I'm putting in an LT4 HOT (.492 lift) cam in my aluminum head L98. I know I need new springs but I'm totally clueless as to what seat pressure means. Can two different springs that are rated for the same lift have different seat pressures? Also, how do I know if I need to shim the springs?

 

First, you should always use shims, or cups of some sort, since they are aluminum heads. Seat pressure is how much force the spring is exerting when the valve is closed. The manufacturer should have given you some specs for installed height, and thats what you want to have your valvetrain set at when you are done. The installed height is the distance from the spring seat to the bottom of the retainer, with the valve fully closed. This usually varies across the head due to machining tolerances and wear of existing parts. If you are really **** like me, you will check every single spring and shim appropriately to get the right installed height and same seat pressure on every valve in the head. That will take tools that you more than likely do not have though.