This is the standard spring that is supplied in all Respons TMS and Deceptor Pro II valves, and is an optional extra for all Mach 2 valves when being set up for atmosphere venting. Use this spring where the valve is set up for atmosphere venting and engine idle vacuum is greater than 19Hg. For full recirc venting and/or engine idle vacuum less than 19Hg use the softer spring 6116.