| gives a lot of insight into what's happening with this particular turbo and perhaps I should have thrown in an additional comparison to elaborate. Dee Ridgeway's TT has a very similar setup to what I am running, however, he has the native GT2860RS with the .64 A/R turbine housing and the 9-blade turbine wheel. His car produces more boost pressure than what you can get out of that turbocharger with the .86 A/R housing - we were seeing around 28psi of boost on his setup. The same thing applies in the case of .64 housing as with the .86 A/R housing, in terms of how the throat area of the housing affects the pressure drop you will get through the turbine, and ultimately, how much energy you can provide to the compressor. Even the .64 A/R housing is too "loose" to provide sufficient pressure drop through the turbine to power the compressor up to its peak operating range, but it does generate a notable increase in compressor pressure over the .86 housing as well as better spoolup response. I will agree that my manifolds likely flow a little better than the inconels but the difference they introduce really only starts to "kick in" out in the last couple thousand RPM of rev the engine is turning. Even then, the turbine housing and wheel I am using is generating significantly more of the "restriction" to flow than the manifolds are. And this is where what you are referring to begins to step in as a consideration (reversion). The point where the exhaust manifold/housing/turbine produce so much backpressure that the reversion counteracts the increase in boost pressure would be noted by a marginal or no increase in power as the boost is turned up along with EGTs increasing substantially. Additionally, this is where the requirement for stronger wastegate springs and more preloading kick in. In the case of the GT28R turbo, that 56-trim turbine wheel is quite small and creates a good bit of backpressure in the manifold/housing that must be overcome by using stiff wastegates with good preloading. But the compressor in that turbocharger will only make ~23psi of boost so on our engines, although it is at that upper limit, is still within bounds and those cars will make more and more power up to the limit of the compressor given ample intercooling. If the turbine section were any tighter in that turbo, you likely would not be able to hold the wastegate shut short of welding it closed.
While I would agree that an efficient IC system can help alleviate the effects of decreasing compressor efficiency as the boost is raised, exhaust back pressure will rise at an increasing rate the further the compressor is pushed outside its efficiency range. If this gets out of hand, I would think that reversion would begin to negate any gains from the increased boost no matter how efficient the IC is.
Sure, you can push stockers to 18 psi, but exhaust back pressure goes through the roof because more power is required to heat the dicharge air from the increasingly inefficient compressor. Thoughts?
Your example is an extreme of conditions and would only be found in cases where there was gross mis-matching of the compressor and turbine wheel. These three cases are more in the other direction of this condition where a turbine section is not producing too much backpressure, but rather, too little and this negatively impacts compressor performance. My setup may be a little on that restrictive side actually - there appears to be a slightly "overabundance" of exhaust energy into my turbine that can be adjusted for to reduce some of the backpressure at higher RPMs and still produce the required shaft power to bring the compressor up to its peak operating condition. My plan in the near future is to have the turbine exducer blades clipped to increase the massflow of the turbine wheel. The trick here is getting the clip just right such that it increases the turbine's throughput without compromising its ability to power the compressor to its peak. Since I have a little overabundance of generated shaft power, I can give a little of that up to improve flow without compromising the peak boost the compressor can make. This ultimately will allow the system to breathe better in the higher RPMs without compromising the low and midrange spoolup/power/torque. Fortunately, most people are using tried and tested turbo designs so it will be uncommon to find a setup where exhaust gas reversion becomes a significant problem. In the case of the stock turbos, the backpressure is likely pretty high due to the low flowrate of that T02 turbine. However, it is only having to drive a compressor that can produce ~20psi of pressure at a max of 20lbs/min. There isn't a whole lot of air having to get through that little turbine (as compared to the 35lbs/min of the GT2860RS) so it flows enough to prevent reversion from totally running rampant, but just barely. Same as the GT28R, it will require preloading the actuator a little more (~10lbs base pressure) to push the compressor to 20lbs, but the setup will make more power at 20psi with proper intercooling vs. 18psi on the same setup. A lot of the "black magic" in the turbocharger industry (namely in the performance aftermarket) is getting the fine balance of parts within the turbo for a given engine setup to produce particular response characteristics. A lot of time is spent in the proper development of a turbocharger to get things just right and most who venture down this path tend to keep their secrets secret because of the time and effort involved. In the case of the GT2860RS on the Z32, the turbo was introduced by JWT with the .86A/R turbine housing just about the instant that turbocharger was available on the market. There was very little to no information available about using the .86 A/R housing on the Z32 at the time of its release and many have come to find that it simply underperforms when compared to the .64 A/R housing with respect to spoolup response and torque production. Further still, hybridizing that turbo and using a 10-blade turbine has shown to further improve spoolup and torque production without sacrificing peak power. Taking it one more step to clip the turbine in that 10-blade hybrid is very likely going to show further improvement in power production in the higher revs without hurting the low and midrange torque production or spoolup response. Right now, my turbine configuration is very close to optimizing the performance of the compressor in my setup; being slightly compromised by a degree of reversion occuring in the high RPMs. 
You can see in these dynoruns of my car that the red lines (~18psi of boost) after 6500RPM, the power flattens out. In the runs following at higher boost, you can see the power falling off at this RPM and falling off as early as 6000RPM in the peak boost run. Exhaust gas pressure IS the cause here but this only happens after 6000RPM at 31psi of boost. Clipping the turbine back a few degrees is going to improve this condition and it would be nice to see the power climb to at least 6500RPM before falling off. But care will need to be taken as I am more interested in the torque production and response through the bottom and midrange. There is some room for improvement on the top end without hurting the bottom and midrange as there is a slight overabundance of energy available to the compressor.
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