Since a larger wastegate has the capacity to bypass more exhaust gas than smaller wastegates, in a given application, larger valves are actually better suited to less boost and lower HP applications. Saying a larger wastegate is necessary for more power is like saying cars with bigger brakes make more power, when in fact bigger brakes assist in making the car slower, not faster. The highest boost wastegate is no wastegate at all, with out a wastegate a turbocharger would produce its maximum boost capability at all times.
CASE STUDY
One example of wastegate sizing constraints is in our very own 80tq project. In various configurations we were forced to change wastegate sizes to achieve different boost control strategies.
Configuration: GT40R turbo with .95AR, stock 30mm Audi wastegate, free breathing 20v 5-cyl turbo. With this configuration, the lower boost limit was 18psi at 5500rpm, boost would quickly taper to over 30psi by 6500rpm on simply spring pressure with no outside control. The reason for this boost taper increase was that the small 30mm wastegate could not bypass enough exhaust gas to maintain springe pressure boost, causing boost to increase. Counter-intuitively, this very small wastegate was perfectly suited to running 30psi or higher at 600 crank HP and above. The only reason to put a larger wastegate in this application would be to lower boost. By fitting a Tial 44mm wastegate we could maintain a 20psi boost curve all the way to redline, a 60mm wastegate could have been fit to run lower boost curves in the 10psi range, or about 300 crank HP. With such a large turbo and a free flowing motor, large amounts of air go through the motor, thus large amounts must be bypassed to control boost. With a smaller turbo, like a GT3071R for example, the stock Audi wastegate is effective at holding 20psi to redline since overall airflow levels are so much less.
