I was sure this topic was discussed before, but couldn't turn it up in a search.
Anyone who's ever worked on a head for the first time may have been surprised at the amount of pressure valve springs exert on the cam. Try turning a lubricated cam in a head - there's an unexpected amount of resistance!
The idea of going with valve springs with less resistance was suggested - as something that would likely return a very small improvement in efficiency.
I happened across a reference that suggests using lighter springs is actually a tactic automakers have used to reduce fuel consumption:
Quote:
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Pulling the engine from the Altima meant adopting some of that car's bias toward fuel economy, smoothness and torque. That's the excuse for a rev limiter that smacks you in the face on every shift. Super-light valve springs designed to get that last fraction of a mile from every gallon of gas mean no more revs until you buy 16 more springs and an ECU.
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Source is a discussion of putting an Altima engine in a Sentra (and then building/tuning it for more power).
2002 Nissan Sentra SE-R Spec-V - Sport Compact Car Magazine
Not that this is particularly useful info. How are you going to get lighter valve springs? Or decide "how light is too light?"