to prevent dust ingress. Fuel line filters, or if necessary, a piece of cloth wrapped around the hose, should be installed.
For the actual measurement, a full-throttle acceleration to approximately 4500 rpm is sufficient, provided the turbocharger system is functioning correctly and providing full boost pressure. In the area of maximum rotational speed, the vacuum is simply read from the LDA (Laser Doppler Anemometer).


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).
It is also possible to replace the standard suction nozzle with other parts (e.g., drain pipes with a diameter of 50 or 75 mm from a hardware store, possibly with angle pieces, etc.). However, significant changes in airflow can cause the mass airflow sensor readings to drop if the airflow profile shifts in a way that places the sensor 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 of the air filter box inlet.

Furthermore, attention should be paid to possible Changes to the intake noise - such as those caused by pipe resonances - must be carefully considered, as they may invalidate the vehicle's operating permit!

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 with each other. You only need to purchase the components of a more powerful version and can achieve a verschandeln solution without any complicated modifications.



2. Intercooling is under scrutiny by engineers and tuners alike.

TDIs are used to increase the density of the intake air, or... Improving cylinder filling with a turbocharger.
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 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. The following difference will be briefly referred to as LLT (Level of Light Transmission) over AT (Ambient Temperature), with the unit being Kelvin (K).

The turbocharger delivers the most power within the engine's maximum power range, and also produces the highest heat 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 1.9L 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 performance.
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 overpressure utilizes all the gaps and spaces between the spoiler grid and the lower lip, allowing it to easily escape instead of forcing its way through the lower lip. 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 intercooler.
"If there are any leaks between the spoiler grid and the lateral air intake (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 the air enters the vehicle and the SMIC (Secondary Modular Integrated Cooler) should ideally be designed as follows:
a) The open area of the spoiler grille and the subsequent airflow path to the low-temperature cooler (LTC) should have a minimum flow cross-section corresponding to the area ratio of the cooling fin fields in the LTC front (rule of thumb: 50% of the LTC front area).
b) A straight and direct airflow path between the spoiler grid and the SMIC (Secondary Air Mass Interface Component), allowing the oncoming air to reach the intercooler with minimal obstruction and enabling the maximum possible airflow for cooling.
c) No leaks between the entry point of the traction cable and the low-voltage control cabinet.
d) Unimpeded outflow of the warm exhaust air behind the liquid cooling device, where the available cross-sectional area should again correspond to the fin surface area in the direction of flow.

In this ideal scenario, the Specific Material Consumption (SMIC) can vary depending on the vehicle model and the maximum... The LD may already be "oversaturated" with cooling air, meaning that minor deteriorations in the cooling air flow do not yet result in a clearly measurable increase in the LLT (Lower Limit Temperature).

However, the reality for series-produced TDIs with SMIC is usually different:
Small grille openings, which are often only partially open, restrict the airflow to the SMIC from the beginning. Depending on the model, even fog lights can act as a significant obstruction to airflow in the cooling path to the SMIC (Secondary Air Mass Cooler), for example, in the Polo 9N, Ibiza 6L, and 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 duct between the spoiler grid and the intercooler cause further losses of cooling air on its way to the intercooler.
Follow: The LLT (Low-Level Transient) 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 Manifold) values are typically found in the 1.9L / 96kW TDI engine configuration, especially in transversely mounted vehicles with a short intake path between the turbocharger and the engine, such as models like the Golf 4, Polo 9N, and platform-related vehicles like the Octavia, Leon, Ibiza, and Fabia.
For vehicles with longer tubes between the compressor, SMIC (Secondary Muffler Integrated Charge Air Cooler), and the 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 (System for Monitoring and Intervention in Combustion), it's usually located on the rear of the outlet air duct. The charge air flowing past there is no longer cooled optimally by the cooling mesh because the wind blowing past the rear of the mesh has already absorbed heat. Therefore, the flowing intake air in the SMIC's airbox exhibits a temperature stratification (well-cooled at the front, poorly cooled at the rear), and the intake air temperature (IAT) 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 (Low-Temperature) readings are approximately 10K lower than in the SMIC (Short-Circuit 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.
For vehicles equipped with factory-installed front-mounted intercoolers (FMICs, in English), the LD/LLT sensor is also located in the intake manifold. "Due to their higher cooling capacity, the FMIC models have the lowest LLT values here, but the real-world difference compared to models with a sensor in the SMIC is not as dramatic as the simple comparison of the log values initially suggests."

