Boost Pressure Comparison: Diesel vs. Petrol

Does the diesel engine IWG turbo produce lower boost pressure than the petrol engine with the same spring pressure setup?

No, that's not necessarily true. In fact, a diesel engine will often produce higher boost pressure than a petrol engine when using the exact same turbocharger and wastegate spring pressure setup.

Why Boost Can Be Higher on a Diesel

The wastegate spring pressure only determines the minimum boost level required to start opening the wastegate flap. The final, actual boost pressure is a result of the total energy in the exhaust stream, which is used to spin the turbine.

This is where the fundamental differences between the two engines come into play:

  • Exhaust Gas Mass Flow: This is the most critical factor. Diesel engines operate without a throttle plate and with a very high air-to-fuel ratio (running lean). This means they are constantly moving a huge volume and mass of air through the engine compared to a throttled petrol engine. This high mass flow of exhaust gas pushes on the turbine wheel with tremendous force.
  • Boost Creep: Because of this immense exhaust gas flow, diesel engines are much more prone to "boost creep." This is a condition where the volume of exhaust gas is so large that it overwhelms the wastegate. Even with the wastegate fully open, it simply can't bypass enough exhaust, causing the turbine to continue accelerating and the boost pressure to rise (or "creep") well past the spring's set pressure.
Engine Type Key Advantage for Spinning a Turbo Common Result
Diesel Very High Exhaust Gas Mass Flow Prone to boost creep, often exceeding wastegate spring pressure.
Petrol Very High Exhaust Gas Temperature (EGT) Higher thermal energy, but lower mass flow at comparable RPMs.

 

In short, while the wastegate spring is set to 1 bar on both engines, the sheer force of the diesel's exhaust gas can easily push the boost pressure significantly higher than 1 bar, whereas the petrol engine's lower gas flow might result in boost that is much closer to the 1 bar spring pressure.

If the spring is soft that will cause opening before its target pressure what the boost situation between these two engine types ?

If the wastegate spring is soft and opens before its target pressure, it will cause a significant loss of boost on both engine types, but the character of the power loss will be slightly different.

A soft spring creates a "leak" for exhaust energy. The wastegate opens too early, allowing the exhaust gas that should be spinning the turbine to escape. The fundamental result is the same for both engines: the turbo cannot spool effectively, and the engine will feel sluggish and underpowered.

Here is how the situation would likely play out for each engine:

Boost Situation with a Soft Spring

Engine Type Effect of the Soft Spring How It Would Feel
Diesel Engine The diesel's high exhaust gas mass flow will immediately start pushing the weak wastegate open at very low RPMs. A large volume of exhaust will bypass the turbine from the moment you try to build boost. The turbo will feel extremely "lazy" and unresponsive. It will hit a very low boost ceiling (e.g., if a 1 bar spring opens at 0.4 bar, you will struggle to ever get past 0.4 bar) and will not climb further, no matter the engine load. The engine will feel flat and weak across the entire RPM range.
Petrol Engine A petrol engine has less exhaust mass flow at lower RPMs. The premature opening of the wastegate means a very large percentage of its available exhaust energy is lost. It will struggle immensely to generate enough pressure to spin the turbine effectively. The engine will suffer from extreme turbo lag. It may feel like a naturally aspirated engine through most of the low and mid-RPM range. You might see a small, insufficient amount of boost come on only at the very top of the rev range when exhaust energy is at its absolute peak, but it will be much lower than the target.


The timing of the turbine flapper opening, comparing both? 

The turbine flapper (wastegate) on a diesel engine will almost always open at a lower RPM than on a comparable petrol engine.

This is true for both a correctly functioning system and one with a weak spring.

## With a Correct Spring

  • Diesel Engine: Because of its very high exhaust gas mass flow at low speeds, a diesel engine builds boost very quickly. It will reach the target pressure (e.g., 1 bar) and open the wastegate early in the rev range.
  • Petrol Engine: A petrol engine needs more RPM to generate the equivalent exhaust energy to build the same boost. Therefore, its wastegate will open later in the rev range compared to the diesel.

## With a Soft (Weak) Spring

The principle remains the same, but the effect is exaggerated.

