Ti AL Sport springs are engineered for precise control under extreme conditions. Manufactured from 17-7PH stainless steel, they provide consistent pressure at high temperature, and resist 'relaxing' at temperatures up to 900deg F (483deg C), well beyond the required temperatures of the actuator. Springs are color-coded, and are sold individually.  Please see the specific diagram for your specific model, to correlate color coding and diameter in order to choose the correct item. Q and QR springs are not chosen by positive (boost) manifold pressure rating, rather, should be chosen based on negative (vacuum) manifold pressure at engine idle.