There is a lot of information floating around out there on all kinds of topics and as my daddy said, “It is always better to get your info straight from the horse’s mouth”. So I guess I will play the horse here and give an easy explanation as to why billet impellers are better then cast aluminum ones and also why Turbonetics Forged Billet Compressor Wheels are even better than standard billet wheels. The eastiest way to learn is to watch the short video embedded but here are the main points;
1. Billet aluminum wheels are much stronger than cast aluminum wheels for a couple of reasons. The first is that there are no porosity issues to have to worry about. This means that there is no chance of minute air bubbles being in the metal. Turbonetics uses a special process called HIP’ing. Hot Isostatic Processing that virtually eliminates porosity in its cast wheels but because the wheel is cast and the metal is still smushed together (laymans terms) the hotter the wheel gets, the faster it spins and the greater the pressure ratio conditions it runs at the greater the chance of metal fatigue, blade straightening or in the worst case bursting. The second is the metal material that can be selected to be used is a much stronger grade than the same cast material. This is where Turbonetics forged billet compressor wheels shine. They are even stronger than standard billet wheels because the grains of metal have been moved in such a way as to align in a specific direction. See article here on forging (http://en.wikipedia.org/wiki/Forging)
2. Because Turbonetics uses Forged Billet to machine the compressor wheel on a 5-Axis mill, the nose and hub of the compressor wheel can be made significantly smaller to allow for a greater blade diameter for a givine inducer size. The nose is where the compressor nut gets fastened down and the hub is the area around the bore of the wheel that the impeller blades are attacheed to. A cast wheel has to have a certain size nose and hub to allow the wheel to be cast number one and secondly it has to be able to be pulled from the mold itself. So simply stated a 61mm Forged Billet turbo will flow more air and have the opportunity to make more power than the same 61mm turbo with a cast compressor wheel.
3. The specific metal material used in Turbonetics forged wheels also gives the wheel tremendous blade strength and lowers the chance of the blades straightening at high speeds (ie. high boost pressures). When the boost pressure is raised many things occur including causing increased friction with the air and thus increasing temperatures, the pressure the wheel is under and finally the great centrifugal force the blades are under spinning at such high speeds. These three factors can cause the blades to contact the compressor housing causing serious damage to the turbocharger and could possibly lead to immediate destruction or greatly decrease the life span of the turbocharger.
4. With forged billet impellers the blades of the wheel can be machined much thinner because they are so much stronger. This allows the wheel to have a greater efficiency range becuase their is less blade thickness to contact the air and heat it up. It also means that with less blade thickness and lower hub area that there is a greater area in-between the blades to squeeze more air and build boost pressure. The more efficient a wheel is the lower the intake temps and everyone knows the cooler the air going into the engine the more horsepower you can make.
5. Being a 5-axis machined part means that this wheel is not cheap as it takes a long time to machine these wheels and it is expensive to program the mill. Hence the increased price of the TNX line of Turboentics turbos but being a machined part allows Turbonetics to continually make improvements in a quicker manner and lower cost method than creating very costly tools and molds for casting compressor wheels. This gives us an advantage in that we can make small changes to the comp wheel and continually improve the airflow and efficiency of the turbo for specific types of applications.
Source: Turbonetics Blog