Nissan 350Z Performance
Controlling Turbocharger Boost Pressure
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Controlling Turbocharger Boost Pressure and the Effect of Exhaust Back Pressure

Turbochargers operate on a very straightforward principle whereby exhaust gasses spin a turbine wheel. This turbine wheel is attached to a shaft and air compressor wheel so that as the turbine spins, it spins the compressor wheel, thus compressing air for the engine.

Whilst the principle is straight forward, the technology employed in modern turbocharger design is very advanced in order to deliver bulletproof durability, crisp response and high outright power.

Turbocharger Boost Pressure Control

A critical aspect of turbocharger operation is the management of boost pressure because this has a direct impact upon the total mass flow rate of air delivered to the engine in addition to controlling where the turbocharger operates in its compressor map. (Click here for a technical discussion on the relationship between turbocharger boost pressure and mass flow rate).

Physically, turbocharger boost pressure is controlled by a waste gate (either internal or remote) and is designed to divert exhaust gasses from the passing through the turbine wheel and turbo housing assembly (turbine). By diverting exhaust gasses away from the turbine, the amount of exhaust gas energy used to spin the turbine is reduced which in turn affects the boost pressure generated by the turbocharger compressor.

The simplest form of waste gate control is one that employs spring pressure alone in the waste gate actuator to control boost pressure. The waste gate actuator is spring loaded and operates the waste gate linkage via an actuator rod.

As boost pressure is generated, force is exerted against the spring-loaded actuator. When the boost pressure increases above the actuator's spring loading, the waste gate rod moves and opens the waste gate swing valve.

This method will limit the turbocharger boost pressure up to the spring pressure rating. For example, if a 7 psi actuator is used, the maximum boost pressure achieved will be 7 psi. Unfortunately, this simple method does not allow the tuner to adjust boost pressure relative to the engine load/RPM.

In almost all modern turbocharger applications, it is vital to allow for variation of turbocharger boost pressure relative to engine load/RPM. For example, on a particular engine it may be desirable to utilize say 10 psi at mid RPM (resulting in higher torque), but 7 psi at high engine RPM. This will also allow the turbocharger to operate within its high efficiency range envelope as specified in the compressor map (see technical discussion on the relationship between turbocharger boost pressure and mass flow rate).

The above graph demonstrates the different boost pressure strategies referred to above. Boost control through a waste gate spring only results in constant boost pressure (given a zero backpressure exhaust system) whereas computer boost control allows for a different and optimum boost pressure profile.

Modern engine management systems for turbocharged engines employ electronic forms of boost control that alter the boost pressure that is sensed by the waste gate actuator via a boost control solenoid. By altering the pressure on the waste gate actuator, one can now effectively control the turbocharger's boost pressure over a range of values and program the exact boost profile required by that engine.

A boost control solenoid valve is placed in line with the boost sensing hose to the waste gate actuator. The boost control solenoid valve pulses at a controlled frequency and effectively bleeds boost pressure from the actuator. The higher the pulsing frequency (or duty cycle), the greater the amount of boost pressure that is bled. In other words, the waste gate actuator senses a boost pressure that is lower than the actual boost pressure - hence allowing for greater boost pressure before the waste gate opens the swing valve.

There are a variety of products available that control the duty cycle of the waste gate boost control solenoid. Typically, the computer is programmed with the desired turbocharger boost pressure or solenoid duty cycle at various points in the RPM range/engine load.

The table below shows an example of the boost control map used to achieve the Computer Controlled Boost Pressure curve in the above graph.

2400 rpm
3100 rpm
3800 rpm
4500 rpm
5200 rpm
5900 rpm
6600 rpm
Engine Load
100%
100%
100%
100%
100%
100%
100%
Boost Pressure
9 psi
9 psi
9 psi
8.5 psi
8.1 psi
8 psi
7.9 psi
Solenoid Duty Cycle
20%
20%
20%
17%
15%
12%
8%

Target boost pressure or duty cycle values are programmed into the engine management computer to vary boost pressure according to the exact requirements of the engine and turbocharger combination.