  • Diesel Engine: The high mass flow will overcome the weak spring almost instantly. The wastegate will crack open at a very low RPM, sometimes just above idle, preventing any significant boost from ever building.
  • Petrol Engine: It will also open earlier than it should, but it still needs to build more engine speed than the diesel to generate enough exhaust flow to open the weak spring. The flapper still opens at a higher RPM relative to the diesel.

Summary of Opening Timing

Spring Condition Diesel Engine Flapper Opening Petrol Engine Flapper Opening
Correct Opens early in the RPM range ⏰ Opens later in the RPM range
Soft/Weak Opens extremely early in the RPM range IMMEDIATELY Opens early, but at a higher RPM than the diesel


Why does the flapper always open in the diesel when the week spring? Is it because there is no throttle plate?

You are absolutely correct. The fact that a diesel engine has no throttle plate is the exact reason why its wastegate flapper is so susceptible to opening with a weak spring.

Here’s a breakdown of why this happens:

1. The Petrol Engine (With a Throttle Plate)

In a petrol engine, the throttle plate controls the amount of air entering the engine.

  • At low RPM or when you're off the accelerator, the throttle plate is mostly closed. Very little air enters the engine, and therefore, a small amount of exhaust gas is produced.
  • Result: There is not enough exhaust gas flow or pressure to push open even a weak wastegate spring. The flapper stays shut until you press the accelerator, open the throttle, and allow a large volume of air/exhaust to flow.

2. The Diesel Engine (Without a Throttle Plate)

A diesel engine is a giant air pump. Its power is controlled by how much fuel is injected, not by limiting its air. The intake is always wide open.

At all times (even low RPM): The engine is pulling in the maximum amount of air it possibly can for that speed. This means it is also constantly pushing out a high and consistent volume of exhaust gas.

Result: This powerful, constant stream of exhaust gas creates significant pressure inside the turbine housing. It is always pushing against the wastegate flapper. If the spring holding that flapper closed is weak, the constant force of the exhaust gas easily overpowers it and pushes the flapper open, even at low engine speeds.

A Simple Analogy

Imagine the wastegate flapper is a swinging kitchen door held shut by a very weak magnet.

The Petrol Engine is like a person gently blowing on the door from across the room. Nothing happens until they walk right up and give it a strong puff (opening the throttle).

The Diesel Engine is like having a large industrial fan pointed directly at the door and running constantly. The weak magnet stands no chance; the door is blown open immediately and stays open.

In short, the absence of a throttle plate means a diesel engine produces a relentless stream of exhaust that a weak spring simply cannot resist.

Turbocharger Adjustment and Low Boost Troubleshooting:

Step 1: Verify a Leak-Free System

Before any adjustments are made, it is critical to ensure there are no air leaks in the intake system after the turbochargers. A boost leak test is the most reliable method to confirm the integrity of all your piping, couplers, and intercooler. Even a small leak will prevent the system from reaching its target boost.

Step 2: Adjust Wastegate Actuator Preload

The most common adjustment to achieve your desired boost is setting the actuator preload. Our turbochargers come standard with a 1.0 bar spring, which typically begins to open the wastegate flapper around 0.7 bar. Adding preload keeps the flapper held shut more firmly, allowing boost to build higher.

To increase boost: You will need to shorten the actuator rod slightly. Loosen the lock nut and turn the rod's end fitting to increase tension. A good starting point is two full turns past the point of initial tension.

For a visual guide on this process, you can review this link: [Insert Your Link Here]

Step 3: Consider a Stronger Spring for Higher Targets

If you have properly preloaded the actuator and still cannot reach your target, the next step is to install a higher-rated spring. If your boost goal is significantly above 1.0 bar, a stronger spring (e.g., 1.2 bar or 1.5 bar) provides a higher base pressure, making it easier to achieve and control high boost levels.

Important Safety & Tuning Notice

Warning: We strongly advise against running the engine with the dump pipe removed as a method to raise boost. This is a dangerous diagnostic practice that can cause uncontrolled boost spikes and potential damage to the turbo and engine.

Our Role: Please remember that our specialty is turbocharger hardware. These mechanical adjustments are one part of the equation. Achieving your final boost target safely and reliably requires proper engine tuning (adjusting fuel, ignition timing, and boost control maps). We recommend working with a professional tuner to ensure your engine runs optimally with the new setup.

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