Different batches of springs often will vary in their actual pressure. Never trust what a catalog states. We always test new springs before use on a Rimac machine at the actual verified installed height with the retainer in use.
Regarding a loss spring pressure with use. Personally I've seen as much as 14 pounds loss on the seat after a dyno session. And not long ago I mentioned a build where the valve springs were set up for a hyd flat tappet application with an initial setting of 130 lbs on the seat, knowing they would lose pressure with run in. The actual pressure after a day on the dyno was 122 pounds.
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'70 TA / 505 cid / same engine but revised ( previous best 10.63 at 127.05 )
Old information here:
http://www.hotrod.com/articles/0712p...tiac-trans-am/
Sponsor of the world's fastest Pontiac powered Ford Fairmont (engine)
5.14 at 140 mph (1/8 mile) , true 10.5 tire, stock type suspension
https://www.youtube.com/watch?v=qDoJnIP3HgE
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