panteradoug posted:
… All things given equal, aluminum heads require the same specs as their iron counterparts ...
Agreed … but are all things equal in this situation?
True, SVO aluminum heads of the 1980s were cast with the same combustion chambers as the factory iron heads of the 1970s. All things are equal between those head castings in terms of combustion chamber dynamics.
But even old school Cleveland heads (iron or SVO aluminum) require more total ignition advance when used with pop-up pistons, compared to the total ignition advance they require with flat tops or dished pistons. The pop-up domes change the combustion chamber dynamics, and the speed of the pressure rise (flame travel) within the combustion chamber.
I am under the impression that TFS heads have revised "Yates/Weslake" style combustion chambers. What are also called "high swirl" combustion chambers. If so, might they not alter the combustion chamber dynamics, and thus require a change in total ignition advance compared to iron or SVO aluminum heads?
I have no experience with TFS heads, so that rhetorical question is all I have to offer on the subject of total ignition advance; however I would like to make a few observations regarding static ignition advance too:
If an engine "diesels" when you try to turn it off, that is a sign of insufficient static ignition advance. If a starter can't turn-over an engine when it is hot, that is a sign of excessive static ignition advance. Ideal static advance obviously falls between those two extremes.
One reason I recommend 16 to 18 degrees static advance with iron heads is because it improves low rpm torque, but it is not so excessive as to overwhelm the starter's ability to crank the engine when it is hot. Manifold vacuum usually doesn't increase much, if at all, above 18 degrees static.
But static ignition is only one of several concerns for setting the idle. A Holley carburetor is designed with an ideal setting for the butterfly opening at idle. The ideal setting exposes (opens) the "transfer slots" below the butterflies a specific amount. Setting the primary and secondary butterflies at that specific opening is a second concern. The idle mixture screws are then adjusted to achieve the highest idle rpm … which will also correspond to the highest manifold vacuum.
I have more than once set the butterfly openings while rebuilding a Holley, then adjusted the idle mixture screws for 1-1/2 turns off their seats, bolted the carb to the manifold, started the engine, and had the engine idle perfectly at 700 rpm with no other adjustments.
However, if the idle rpm is too high when everything is optimized, which variable will you alter to lower the idle rpm? Folks argue that one. I would suggest decreasing static ignition advance, and increasing centrifugal advance.
Food for thought. I hope it helps.