And at 18psi, we have seen 500whp as predicted. With RB/ASRs at 16psi, we really do see ~460-470whp. And the numbers are right in line with what we've seen on the dyno. At 14psi of boost, an FBO stock turbo car running full advance with low IATs would probably make 390whp, which is nearly 50whp LESS than the ideal theoretical power output of 437whp! So no surprise that upgrade twins (RB/ASR/TD) make a big difference and are far more efficient at higher boost pressures. One thing we can easily see is how quickly the stock turbos lose efficiency running boost much past stock levels. If any particular car (single turbo or twin) isn't making the projected power levels, there is a system inefficiency that is limiting power. So the above numbers tell us the "ideal" WHP we would expect to see at different boost pressures. So looking at different boost/absolute pressures, we can calculate the following:īoost_Abs Press_Theoretical WHP 15.23 HP/Abs Pressure (psi) 300hp/19.7 = 15.23hp per 1psi of absolute pressure. Which means that absolute pressure at the intake manifold is 19.7psi. 5 psi of boost means 5psi on top of atmospheric pressure (14.7psi). So in this case of the n54, we've seen most FBO cars, in good conditions, make right around 300whp at 5psi of boost (stock tune/valet mode). Efficiency that we would want to emulate at high boost pressures with a single turbo. This is especially important because we know that the entire turbo system operates at very good efficiency at this boost level. The FBO part is important since it means that their are no inlet and exhaust restrictions to speak of. To start, we need to look at what an FBO car running stock boost (~5psi) makes on the dyno running full advance (no knock retard). But clearly, this is just a basic calculator and ignores subtle loss components. It also assumes that there aren't any other power-dependant losses (tire distortion, driveline friction, etc.) And for the most part, there isn't. remain constant regardless of boost pressure. We must also assume that things like inlet temp, exhaust back pressure, etc. And that power output is directly proportional to airflow rate which is proportional to inlet pressure. This very simple calculated assumes that the engine is nothing more than a pump. If not, you know there is a problem and that something is limiting power (i.e., restrictive cylinder head design, direct injection shortcomings, exhaust restriction, inlet restriction, etc.) Making these calculations is useful to determine if you are making the power that you should theoretically be making. There is a very simple basic way to calculate this. Variation to ISA \(\left(\Delta \mathrm_c\right)\)There is some talk about expected power at different boost pressures. Design conditions Flight Mach number \(\left(M_\infty\right)\) Where \(\nu(M)\) is the Prandtl-Meyer function evaluated at a position with Mach number \(M\)ĭescription of what the calculator does goes here. The net turning may be calculated as follows: The left and right ends of the expansion fan are defined by the angle of Mach waves in the upstream and downstream region, respectively. The expansion fan (Prandtl-Meyer expansion) generated at the expansion corner in the figure below will gradually change the flow direction and the end result is a net turning of the flow such that it follows the wall downstream of the corner. In the figure below, the sharp expansion corner (left figure) will lead to the generation of a centered simple wave or expansion fan and the gradual expansion (right figure) will generate a simple wave (that might be centered depending on the shape of the wall) Thus, the integrated effect over the expansion region is also isentropic. Each Mach wave contributes with an infinitesimal change in flow direction and flow properties without introduction of losses i.e., the process is isentropic. The expansion region constitutes a collection of an infinite number of Mach waves. An expansion region or expansion fan is generated in a supersonic flow when the flow direction needs to change such that the flow area increases - the flow turns out from itself.
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