Cast vs MFS Compressor Wheels
Machined from Solid’ compressor wheels are the latest in a long line of developments from the OEMs to enter the aftermarket. Here we explore the evolution of the compressor wheel to determine if there are any benefits to using MFS wheels on standard cast wheel applications.
Cast compressor wheels are crucial turbocharger components. With over 15 million turbos produced globally each year they have provided the durability and dimensional precision that, up until now, the majority of turbocharger applications have required. If a cast wheel is used by an OEM there is no particular advantage to using an MFS wheel, unless there are known application issues that could affect the integrity of the compressor wheel.
Traditionally, compressor wheels are produced from aluminium because of its low density weighing only one third of the weight of steel. It is also a relatively simple and inexpensive process to cast compressor wheels, but a major disadvantage is that cast aluminium is inherently not as strong as other manufacturing techniques. To create a stronger wheel post production processes are required, which include heat and solution treatments.
A high proportion of new turbochargers are spinning faster than ever before, with higher pressures, and as a result are subjected too much higher stresses which are beyond the limits of cast aluminium. Consequently, alternative materials and manufacturing processes are used.
If the compressor wheel material is not as strong as it should be it will eventually show signs of fatigue, because the blades are exposed to a continuous cycle of positive and negative stress caused by the wheel spinning fast and then slow.
Variations in Design
In response to the ever changing operating conditions there have been significant developments in compressor wheel design over the years.
Flatback: Is the earliest design of compressor wheel and is still used by some manufacturers.
Superback: This design was introduced due to the increased speeds that turbochargers rotate, which increases the force on the compressor wheel significantly. In particular the exducer diameter of the compressor wheel suffered the most. The Superback adds more material to the highest stressed area, therefore coping with higher loads.
Deep Superback: An exaggerated design of the Superback which strengthens the wheel further by adding more material around the highly stressed hub.
Deep Superback – extended tip: This design promotes greater airflow providing a faster boost response at lower engine speeds. The extended tip design increases the efficiency of the compressor wheel at higher boost pressures.
‘Machined From Solid’
Taking the design process one step further, the OEMs introduced a new method of manufacturing compressor wheels known as ‘Machined from Solid’ (MFS), primarily due to cast aluminium not being strong enough for higher operating conditions.
By using a forged aluminium bar, it is possible to use a much stronger aluminium alloy than can be used in the casting process. By using a stronger material, the wheel has a much longer life in comparison to cast wheels as it can carry much higher loads.
In addition, MFS wheels are ideal for low production runs, enabling manufacturers to respond quicker to new blade design technology as there is no delay due to casting tooling. The wheels are produced using sophisticated 5-Axis technology to carve out the blades from a solid bar of high strength aluminium alloy, providing superior durability.
Each wheel is precision balanced on fully automated balancing stations with autocorrection – although the precision of the machining operation often means that the wheel doesn’t need any balance correction. To create an even stronger wheel on specific high stress applications, titanium can be used, which prevents failure in applications susceptible to high cycle fatigue.
MFS and the Aftermarket
To conclude, if the OEM turbo is designed with a cast compressor wheel then there will be little or no advantage to using an MFS wheel other than if the application often suffers failure through fatigue – in which case an aftermarket upgrade would be appropriate.