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Re: 1951 Packard 200, 288 engine. The timing calls for 5 or 6 degrees BTDC. There is vacuum to the vacuum advance. Do I disconnect the vacuum line, set the timing then re-attach the line? Or, leave it connected when setting the timing? The vacuum appears to be "ported" vacuum not manifold vacuum.
Thanks for any answer.
Regards, Steve.......

I believe that if the idle speed is within reason, it makes little or no difference whether the vacuum is attached or not, but just try it both ways and see for yourself. I know many makers recommended disconnecting the line, and I don't recall if Packard ever had a specific recommendation. Easy enough to find out, just browse the shop manual.

My experiences are that with the changes in gasoline over the years and things like cylinder heads that have been milled, the original timing recommendation may not give the best performance. I much prefer to drive up a hill with a bit of throttle applied and the window open, and do it by ear to give just a hint of preignition under those conditions. This is more dynamic than just using the original # when gasoline was far different and the compression ratio may have bee different. It may or may not be more advantageous on your particular car.

I didn't really answer your question directly, but hope this proves useful to you.

Nothing mentioned in service manual or Motor's about disconnecting vacuum but Packard did say keep the engine RPM below 450-500 so the centrifugal advance does not come in.

preignition under those conditions.

Owen, Does this mean: Timing calls for 6 degrees.....Set instead at a number less than 6 degrees down to 0 degrees?

HH56, Thanks for checking your books as to whether I need to disconnect vacuum or not.
Steve.....

Owen, Does this mean: Timing calls for 6 degrees.....Set instead at a number less than 6 degrees down to 0 degrees?

Not exactly, it means set it at the point where driving experience indicates it should be, regardless of what the number is. You can also get the same result by setting it at 6 degrees BTDC and then do a road test including driving up a reasonable hill with some heavy throttle - if you don't hear any hint of preignition, then you should probably advance it a bit more. If the road test gives easily audible preignition on the hill, then you need to retard it a bit, and then retest. All you're doing is optimizing the timing for the gasoline you are using, and the condition of the engine.