As I mentioned, the 1.9-liter TDI engines with 96 kW generally handle the highest LLT (likely referring to a specific type of load or stress test) well, 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 intercooled 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 coolant temperature (Vmax-LLT) at the coolant outlet (LLK-Ausgang) could easily exceed 100°C even without any modifications, especially when driving at full throttle (BAB-Vollgas) for extended periods. However, depending on the software, the engine control unit (ECU) initiates an emergency shutdown at approximately 80°C of coolant temperature (CLT), reducing boost pressure and fuel injection volume, among other things, to protect the variable geometry turbocharger (VTG) from excessively high exhaust temperatures.
Therefore, many 96kW TDIs from the Polo/Ibiza/Fabia platform (which have particularly poor airflow to the intercooler) already operate near the threshold of power reduction at higher highway speeds in the summer, even without any tuning modifications. If, for example, the SMIC (surface cooler) is no longer fully effective due to dirt accumulating on its fins, the driver will experience a loss of power, which gradually diminishes the driving pleasure of the TDI engine (as the SMIC fins become increasingly clogged). See, for example, /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. With BAB full throttle (in 4th gear). Continuously from approximately 2500 to approximately 4300 rpm (or higher), log the boost pressure and intake air temperature using VAGCOM, using measuring blocks 11 and 7.
2. If the LLT (Lower Limit Temperature) rises significantly above 80°C, the SMIC (Surface Mount Integrated Circuit) should be checked for contamination and thoroughly cleaned, even if it doesn't appear to be severely damaged 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, especially in summer, with regard to the durability of the engine and turbocharger.
The aforementioned protective functions of quantity and charging 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, of course, increased during chip tuning. Therefore, even with active fuel and boost pressure reduction, tuned engines operate with higher fuel and pressure levels than normal, meaning that the factory-specified limits for 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 performance enhancement, 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 pieces and installation costs), which can reduce the LLT (Leakage Limit Threshold) 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 intercooler (SMIC). To achieve this, the standard air intake routing to the SMIC must be modified as described above. Points a) through d) should be reviewed or revised.
Depending on the contents of one's personal stock of materials, such projects can often be done at almost no cost, for example, using old plastic containers as raw material for air ducts, a hot air blower to shape the parts, foam and/or tape to seal gaps, etc.

The less the standard cooling air routing complies with points a) to d), the greater the potential for improvement. An example using the Polo 9N1 is shown in the attachment.
The discerning reader will note that the lowest LLT (Lowest Load Temperature) is only achieved when there is no NSW (New Solar Wheel) in the cooling air stream. Therefore, its removal is at least recommended for the summer months! For those who are bothered by the appearance of the empty NSW (New South Wales) license plate recesses in the plastic grilles, "No NSW" spoiler grilles are available 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. However, you should pay close attention to their permeability to ensure that the final result doesn't lead to a higher leakage rate than with the original parts! The grids should therefore...
- exhibit a minimal surface area of the webs 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), thereby reducing the cooling airflow through the heat sink.
Ideally, something like a rabbit hutch grid would be .

Those seeking the optimal balance between design and LLT (Low-Level Test) should include LLT log data in each version, as mentioned above. As a (theoretical) optimum, one should log the LLT (charge air temperature) once completely without any intercooler grilles. Then, you can subsequently test each grille variant to determine how much charge air cooling is sacrificed.
If the SMIC (System for Microclimate Control) is already saturated with cool 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 - lower limit temperature).

To obtain comparable data when logging LLT (likely referring to a specific engine parameter), it is crucial 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 (Pmax).
According to the author's experience, the best comparability can be achieved with the least effort by maintaining a constant high speed in 4th gear, with the engine running from approximately 1800 to 4300 rpm. Depending on the transmission, speeds of over 150 km/h can be reached, so it's advisable to choose a section of unlimited-speed highway with low traffic volume for this purpose.
In lower gears, the heat buffering effect of the coolant becomes noticeable compared to the faster engine speed-up, as the coolant temperature (LLT) is lower. Furthermore, the trend and final value of the coolant temperature are significantly influenced by the driving profile of the last approximately 20 seconds before the measurement.

Assuming that the high LLT (Low-Level Transmissions) values 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 or
-> can be used in conjunction with tuning measures to conserve materials (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 implementing any improvements to the intercooler.
In that 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 intercooler air ducts?
"Someone with malicious intent might believe that usable tuning potential is 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 opt for the more powerful factory-installed engine." Bad luck for those who already have the most powerful factory-installed engine in their model.