From kratos:
The first thing that has to be considered is that the engine was designed in such a way that it's meant to be most efficient within a specific rpm range. This basically means that the valve timing and camshaft profiles are part of an equation that allows the engine to perform and breath it's best within that rpm range. Another thing to consider is that as the RPM increases, it gets harder to get the optimal amount of air and fuel into the cylinder and burn it at an optimal rate. The faster the engine revs, the less time there is to induce, compress, burn, and exduce. This in turn affects your ignition timing, which means you start the burn process earlier than what's ideal so the flame dies just before the exhaust valve opens. Unfortunately, there is no way to install an aftermarket camshaft on the intake side due to the valvetronic and currently there are no exhaust cam options. Another thing to consider is the flow capabilities of the turbo system. At our peak output of 1150whp our max boost duty cycle is 94% which produces 41psi. The turbos are able to maintain 41psi all the way to the rev limiter. We also measure the shaft speed of our beta systems with custom shaft speed sensors installed on the compressor covers. This tells us the shaft speed the turbos were designed to operate within. If we go above that shaft speed, the turbos will create more heat per pound of boost and be less efficient which is what we do not want. According to our data, we are close to the upper limit of shaft speed we would like to stay within which means the drop is not related to compressor. That leaves us with the turbine wheels which were designed to support just above 1200whp as a pair. Taking all this information into account we feel it's a combination of limitations between the two. Flow capabilities being reached sooner as a result of camshaft lift and duration limitations.
