An Inside Look at the New Garrett GTX Turbochargers
By Mike Kojima
One of the current rages in the turbocharger aftermarket is the billet compressor wheel. The traditional compressor wheel for most turbochargers is made of cast aluminum. Cast aluminum is a reasonably good material to make a complex part such as a compressor wheel out of economically in large scale production. However, cast aluminum is not as strong and does not have as good mechanical properties as billet aluminum.
One of the problems cast wheels can have are small voids and other internal defects. These voids can cause points of stress concentration inside the compressor wheel which carries a lot of load due to the high rpm it turns at (up to and over 150,000 rpm in some cases). These areas of high stress are called stress risers and can result in wheel failure. The other issue is that the mechanical properties of cast aluminum are limited as far as strength. For these reasons a cast wheel has to be designed with these issues in mind and the blades are made thicker and the center hub chunkier and stronger.
Some high performance cast wheels are post processed with Hot Isostatic Pressing or HIP which is a process that subjects the wheel to extremely high pressures and temperatures (around 10,000 psi and 900 degrees) in an inert atmosphere, usually argon. This process reduces internal voids and improves the grain of the material increasing strength.
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| The Garret GTX wheel is a 5 axis machined work of art, the 5 axis CNC machining gives the degree of freedom needed to design a wheel with no compromises in aero or stress. |
Billet wheels have even better strength than HIP treated cast wheels. A billet is a piece of raw aluminum stock that has been either roll formed or forged into shape. This highly compressive mechanical working of the material greatly reduces internal voids and improves the metal's grain making it finer and more homogeneous. This greatly increases the strength of the raw material. Typical billet compressor wheels are CNC machined from rolled bar stock.
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| The Garrett GTX series turbos use the same excellent dual bearing, oil damped, cartridge center section as the older R models. Garrett's world class OEM level engineering combined with new uncompromising compressors with designs not constrained by the mass production volume and price requirements gives GTX turbos a huge leap in performance. |
