Introduced 7/15/12, these Hi-Tension valve springs for the box stock class are designed to PASS ALL 2013 TECH SPECS and provide significantly more nose pressure for extended periods on engines turning 6300+ RPM as compared to ALL other springs that pass tech. They are made from a special wire that does a far better job of resisting spring tension loss when the springs are consistently compressed to solid height (aka excessive valve float). If you are running a blue plate or an unrestricted box stock class engine, these springs set the standard for "legal" performance box stock springs. Sold as a pair. As with any aftermarket part with so many rule specs, it is very difficult to keep within specs, and achieve maximum performance. These springs are produced by the thousands, and although we have never found one to be out of spec. It is a good idea to check them anyway. Manipulation of these springs to increase their tension MAY be a bad idea. Their tendency to maintain tension after prolonged use means that you should not depend on heavy use during a race to weaken the spring in hopes that it will lose enough tension to pass tech.