I noticed a car like a SRT4 or audi has a average PSI for the turbo of about 8-15 PSI and a truck like a F-250 can have up to 30 PSI safley. Why is this?
The reason is that since Diesel engines have to produce a high compression ratio to ignite the oil, higher CR means more torque output to "pull" the cargo often loaded onto turbocharged trucks. About petrol engines, they need turbochargers because the low power in the naturally-aspirated version of the engine is not enough to be multiplied for power increase by means of a supercharger. Hence, a turbocharger is used in such cases.