There are occasions however when making substantial changes to either the wastegate spring tension or the boost mapping program that results in little or no change to the turbocahrger boost pressure.

For example, let's consider a turbo system that employs waste gate spring tension alone as its simple boost control mechanism. If one had a 9 psi actuator spring in the wastegate, it would be reasonable to assume that 9 psi would be available over the turbocharger's operating range. In a similar fashion, if that wastegate spring was upgraded to 13 psi, it would be reasonable to expect 13 psi from the turbocharger's operating range.

However, when one is presented with the boost curves shown in the graph below, the results are not as expected.

As can be seen above, the higher 13 psi actuator spring does nothing to raise the total boost pressure - but rather delays the initial opening of the waste gate - hence allowing boost to build slightly sooner than that with the 9 psi actuator.

This raises the question... Why did the boost pressure remain the same?

Exhaust Back Pressure

Since a turbocharger is effectively an exhaust gas driven compressor, it relies heavily upon the available exhaust gas energy to deliver charge air to the engine. Specifically, it relies upon the differential of exhaust gas energy across (between the turbine entry and exit) the turbocharger.

All of the above boost control discussions were based upon having sufficient energy differential to achieve the desired boost pressures. If however, a restrictive exhaust system is utilized, a great deal of backpressure is built up after the turbocharger (and to a lesser degree before the turbocharger) and there may not be sufficient differential across the turbine. In this case, regardless of the boost control mechanism employed, the turbocharger may not be able to achieve the target boost pressure. This is often seen with NA vehicles that are later turbocharged whilst using the stock exhaust system.

For the sake of simplicity, let us consider exhaust backpressure alone and ignore total exhaust gas energy as a measure of a turbocharger's ability to deliver charge air to the engine.

As an example, let us consider a turbocharger system that experiences 30 psi backpressure before the turbocharger's turbine and 5 psi backpressure after the turbocharger's turbine. This effectively provides the turbocharger with 30 psi - 5 psi = 25 psi pressure differential across the turbine.

If a zero back pressure exhaust system is utilized, then the backpressure after the turbocharger is 0 psi. This results in 30 psi - 0 psi = 30 psi pressure differential - hence greater potential for the turbocharger to deliver higher boost pressure.

Conversely, if a very restrictive exhaust is utilized, the backpressure after the turbocharger may be as high as 10 psi. This results in 30 psi - 10 psi = 20 psi pressure differential across the turbocharger’s turbine - resulting in much lower potential for the turbocharger to deliver boost pressure to the engine.

The following table summarizes the above results:

Pre-Turbo back pressure Exhaust back pressure Pressure Differential Boost Pressure Potential
Stock Exhaust System
30 psi
10 psi
20 psi
Low
High Flow Exhaust System
30 psi
5 psi
25 psi
Medium
Zero Back Pressure Exhaust System
30 psi
0 psi
30 psi
High

The above discussion had assumed constant backpressure values. However, since backpressure varies according to engine load and engine RPM, a turbocharger unit's performance can change accordingly.

There are many cases where the exhaust system backpressure is the cause of lower boost pressure. As we will see this typically occurs at higher engine RPM and regardless of the waste gate control mechanism, the pressure differential across the turbocharger is not sufficient to achieve the desired boost pressure level.

The backpressure of an exhaust system is relative to the mass flow rate of exhaust gas passing through the system. The higher the engine's power output, the greater the mass flow rate of exhaust gas - hence the higher the back pressure from the exhaust system and indeed through all components after each exhaust port.

Typically, the higher the engine RPM (at full engine load), the greater mass flow rate of exhaust gas through the exhaust system - hence higher backpressure. Let's consider the pressure differential at two different engine RPM points.

Using a restrictive stock exhaust system and testing at 3000 RPM and full engine load, the backpressure before the turbocharger may be 29 psi. After the turbocharger however, the exhaust backpressure may be as little as 2 psi. This provides a pressure differential of 29 psi - 2 psi = 27 psi. With this differential value, there is good likelihood that the turbocharger will deliver the target boost pressure.

