Guys,
I know this subject has been beat to death. So if you don't want to partake in this discussion that's fine I don't blame you. All I ask is that you be respectful as I want to better understand this and would much rather do this in a pub over a bear with a pencil and paper. I want to know if this makes sense or if there is wrong logic etc. I am just trying to learn.
OK so here is my thinking. Manifold vs. Ported vacuum for vacuum advance distributors is engine and application specific. My engine/application is problematic.
The Car:
The engine wants roughly a base 42° of advance at idle with 15Hg of Vacuum. It cannot obviously handle that much without detonation under heavy acceleration/WOT conditions. 34° max is pushing it under load. But for the sake or argument let’s keep it at 34°.
Engine losses like 300 RPM min from P to in gear.
The engine is a high compression 12:1 TRW piston and will assume its roughly 12:1 not assuming anything about the deck or gasket height etc.
The cam is a solid mechanical lifter with a high lift and long duration aggressive design. Unfortunately I don’t have the specs.
The torque converter appears to be a lower flash tight unit.
Power brakes
J heads with solid lifter springs
Currently MSD box and a MSD non-vacuum distributor.
Banter:
So you can see I am trying to make this 4 speed drag/strip motor run in a heavy automatic car with a tight converter on 91 Octane in Arizona.........
I know that if I had money right now to throw at this I could rebuild the motor and lower the compression and put a better cam in it for street use and a higher stall converter but for now I want to see if I can tame (not detonate and not overly de-tune/retard the motor to much) this set up.
I already have slowed the distributor’s curve way down and that has helped a lot.
Given this set up it appears to me that a vacuum advance distributor set up with manifold vacuum would help me.
Here's my rationale:
Again my engine wants a higher base tune at 42° (I also assume at cruise) maybe more and 34° total under acceleration/load and WOT. If my vacuum advance gave me 8° then I would get 42° of advance at idle and 34° under acceleration/load and WOT. The benefit would be a better idle with more vacuum and a more efficient burn. More vacuum for my power brakes as well when I take my foot off the brake and vacuum builds. Ported will not do this since there is no vacuum at idle.
This is my rationale for saying it seems to me that manifold vacuum is a better choice for my set up than ported.
Does this make sense? Is there a reason that ported would be better? Am I missing something?