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HKS actuators use a progressive spring, the initial spring rate is relatively weak. This is based off the results another member posted about, I did not verify this myself, but I see no reason he lied about the result. IIRC he basically tied the HKS actuator to a stock JWT actuator and tried to pull them apart, the JWT actuator would pull the HKS actuator upto a point, then the HKS actuator started pulling on the JWT actuator. The Forge actuators are a simple linear spring, which the user can swap with different spring rates. The HKS body is sealed so the internal spring cannot be changed. With a progressive spring the HKS actuator should provide a greater range of boost control. However the soft initial spring rate could cause the wastegate to leak partially when running high boost, slowing down boost response. A stiffer linear spring offers less range of boost control, but should give a better response when running high boost. When I first used the HKS actuators, boost response was as follows: boost would ramp up to 7psi relatively quickly, then build slowly from 7psi to whatever is the final boost. This is consistent with the progressive spring theory. When I switched to Forge actuators I no longer saw this step in the boost response. It seems that the HKS actuators only work well when they are pre-loaded sufficiently that it is working in the stiffer spring rate region. IMHO that is not ideal, since that would eliminate a lot of the actuator travel.
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