what is the correct ignition timing for a 1976 280SE?

The manual I have is American and doesn't list a 1976 280SE with points distributor. The points distributor it shows has a vacuum advance and vacuum retard connection.
My car has a vacuum retard only. At idel, with the vacuum line connected, timing is set at TDC. If I disconnect the vacuum line, it goes to 10 deg BTDC. With vacuum connected, advance goes to 30 deg BTDC at 3000rpm. With vacuum disconnected, it goes to 40deg BTDC at 3000rpm.
So, my questions are: what should timing be set to at idle with and without the vacuum connected? Should the vacuum retard advance instead of increase it? It seems intuitively wrong. It means if I put my foot down suddenly, advance increases instead of decreases. I checked this with the timing light, and a sudden blip on the throttle increases advance by 10 deg!
Thanks,
Steve
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And so it should!
You're forgetting that when you drive the car and the engine is pulling at 3,000 rpm the throttle is open more than at 3,000 rpm in neutral, so it will then have lower manifold vacuum and a greater spark advance. Same as when the accelerator is "floored" - vacuum falls and spark advances in anticipation of higher engine speed.
OK, now how to set the distributor. I don't have the specs for your engine so generically I'd set it as far advanced as possible without hard (cranking) starts or pinging. Too much retarded will start instantly but limit performance and have poor economy.
My '79 280E was set to TDC with vacuum disconnected, if that helps.
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I've always had a difficult time wrapping my brain around the idea of how manifold vacuum works. It seems to me that at high RPM there'd be more suction through the engine's induction system, and therefore more vacuum through the manifold. But I gather that isn't the case. What am I missing?
Geoff
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Vacuum are at the max when engine speed is constant. Whenever there is a change in RPM as in acceleration, the vacuum drops.
Vacuum at max is most efficient stage of burinng gas... because the timing is advanced to the point where they have ample of 'time' to burn and yield maximum efficiency/power.
Vacuum at lower point will retard timing which gives gas short time to burn. This is necesary in order to accelerate because you want maximum acceleration performance... meaning you want less resistance to move piston faster and you want to ignite the gas when piston is at the top of the stroke to push the piston downward faster... as oppose to igniting the gas before the piston reach the top of the stroke.
This is a simple explanation of vacuum concept in normally aspirated gas engine. Turbo or supercharged engine does not create vacuum as you know... they are pumping air into the engine so no possibility of engine creating suction...
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At cruise or idle, the throttle is letting less air into the engine than on full throttle. This means that the engine is pumping the intake manifold out as hard as it can to breathe - very low pressure = higher vacuum.
Opening the throttle lets more air in, the pressure inside rises and the engine can breathe better - producing more power, which = lower vacuum.
Hope this helps. Don
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Crystal clear. Thanks!
Geoff
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The spec should be written on a placard in the engine bay above the headlight... long black strip.
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