| give me a chuckle. The 62-trim 60mm compressor wheel used in the GT2860RS is capable of a maximum pressure ratio of ~3.15 or so, which is about 31lbs of pressure. If you run the turbo up to this pressure ratio, it will not be operating in its peak efficiency as you note. However, what appears to be overlooked so often is the fact that our vehicles are equipped with intercoolers, which in the most simplistic explanation, add efficiency back to the system. When talking about turbocharger compressor efficiency, what is being referred to is the compressor's ability to compress the air with a given amount of temperature rise in the process. Inherently, compressing air has a byproduct of increasing its temperature. Even at 100% efficiency, when you raise the pressure of a gas, its temperature will also rise proportional to the increase in pressure. However, since a centrifigal compressor does not operate at 100% efficiency, there is additional heat introduced in the compression process above the adiabatic rise in temperature associated with compressing a gas. As an example: Lets say you have a hypothetical compressor that works at 100% efficiency and we run it at a pressure ratio of 3.15. Efficiency: 100%
PR: 3.15 (31.61psi)
Ambient temp in: 70F (31.61C)
Discharge Temp:: 275.62F (135.35C)
Work Performed: 67HP Now that same compressor operating at 65% efficiency at the same pressure ratio: Efficiency: 65%
PR: 3.15
Ambient Temp: 70F
Discharge Temperature: 386.34F (196.86C)
Work Performed: 103.07HP You will note that the compressor operating at a more realistic 65% efficiency at this pressure ratio is adding an additional 110.72F temperature rise to the discharge gases. That is an appreciable rise in temperature and you certainly could not push air into the engine at 386F and expect it to run without detonation, however, properly sized intercoolers will vastly reduce this temperature prior to entering the engine and the increase in pressure (vs running a lower boost level) is going to produce more power, hands down, as you are moving more air mass through the engine. Good case and point would be my TTZ. I am using the same compressor section as in the Sport700/GT675RS but my turbine section is capable of producing the required amount of work to drive the compressor up to its maximum pressure ratio, not to mention, driving them to higher pressure ratios much quicker than the other two turbos mentioned. Additionally, the intercoolers (MASSIVE SMICS) are capable of bringing the discharge temperature back down to reasonable levels so that detonation does not occur. I put together a turbine calculator spreadsheet some time ago for use with calculating the thermodynamics involved in small gas turbine engines. Some of the information above was produced from that spreadsheet. The power requred to drive the compressor wheel is something of great importance when designing a jet engine and I noted it to make a second point about the .86 A/R housing on this turbo. In short, the throat area in the .86 A/R housing is too large to create a sufficient pressure drop through the turbine to extract the required power to drive the compressor wheel above a pressure ratio of about 2.75 (25.73psi) - 82.68HP is required to drive this compressor at that pressure ratio with a 70% efficiency. Required pressure ratio across the turbine is 2.01 and the square root of that is 1.42, which the turbine housing has to generate. You are only going to get a maximum of about 80C drop through the turbine with an exit velocity of 1403ft/s and 31.40cubic feet/lb density out of the housing. Regardless of what you do with the wastegate actuator spring, the turbine will NOT be able to extract enough energy from the exhaust gas to drive the compressor up much higher than about 24-25psi. And these calculations are based on zero backpressure after the turbine, which is almost never the case. Back to the efficiency: my car made 604RWHP at 24psi of boost pressure on 93-octane pumpfuel without detonation on two runs. This would not be possible without intercoolers, and likely not even possible with anything other than the MASSIVEs. It made 693RWHP at 31psi of boost pressure on racefuel. What I am trying to drive home here is that although you may not be running the turbocharger within its peak efficiency condition, with proper intercooling, you can literally run the compressor to its limits and produce more power than you would if running it at lower boost levels. One way to tell if your intercoolers are up to the task you are demanding of them is to watch what the power does as you slowly notch up the boost: if you get to a point where you make the same power or less as you turn up the boost, you are reaching the limits of your intercooling, NOT the turbocharger. So long as you keep the charge temps cool, any increase in manifold pressure is going to produce more power and torque. To drive this point home even more, take a look at a stock turbo compressor map: 
The peak efficiency ends at 1.9 pressure ratio, which is 13.05psi of boost. We all know that a ZTT with 555cc injectors and intercoolers can run 18psi of boost and it will make far more power than running the stockers at 13.05psi - even IF you were to jack the timing and lean out the mix at 13.05psi you would never achieve this kind of power at such low boost. So, there's a bit more to it than just the turbocharger's efficiency. ;-)
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