Different Timing For Different Distributers

I set the timing on my 1969 F4 and noticed the FSM lists two choices (5 BTDC or 0 TDC) depending on the distributer. Mine is the Prestolite IAY-4401A, which shows 0TDC for timing. The FSM also lists a different idle speed. Any thoughts on why they would be different? It was at 5BTDC and I set it at 0TDC and don't notice any difference in running or starting. I do not have the exhaust emissions controls, if that helps.
Thanks!

 

Distributers have a build in set amount of centrifugal advance, some are higher than others, if yours has a lot and you set your initial timing too high you will be over advanced in the upper rpms. I agree with glen though, you should be fine. if you notice any pinging cruising, I would reduce the initial setting until its gone.

 

Did emissions requirements begin as early as '69?
I recall them being a PITA in the '70s, but I don't remember when they started.

 

I know they were in California. Had a 1969 Toyota truck with a smog pump (air injection).

 

my guess is one has vacumme advance and one doesnt. Or if both listings are for 69 then maybe emmisions for california or something like that

 

In that era most cars had at least an air pump with a manifold bypass valve, which if it didn't work could create some hellacious backfiring, an EGR valve and of course PCV systems.

 

I never had problems with it at 5BTDC, so I think I'll switch it back.
Thanks everyone!