Observing the intermediate results of an in progress tune of an SRM RS6/K24 equipped S4 it was clear the car was developing boost much sooner than the typical S4 with these turbochargers.
In the chart below the lines are of the subject car and the colored dots are data from other S4’s. It’s clear that the subject S4 is generating boost on average about 250 rpm sooner than past averages.
During discussion to try to discover what differences this car may have that could led to such a result, one possibility proposed linked the outcome to a higher than normal preload on the turbocharger wastegates. The normal preload setting for these turbochargers is in the 14-16 psi range, in the case of the subject car the setting is 18 psi.
If this cause is correct it has some noteworthy ramifications. To try and determine if raising preload more generally has a similar effect I looked to some data I collected with my S4 showing the boost onset with FrankenTurbo F4H turbochargers before and after the addition of helper springs to the BorgWarner K03 wastegates that I was using.
The solid lines of the chart above are for the unaided K03 wastegates and the dashed lines are with high spring rate helper springs.
It appears that the addition of the springs slightly improved the boost onset when boost levels reached double digits, but when accounting for the Intake Air Temperature, it is clear that the IAT’s were higher in the unaided case. Higher intake air temperatures result in reduced spool up, thus the slight difference in boost onset can be explained by the difference in IAT.
In the case of my S4 with the F4H turbochargers, increasing the wastegate preload did not appreciably change the rate of boost onset.