At 6,600 RPM however, the pre-turbocharger backpressure may be as high as 32 psi but with exhaust back pressure of 13 psi. This results in a pressure differential across the turbine of only 32 psi - 13 psi = 19 psi. This value is a great deal lower than the 27 psi seen at 3000 RPM and may indeed be too small a pressure differential to achieve the target boost pressure level.

The results are summarized below:

Stock Exhaust System Pre-Turbo back pressure Exhaust back pressure Pressure Differential Boost Pressure Potential
3000 RPM
29 psi
2 psi
27 psi
High - likely to achieve target boost pressure
6600 RPM
32 psi
13 psi
19 psi
Low - unlikely to achieve target boost pressure

With this exhaust system the turbocharger will most likely achieve the target boost pressure level at low to mid engine RPM, but as the exhaust system back pressure increases, the turbocharger boost pressure will taper down.

Another advantage of a free flowing exhaust system with zero backpressure is that the turbocharger will respond quicker in on/off throttle applications, partial throttle (20%+ throttle) as well as delivering positive manifold pressure (boost) earlier in the engine's RPM range. This again is due to the lower exhaust backpressure and the resultant greater pressure differential across the turbocharger's turbine.

At this stage of the discussion, it should be noted that there are other exhaust gas variables that have an affect on a turbocharger's ability to deliver charge air to the engine. Exhaust gas speed, temperature, etc add to the total energy available to drive the turbine. Moreover, indeed, the total energy differential provides the driving force for the turbine. Whilst the above discussions concentrated on pressure differential alone (for the sake of simplicity), the same approach can be used for any of the other individual variables or indeed, for total energy.

Generally, when the best possible turbocharged engine performance is desired, a zero backpressure exhaust system is the optimum choice, as this will allow the turbocharger to achieve the target boost pressure values throughout the entire engine operating range.

As importantly, a zero back pressure exhaust system will also place less thermal load on the engine's combustion chambers, valves pistons etc. by removing hot exhaust gasses more efficiently than a restrictive exhaust. This new discussion however is best left for another time.

Fuel Octane and Boost Mapping

Modern boost mapping gives the tuner great flexibility to fine tune the boost map in order to take advantage of better quality fuels. Of course, there are other mapping issues such as ignition timing advance/retard and air fuel ratio (Click Here for further discussion on engine mapping) that are tuned accordingly, however boost mapping is just as important in terms of additional engine power and torque and engine safety.

For example, let us consider the following boost curve that is optimum for 91 octane fuel.

The target boost pressure at high engine RPM is 5.5 psi and at this boost pressure, the engine delivers the desired outright horsepower. However, with computer controlled boost mapping, the boost pressure is raised at lower engine RPM because in this case it is safe to do so (with appropriate fuel and timing mapping) and the result is higher mid RPM torque than would be achieved with a constant flat boost curve set at the maximum of 5.5 psi.

When better quality fuel is used, the optimum boost curve may be different with higher turbocharger boost pressure.

In this case, the maximum boost pressure at high engine RPM may be raised to 7 psi for example and the peak boost pressure at lower engine RPM may be raised as well to deliver even greater power and torque across the RPM range. In addition, the shape of the boost curve may be changed to better suit the engine/turbocharger/fuel combination.

Finally, let us consider a possible boost curve for the same engine running on 94 octane fuel in addition to a zero back pressure exhaust.

As we have seen, in the first part of this article, exhaust backpressure has a significant impact on the ability of a turbocharger to deliver high boost pressure - particularly at high engine power levels where the volumetric flow rate of exhaust gasses is high.

In this example, the optimum boost pressure curve may indeed be a flat line (by coincidence in this case - but a worthy example) and given that the exhaust system has sufficient capacity, the boost map programmed into the engine management computer is set to again deliver the optimum boost pressure curve for this application.

Using modern computer controlled boost mapping, the optimum boost curve is programmed to suit the particular engine/turbocharger/fuel combination throughout the entire RPM range. This ensures that the engine delivers the maximum safe power and torque at all times.