Boost creep is typically a slower increase of boost pressures due to the current wastegate system unable to control the exhaust gas flow.  The stock turbo and wastegate system was designed to work with all stock components.  Change any one of these components, and the end effect will change.  Almost always, if you increase the efficiency of the flow of the air intake and / or the exhaust gas, you will change the boost pressure of the wastegate and eventually the turbo.

The wastegate is a mechanical device that has no idea what the absolute pressure the turbo is making at any one time.  All it knows is that if the pressure generated by the turbo compressor section is enough to compress the spring in the wastegate actuator, the actuator will start to move the flapper to bypass exhaust gases in the turbo turbine section; the wastegate, in this case, is an "internal" type which is integrated into the turbo turbine housing.  If the exhaust gases flow more than "stock", then the wastegate cannot effectively bypass the increase exhaust gas flow, and boost pressure increases.  At a certain point, the exhaust gas flow increase so much that the wastegate becomes ineffective to hold any stable boost level, and boost will rise uncontrollably.  Luckily, unlike a sudden wastegate failure, boost creep will usually raise boost pressures at a slower rate.

To combat boost creep, we need to make the wastegate system more effective.  In practical terms, we need to enlarge the wastegate orifice and passages.  Luckily, we can port the wastegate to handle mild cases of boost creep.  By enlarging the existing wastegate orifice and porting the wastegate passages for more effective exhaust gas flow, we allow the wastegate to work more effectively, minimizing boost creep itself.  In extreme cases, an external wastegate can be retrofitted for ultimate boost control over the stock wastaget system.  An external wastegate retrofit is costly and not really recommended with the stock turbo (turbine housing), as a custom or aftermarket turbo exhaust manifold would be more cost effective (with a full turbo upgrade).
FC3S Pro: How-To's, Zenki FC3S Turbo Wastegate Porting
FC3S Pro: How-To's, Kouki FC3S Turbo Wastegate Porting

Ensure any boost controllers are working properly!

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