[dmjlambert]

Bringing this back to ask another couple questions please. This is for my understanding, not because my engine is running poorly. OK, so my GM HEI on a 350 engine has initial timing set to 7 BTDC. That is the most number of degrees BTDC I can go without causing knocking. The vacuum advance gives 23 degrees of advance when it is connected to vacuum. I have it connected to manifold vacuum, so that means I get 30 degrees BTDC timing in idle.

The tune-up sticker says to set initial timing to 4 degrees BTDC, so I suppose that 3 extra degrees I'm giving it is what it can run with modern gas, the modifications done to the engine, and tweaking the conservative settings the truck rolled out of the factory with. But then I realize the tune-up sticker was not printed with HEI in mind, and the stock distributor should have had 13 degrees of vacuum advance according to the manual.

Now for the questions: How can anybody advise set initial timing at a particular value, such as 12 to 14 degrees BTDC without knowing how much advance the vacuum advance gives? Obviously that would not work on my engine.

Is adjustment of timing just something that should be done by ear and the follow up with a light just so you know where it is set for future reference?

Would it be advantageous in any way to have a vacuum advance that gives a smaller amount of vacuum advance, such as 15 degrees, and then have a more initial advance, such as 15 degrees, so the total amount of advance at idle is still 30 degrees ( same as 23 degrees from the vacuum advance + 7 degrees initial that I have now).

Note, this is not a performance engine, just rebuilt stock except newer heads, HEI, and a 1985-86 Quadrajet. And it's not driven by a lead-foot, just going to the store and work and back on city streets most of the time and on the freeway occasionally.