Billet wheels have been getting a lot of attention lately. Manufacturers claim that they are much stronger and will last much longer than the typical cast wheel, and some claim higher boost potential as well. But one look at the high cost of a billet compressor wheel and most enthusiasts have to stop and ask themselves: Are they really good enough to justify the extra cost, or is it just another marketing trick?

A 'billet' compressor wheel is a fully machined wheel, which refers to the process by which it is made. In loose machine shop terminology, a 'billet' is nothing more than a rectangular chunk of metal. In order to turn a billet into a useful product, it must be shaped by the mechanical process of cutting away material. In contrast, a 'cast' compressor wheel is made by pouring molten metal into a cast mold. So the names themselves refer to the way each type of wheel is made, rather than the material used to make them.

That isn't to say that there's no difference in the material used for each type of wheel, because there is. The process of casting requires material with good properties of castability. For example, castings are poured from hot liquid that eventually cools, and this requires the use of a material that is resistant to the formation of cavities during the cooling process. A casting also shrinks as it cools, and thick areas shrink at a different rate than thin areas. Unless a suitable material is used, this could lead to warpage and cracking. There are other considerations, but it's fair to say that the casting process places certain restrictions on the type of material used.

Fully machined wheels don't have the same restrictions, and stronger materials can be used to make them (the materials don't have to be stronger, however: any fully machined wheel is technically a 'billet' product, regardless of the material used to make it. This is an important point, because uninformed consumers could easily be tricked into buying an inferior part simply because it is labeled as a 'billet' part). This means that some billet wheels can assume more radical shapes than cast wheels, and they can better withstand the stress of centrifugal force at extremely high RPMs. They are also typically less susceptible to fatigue than cast wheels. On the surface, it would seem that these qualities would make them a clear winner over cast wheels. But this is only true if additional strength and a more radical profile are needed.

Turbo manufacturers spend untold millions on research and development, and a lot of that includes perfecting the shape of their wheels. Aerodynamic engineers are continually designing new wheel profiles and testing them using complex fluid dynamics and finite element analysis techniques, and the result of all that effort has proven that blade shape and hub diameter are the most critical elements of wheel efficiency. Strength is great, but anything beyond what is necessary is wasted. Barring a new profile that requires the use of a higher strength material, there is no reason to believe that the higher strength is an advantage. After all, compression wheel failure is very rare, even with cast wheels.

According to Garrett, '. . . our engineers ran identically-designed compressor wheels on our gas stands. We removed as many confounding variables as possible; there were no vehicles, no engines, just a test cell and a turbo. The only difference between the wheels tested was the manufacturing process used to create them. One was a fully machined wheel and the other was the standard, high-quality cast wheel . . . no noticeable gains in performance can be seen when testing a machined wheel vs. a cast wheel of the same design.'

This also reflects a principle of physics discovered centuries ago by Galileo: the shape of an object'”and shape alone--determines how it interacts with air. As long as it doesn't fall apart, a fan blade made of dried macaroni can move air as well as one made of titanium.

Manufacturers employed the billet manufacturing process for OE diesel applications where fatigue was a problem. These engines were required to sustain constant changes in RPM and load, and cast wheels weren't up to the job. This is not usually a factor in the reliability of automotive turbochargers, however. In common automotive application, the greatest advantages to the billet process goes to the manufacturers themselves. It takes a lot of time and money to retool a manufacturing process for a new casting, and the expense can't be justified for a production run of only a handful of parts. On the other hand, it's impossible to test a new part until it has been made. Rather than go through the time and expense of setting up for a production run on a part that may be useless, using the billet manufacturing technique allows for the production of a single part with only the reprogramming of a CNC cutting machine. This helps speed up R&D while allowing the company to bring a new product to market faster. It also lets the company produce a single compressor wheel for a custom application far cheaper than casting it.

In conclusion, billet manufacturing allows greater flexibility in blade profile and shape. The materials used are usually stronger, allowing for a wider range of blade shape. The increased strength may offer a longer life and allow for higher RPMs. But with identical wheels, operated under identical conditions, there is no advantage to the billet wheel simply based on the material used.

IMAGE: By S.J. de Waard Own work. CC BY 2.5 , via Wikimedia Commons