Turbos at high altitude

[Chevy2g3]

I was told there was an article on it but I've had no luck finding it. Here's my issue: yesterday I went out to try to tune my car at 40+ psi. I have about 36 psi worth of springs in my wastegate plus the boost controller. Basically I was only able to boost my holset hx40 7 blade to about 38ish psi and would not go any higher. I was told altitude affects max boost of a turbo. Does anyone happen to know anything about this kind of thing?

 

[knochgoon24]

If you look at a compressor map, it has pressure ratios. 40psi(g) at sea level would be a pressure ratio of 3.7:1. But at 5,500 ft, the pressure ratio is instead 4.3:1. This pushes you even further out of the efficiency island. And when you look at the lower axis, you'll also usually see that the airflow is measurement has density involved.

Larger version: http://www.vortechsuperchargers.com/maps/YSi-Trim_Compressor_Map.jpg

This one has an equation to calculate corrected airflow based on the barometric pressure (BP), aka the pre-turbo pressure, and the intake air temperature, pre-turbo (Ti). The map isn't for an HX40, but if you run some numbers, you can get some approximate conversions.

And this link has some good info on reading Holset compressor maps.
http://www.dsmtuners.com/forums/turbo-system-tech/362599-how-read-hx35-hx40-compressor-map-s.html