Normal air pressure at sea level is 14.7 psi and 12.2 psi at 5000 ft. The turbo can't spin faster to make additional boost unless remapped or running a boost controller, the waste gate is going to open to prevent going above the OEM setting there is no way around that other than modifications. However the turbo may spool faster depending upon the intake, turbo and exhaust design but that won't make up for the losses cased by elevation, but they are much better than NA at altitude and were originally invented to increase propeller aircraft performance and overcome NA engine limitations at altitude.
With a modern ECU controlled turbo, wastegate and recirculating blow-off valve the engine will likely be into boost easier (quicker spool), at higher levels with less throttle input and for longer when at altitude for any given throttle input but it will still make less power and consume more fuel regardless than when compared to sea level. Will the driver notice, 99% probably not, and in colder months due to increased air densities it would be even hard to decern. A ECU turbo has no need to be tuned to 5000' or any other elevation the IAT/MAF/MAP take care of that.
Exhaust gas drives the turbo, input air pressure is irrelevant in terms of through put flow as the system will only suck in what it can by design, the turbo will only spin as fast as the exhaust gas can exit the engine and exhaust. Due to the lower density (ie pressure) of the air, there are losses, no way around that. The back pressure on a turbo climbs the higher the boost goes and ambient pressure has little effect on it in a turbo, the same is true for NA minus the boost, more flow increases the backflow, best combat to it is maintaining a high enough EGT and free flowing exhaust to aid in scavenging (less air pressure typically means less heat and thus less scavenging ability) a good exhaust design will also allow the turbo to spool faster but will have no effect on max boost by itself.
Also when at altitude and running high boost the turbo is more prone to blowing oil through the seals into the intake tract as the pressure differential climbs across the turbo due to less ambient air pressure on the compressor side and the increase in back pressure at high boost on the turbine side. Over time this will destroy the seals and when one lets go immediate and catastrophic turbo failure is a guarantee and at that point you better pray the intercooler catches the compressor wheel debris and the uncontrolled oil flow towards the cylinders top end it doesn't hydrolock the engine.
With all that said, I wouldn't worry about any of this on an unmodified OEM designed system.
For this platform this equates to ~9.4% loss @ 5000' given same ambient temps which is about 30Hp/44 LbFt, which I wouldn't call insignificant many pay 1k or more in hopes to achieve a gain with those numbers.
With a modern ECU controlled turbo, wastegate and recirculating blow-off valve the engine will likely be into boost easier (quicker spool), at higher levels with less throttle input and for longer when at altitude for any given throttle input but it will still make less power and consume more fuel regardless than when compared to sea level. Will the driver notice, 99% probably not, and in colder months due to increased air densities it would be even hard to decern. A ECU turbo has no need to be tuned to 5000' or any other elevation the IAT/MAF/MAP take care of that.
Exhaust gas drives the turbo, input air pressure is irrelevant in terms of through put flow as the system will only suck in what it can by design, the turbo will only spin as fast as the exhaust gas can exit the engine and exhaust. Due to the lower density (ie pressure) of the air, there are losses, no way around that. The back pressure on a turbo climbs the higher the boost goes and ambient pressure has little effect on it in a turbo, the same is true for NA minus the boost, more flow increases the backflow, best combat to it is maintaining a high enough EGT and free flowing exhaust to aid in scavenging (less air pressure typically means less heat and thus less scavenging ability) a good exhaust design will also allow the turbo to spool faster but will have no effect on max boost by itself.
Also when at altitude and running high boost the turbo is more prone to blowing oil through the seals into the intake tract as the pressure differential climbs across the turbo due to less ambient air pressure on the compressor side and the increase in back pressure at high boost on the turbine side. Over time this will destroy the seals and when one lets go immediate and catastrophic turbo failure is a guarantee and at that point you better pray the intercooler catches the compressor wheel debris and the uncontrolled oil flow towards the cylinders top end it doesn't hydrolock the engine.
With all that said, I wouldn't worry about any of this on an unmodified OEM designed system.
For this platform this equates to ~9.4% loss @ 5000' given same ambient temps which is about 30Hp/44 LbFt, which I wouldn't call insignificant many pay 1k or more in hopes to achieve a gain with those numbers.
