Garrett GT28 Turbocharger TMS Astra VXR stage 5 engine
Details of the Garrett GT28 turbocharger specified by Thorney Motorsports for use in their stage 5 Astra VXR engine producing 400+bhp.
Garrett GT28 Turbocharger
How a Turbo System Works
Engine power is proportional to the amount of air and fuel that can get into the cylinders. All things being equal, larger engines flow more air and as such will produce more power. If we want a small engine to perform like a big engine, or simply make our bigger engine produce more power, our ultimate objective is to draw more air into the cylinder. By installing a Garrett turbocharger, the power and performance of an engine can be dramatically increased.
So how does a turbocharger get more air into the engine? Let us first look at the schematic to the left
1 Compressor Inlet
2 Compressor Discharge
3 Charge air cooler (CAC) / Intercooler
4 Intake Valve
5 Exhaust Valve
6 Turbine Inlet
7 Turbine Discharge
The components that make up a typical turbocharger system are:
- The air filter (not shown) through which ambient air passes before entering the compressor (1)
- The air is then compressed which raises the air's density (mass / unit volume) (2)
- Many turbocharged engines have a charge air cooler (aka intercooler) (3) that cools the compressed air to further increase its density and to increase resistance to detonation
- After passing through the intake manifold (4), the air enters the engine's cylinders, which contain a fixed volume. Since the air is at elevated density, each cylinder can draw in an increased mass flow rate of air. Higher air mass flow rate allows a higher fuel flow rate (with similar air/fuel ratio). Combusting more fuel results in more power being produced for a given size or displacement
- After the fuel is burned in the cylinder it is exhausted during the cylinder's exhaust stroke in to the exhaust manifold (5)
- The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure
- A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas' energy to provide the power necessary to drive the compressor
What are the components of a turbocharger?
The layout of the turbocharger in a given application is critical to a properly performing system. Intake and exhaust plumbing is often driven primarily by packaging constraints. It is important to understand the need for a compressor bypass valve (commonly referred to as a Blow-Off valve) on the intake tract and a Wastegate for the exhaust flow.
Blow-Off (Bypass) Valve
The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo's compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the intercooler. When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.
On my Corsa VXR I use the Forge RCV. This recirculation valve (or compressor bypass valve CBV) is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged vehicle when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.
Wastegates
On the exhaust side, a Wastegate provides the means to control the boost pressure of the engine.
There are two (2) configurations of Wastegates, internal or external. Both internal and external Wastegates provide a means to bypass exhaust flow from the turbine wheel. Bypassing this energy (e.g. exhaust flow) reduces the power driving the turbine wheel to match the power required for a given boost level. Similar to the BOV, the Wastegate uses boost pressure and spring force to regulate the flow bypassing the turbine.
Internal Wastegates are built into the turbine housing and consist of a "flapper" valve, crank arm, rod end, and pneumatic actuator. It is important to connect this actuator only to boost pressure; i.e. it is not designed to handle vacuum and as such should not be referenced to an intake manifold.
External Wastegates are added to the exhaust plumbing on the exhaust manifold or header. The advantage of an external Wastegates is that the bypassed flow can be reintroduced into the exhaust stream further downstream of the turbine. This tends to improve the turbine's performance.
Journal Bearings vs. Ball Bearings
The Garrett GT28 turbo specified by TMS for use on the stage 5 Astra VXR engine features roller (ball) Bearings, uprated seals and shafts.
The journal bearing has long been the brawn of the turbocharger, however a ball-bearing cartridge is now an affordable technology advancement that provides significant performance improvements to the turbocharger.
Ball bearing innovation began as a result of work with the Garrett Motorsports group for several racing series where it received the term the 'cartridge ball bearing'. The cartridge is a single sleeve system that contains a set of angular contact ball bearings on either end, whereas the traditional bearing system contains a set of journal bearings and a thrust bearing.
Turbo Response - When driving a vehicle with the cartridge ball bearing turbocharger, you will find exceptionally crisp and strong throttle response. Garrett Ball Bearing turbochargers spool up 15% faster than traditional journal bearings. This produces an improved response that can be converted to quicker 0-60 mph speed.
Manifolds
Manifold design on turbocharged applications is deceptively complex as there many factors to take into account and trade off
General design tips for best overall performance are to:
- Maximize the radius of the bends that make up the exhaust primaries to maintain pulse energy
- Make the exhaust primaries equal length to balance exhaust reversion across all cylinders
- Avoid rapid area changes to maintain pulse energy to the turbine
- At the collector, introduce flow from all runners at a narrow angle to minimize "turning" of the flow in the collector
- For better boost response, minimize the exhaust volume between the exhaust ports and the turbine inlet
- For best power, tuned primary lengths can be used
At the heart of my Astra VXR engine is the custom turbo and equal length manifold design. The manifold is built by the same company that supplied Red Bull Racings F1 manifolds so the quality of construction is exemplary. It is a true equal length which is crucial in maximising airflow from the larger turbo, no point creating power via boost if its wasted in blocked airflow.
A lot of the information above has been extracted from the Garrett website. For a lot more information on turbo design and technology visit their website.
