I've been playing with my ignition timing lately, trying to see how far I can advance it with 94 octane, and I noticed that the green/black check valve was oriented with the green side facing the ignition computer. I seem to remember that it should face the other way. I ran it that way, and ended up with some seriously hot oil and elevated coolant temps! So I broke out the timing light.

With all vacuum to the computer disconnected, I ran the engine up and down - timing seemed to change based solely on RPM, maxxing-out around 30* advance. I connected the valve with the green side facing away, and now the advance climbed rapidly off the timing scale, maxxing somewhere around 50*. Running the valve in the original orientation (green towards computer), the behavior seemed to mirror the no-vac connected case, maxxing out around 30* advance.

Can anyone out there confirm the OEM-spec orientation of this valve? And what exactly does this valve do? It seems to allow airflow in one direction, and not the other, but I don't see how this would really be desireable in this case. Is there a reason why I wouldn't want a direct vac. line from the computer to the manifold? And what should the max ignition advance be at higher RPM? I confess, I'm somewhat ignorant about ignition timing curves...
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Speed Racer, '83 240 R, '74 164 E, '93 940 OL1 (Manchester, CT)