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The drive for better consumption is documented in the factory literature and prominent in the advertisement of the period. The changes were primarily driven by the requirement that all cars run on unleaded, lower octane fuel. Higher octane unleaded fuel was not available. Once you understand the timeline, the changes in the factory's approach is obvious.
Output is measured at WOT, vacuum advance is irrelevent. Vacuum advance is a device to improve part throttle cruise efficiency. The US 911SC has both vacuum advance and retard. The retard is strickly an emissions device, uses manifold vacuum, and is only really active at idle. Most SC cold start better and run cooler at idle with it disconnected and the idle speed readjusted.
I think the best way to answer the OP question is, once the fuel spec drop occured, the 70's and 80's models were detuned timing wise. If you have higher octane fuel, these models respond to advanced timing. Once the emission strategy advanced to the feedback converter, the factory increased compression to improve both output and efficiency. Even when the fuel spec rose, they chose to keep raising compression rather than timing to post better consumption figures.
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Paul
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