| :VE and peak achievable boost are not related at all. I don't agree with this at all. Boost doesn't necessarily mean power. You are right; boost does not necessarily mean more power. I didn't imply that it did, although, with proper intercooling, you will make more power from a given setup as you increase the boost level. The effectiveness of properly sized intercoolers will vastly supercede the drop in efficiency of the compressor wheel as you go above its highest-efficiency operating range. But, this isn't the topic at hand.. :) In order to generate pressure, you will have to have some degree of restriction on the compressor, yes, however and more importantly, boost is also a function of how quickly you can spin the compressor wheel with respect to the amount of restriction on the compressor. If you lack the shaft rotation speed to bring the compressor into these higher pressure ratios, you just aren't going to make more boost. A higher VE engine will move more air, which means it has a higher massflow requirement at a given RPM as compared to a lesser VE engine, but the variations in VE do not affect the compressor's operation in terms of what it is capable of. An example: I know you are aware of all the work and upgrades I have done on my personal Z - upgrades that are having a positive impact on the volumetric efficiency of the engine. All of these upgrades combined probably put my engine at the highest VE of Dee's and Seb's car. Please consider that fact and then try to explain why I can produce over 30psi out of the same compressor that Dee and Seb are using in their cars while they have not been able to produce this kind of boost level. What is happening with Dee and Seb's setup is very simple: the compressor is not being accelerated quickly enough to allow it to reach the compressor's maximum pressure ratio before the airflow requirement/engine RPM has already passed beyond the region of the compressor's capabilities to produce that kind of pressure. The performance of the compressor, in terms of what its maximum pressure and airflow will be at a given engine RPM is completely dependent on the hot section of the turbocharger's design. The .86A/R housing is simply too big and it is allowing too much of the exhaust pressure to escape through the turbine scroll rather than being directed into the nozzle of the housing. This is why Seb cannot achieve higher boost than he is getting - it has nothing to do with the VE of his engine. This larger housing is preventing the turbine from absorbing enough energy to gain sufficient shaft speed to drive the compressor into its upper operating range. The .64 housing is better, but it too does not appear to be capable of generating sufficient shaft speed to allow the compressor to get there either, although it is higher than what the .86 will achieve. The setup I have on my personal Z with the same compressor/housing as Dee and Seb, is using a slightly smaller A/R (.63 vs. .64) but I am also using a turbine wheel with one additional blade on it. That means one more blade is passing by the nozzle per revolution to absorb that energy, and given the spoolup characteristics and power output of my setup as compared to Dee's and Seb's car, you can see exactly how the differences in the hot section of the turbo are what affect the peak total boost, NOT the VE of the engine.
You can also look at the torque curves and the peak torque values and see this as well: Seb's car makes less peak torque than Dee, and they both make less torque than I. This is because the compressor simply isn't spinning quick enough in Seb's or Dee's car to achieve the same level of boost as I am. Now, I see exactly what you are saying about how differences in VE will affect where you fall on that compressor map at different RPM's and PRs, but I think you have an exaggerated understanding of how much of a difference the changes in VE that are attainable will affect where you fall on the map. These differences in VE aren't large percentages. Given the information about the differences in the turbine housings between these three cars I Am referring to, and keeping in mind that there are several other differences in those setups, it is pretty clear that this one single component of the entire system is what is responsible for the differences in spoolup characteristics as well as the peak boost we have been able to achieve.
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