Why on Earth would ATI spend a great deal of time and money developing a billet 7075 T-6 aircraft aluminum, CNC-machined impeller? The answer is simple: to give you more boost. Because the billet impeller has significantly greater material strength than cast impellers used in other superchargers, it can be spun substantially faster to produce more boost and hp for your Mustang.
Superchargers, of course, do not produce boost with just an impeller. You must also have a transmission capable of spinning the impeller at the desired speed. The ProCharger transmission's (gears, bearings, seals) are the strongest and most durable ever built. Without this, the billet impeller would not even be needed, because transmission meltdown would occur, as is often seen with other superchargers, well before the physical limitations of even a cast impeller could be reached. However, armed with this exceptionally powerful high speed transmission, ATI is able to take full advantage of the high tech aerospace billet impeller.
A big part of supercharger flow and boost is based on the tip speed of the impeller. A simple way to find tip speed at a given rpm is to take that rpm and the impeller's circumference (2'r), and convert this into feet per minute, or miles per hour. An impeller, any impeller, can only go so fast (in feet per minute, or mph), before it will bend and break. This critical speed is determined in large by an impeller's material composition and the manufacturing process used to create the impeller. Thus, a bigger impeller, with a larger diameter and circumference, will reach its critical speed at a lower rpm than an impeller with a smaller diameter and circumference. So although a larger impeller may create a lot more boost and flow at a given rpm, its rpm limitation may ultimately make it no more powerful than a smaller, faster turning impeller due to the previously stated tip/critical speed relationship.
Historically, all centrifugal superchargers have used some form of cast impeller. Therefore, everything else being equal, superchargers with larger, slower turning impellers or alternatively, smaller, higher speed impellers both face essentially the same critical speed (and airflow and boost) limitations. Now, there are some other minor factors which can mildly affect overall performance, such as impeller to wall clearancing, number of impeller blades, and blade design.
However, these design issues can only give mild improvements to the basic performance of a cast impeller. By using the strongest production aluminum available to mankind, ATI is able to substantially change the rules. The material strength for most aluminum castings is 32,000 - 36,000 pounds (tensile), as compared to 7075 T-6 aircraft aluminum, which is 83,000 pounds (tensile). In fact, this aircraft aluminum is so strong that in many categories it actually has the same strength as steel. The end result is that the ProCharger impeller design retains the optimal boost curve for a streetable supercharger while also achieving boost levels never previously achieved by a warrantied supercharger. The ProCharger's larger impeller also allows it to operate at a moderate rpm and still create boost and airflow levels which exceed other manufacturers' units when tapped out. And for those customers who demand all-out racing performance, the powerful billet impeller's composition means that it will not bend or break even under super high boost/flow conditions. The billet impeller also weighs approximately 25% less than cast impellers, which serves to reduce transmission load and noise. Additionally, all surfaces are machined to further improve adiabatic efficiency, the impeller is better dynamically balanced, and has no internal casting cracks or flaws.
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