An update.
I corresponded with Len Hoffman, owner of Hoffman Automotive Machine,
http://www.hamheads.com/ and he gave me some insight on what could be going on. Reposting with permission.
Quote:
In our experience with these heads when we've seen a broken spring it was always accompanied with a failed follower. We believe the follower fails first in this circumstance and takes out the spring.
IMO making a jump to dual springs would be an overreaction for a stock engine. We've installed countless new Genuine Porsche springs on track cars running sustained hi revs in Spec Boxster and Spec 996 and have never seen a spring fail on its own. Now, if they lose pressure (fade) they can contribute to a hydraulic lifter pumping up and the whole mess goes kaboom.
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He later clarified that springs get weaker both from age and/or use, and he would never put used springs into a Motorsport (including autocross) situation. It sounds to me like if your car is 20 years old, and sees track time, it probably needs new springs, but if you have a street car that doesn’t see extreme use you could maybe run old springs and get away with it.
So, I am going to skip the double springs and buy a set of new Porsche Genuine springs.
Regards,
Silber