Exactly.
“400lb/in”
Not the same thing as “the recommended spring pressure is 400lb”
400lb/in is the spring “rate”
The cam is a fast rate of lift cam, and they are recommending a fairly high rate spring(for a .500 lift hyd cam) to be used with it to help maintain control of the valvetrain.
The spring rate is the amount of change over the specified range.
The recommended spring will show a load change of 400lbs in 1 inch of travel. 400lbs per inch.
