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78 280z, just completed rebuild (flat top pistons and MSA Stage III cam, ported intake, shaved head).
I know the stock timing is 8 deg BTC, what should the timing be at with the Stage III cam?

Stock timing is 36 degrees of total advance. What you meant to say was 'idle timing' was 8BTDC, and that's with vacuum advance disconnected. When you connect vacuum advance idle timing STOCK is closer to 15 BTDC, but TOTAL timing remains unchanged.

And since you aren't running Methanol, total timing for your engine with higher compression and larger cam will not change, providing you run sufficent anti-knock compounding in the fuel. If you don't, or buy low octane fuel as a habit, you likely will have to retard the timing of the ignition events, and/or have a custom recurved distributor set up to match your fuel/engine combination.

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Tony D: "Knowledgeable but Caustic"... rationull
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Thanks Tony.

I would take the distributor apart and reduce the mechanical advance slot lenght with a little welding.

A cam that big would like at least 20 degrees of initial timing. 25 to 30 would be better. Then 36 total mechanical seems to be a good number. Then use the vacuum advance on the stock location (ported source) if you use 25 to 30 initial.

The late intake valve closing point of the stage 3 cam should reduce cylinder pressure enough with 10:1 cr to prevent detonation with full timing using 93 octane. But pay close attention to sign of detonation. pinging and/or power falling off.

What fuel system are you using? if you are using the stock efi you may need to make adjustments to the stock efi (afm, fuel pressure, tps) to compensate for the high performance cam in the engine. Would be a good idea to install an O2 sensor bung in the exhaust system for tuning. I feel carbs would be better with that cam.