[naturalextraction]

Thanks for the info links to the Neptune info CarlosSW2. pgfpro-in regard to idle: your turbo is helping at higher rpm related to pressures even at ambient although you may not read any psig. It is still helping to push air. It is also helping the fuel to burn when the natural vacume of the engine otherwise would not sufficient enough to allow for lean mixture to burn. Thus why at idle it suffers.
This bringing me to air temperature and potential damages. (I've been studying and experimenting related to different fuel distribution methods and management for the ICE for over 4 years now. I in list the help of some of my physicist and mathematical friends from Sandia labs when the information or findings exceed my temporary limitations. I have access for testing at the labs Los Alamos or Sandia and use of typical dynometers rear and engine.) Lean burn will not hurt your pistons from the standpoint there is not much to burn. This in keeping the combustion chambers within an "ideal" temperature range. What damages them under a lean burn condition is fuel droplet sizes, particularly when the chamber is hotter and this is where timing is a factor as well as the fuels ability to burn, and flame front travel. I've run two engines at the 18:1 to 21:1 just fine and maintain some levels of usable power in both a motorcycle and car. (car being a 1972 Duster 318). After 36K miles the engine was torn down to find no burnt valves, very clean piston tops all while running the engine mostly on low grade unleaded. (Stretched head bolts were the only culprit)
While droplet size from modern day injectors are quite small, in a hot chamber they still can create pinging or complete detonation damaging pistons or valves. Thus why engineers developed DPI for better fuel distribution with higher fuel pressures. Other factors that create anomalies from injected engines would be pump pressure consistencies due to possible voltage drops at pump, restrictions, fuel density changes due to ambient temperatures, to name a few. Your management system is constantly monitoring and trying to make changes quickly but can not compensate as quickly or as well under leaner conditions. Leaner conditions basically require changes to the fuel itself, delivery, air temperature controls and more accurate consistencies in all areas to the induction and fuel delivery systems. Delivery and firing timing becomes much more critical. Without your turbo you would expect to have the same issues you experience at idle but at higher rpms without other control devices. I personally do not recommend inter coolers in relation to obtaining economy in the ICE. Their good for power but not for economy. That gets into a whole other discussion regarding adiabatic/thermal issues and other control issues. Keep in mind the basic efficiencies of most ICE's to be at ->30% anyway. Most of it is lost to heat energy. This being why I like the volumetric efficiency gains turbos make while using expended heat energy. Thus bringing up the overall efficiency factors of the ICE, although not by a big margin.
Also keep in mind cylinder chamber temperatures are much lower due to the fact you are NOT burning the same volume of fuel. Note your exhaust temperatures under those conditions. But the fuel cannot change its burn rate under those conditions. It's the fuel that creates the problems primarily, keeping in mind the other issues stated above.