Well m not engineer but i have been building drag cars since 2002 so i know how the turbocharged car work, even the unichip tuner guy told me the same thing if the same cars which has been tuned to Khi sea level it wont perform same in high altitude as he has seen many cars struggling in high altitude due to different tune map. I have Audi A3 1.8tfsi which is tuned as well with bolt-on parts, i have dynoed here in KHi....if i dyno in Isb the power wont be same it will be little in power different due to high altitude . A friend of mine showed me his car drag timing was 13.2sec 1/4mile in Durban, SA but in Johannesburg, SA his car timing was 13.6-13.7sec btw he got A3 1.8tfsi tuned same as my A3 1.8T but with different setups. So you see high altitude reduced the performance of the car due to less dense air mean less oxygen, here's why: If a turbo is rated to deliver 5 PSI of boost, that means it will deliver 5 PSI above the atmospheric pressure. At sea level that is around 14.7 PSI. At high altitude, the same turbo may deliver 5 PSI of boost, but relative to the lower atmospheric pressure of say 12.3 PSI . This means that the same system is delivering less air to the engine because overall there is still a difference of 2.4 PSI for the system (decrease in density of air). Similarly weather/climate also affect the power, in winter my car was making more power as compare to summer as warm/hot air is also bad for turbocharged car or any foreced induction cars.
