The downside to a fully machined wheel on a 5-axis is cost; not only is the unit cost per wheel more expensive, it also takes longer to make a wheel too. When Garrett needs to make 8 million turbos a year, with a projection of that almost doubling in about a decade, wheels need to be made quickly. At the projected demand, that means a factory working 365 days a year, 24 hours a day, needs to make one compressor wheel about every 2 seconds! However, in order to move a wheel to production quickly and for expected low-volume runs, machined wheels make more sense than creating an expensive cast tool.
A compromise between a cast wheel and a 5-axis machined wheel is a flank milled compressor wheel. Flank milling involves the cutting tool only coming in from the flank, or side. This has benefits in cost and time of milling the wheel. The downside is the blade geometry has to be compromised as now the cutting tool can only come in from the side instead of from various angles as on a 5-axis machine.
The GTX wheels in the mid-frame sizes were spec’d out with certain performance targets in mind. As some have noticed, the 71/76/82mm GTX wheels are now a 58-trim as opposed to the common 56-trim of the previous GT series wheels. The trim of a wheel is one knob that can be turned to adjust the flow of a wheel with a larger trim flowing more; the increase in flow is nearly a linear relationship with increase in trim size. So a 58-trim wheel flows about 3.5% more than a 56-trim (=58/56). While a mid-frame GTX wheel in 56-trim still flows significantly more than the standard GT series wheels, it did not quite meet the performance target. The demand from the customer was for more flow, so Garrett gave it to them! As mentioned before, there are compromises with everything; the bigger trim did reduce the surge margin a tiny little bit as compared to the previous wheels, but the GTX wheels still have significantly more map width and flow.
|Another example of of just how much better the GTX wheels are performance wise: more flow, higher pressure ratio and more surge margin, a much better performer! This is the very popular 3076 mid size turbo which works great on your typical streetable 500 plus hp 4 cylinder engine.
There are a few other knobs to be turned that affect the performance of a compressor wheel. These include the tip width of the compressor wheel at the exducer, the location and angle of the ported shroud, and also the diameter, height, and type (vaned or vaneless) of diffuser section of the compressor housing. Each of these can be adjusted to optimize the surge margin, efficiency, pressure ratio, and choke flow. Those are topics for another day.
So what makes the GTX wheels unlike other billet wheels? The GTX wheels are not the same design as cast wheels, only fully-machined. There have been real, measurable improvements in the new designs. They have new aerodynamics for higher pressure ratio capability and greater flow, are made from forgings for increased strength, and are FEA stress analyzed and optimized for OEM-like reliability while being created specifically for the aftermarket where they will be pushed to the limit.