Here is my best reasoning why the secondaries are needed for this intake manifold (both uim and lim):
-The fuel injectors are in the primary runners
-The secondaries are slightly larger in diameter than the primaries.
-Fuel doesn't travel in the secondaries.
-When there isn't a whole lot of air running through the primaries, the swirl and tumble of the air doesn't pick the fuel up properly, leaving the fuel not as suspended (big droplets and it sprays on the sides of the walls.)
-Most of the air goes through the secondaries at any RPM (if the secondary butterflies are open)
-SINGLE runner, like on the 3L would eliminate that problem, and possibly provide a better running path for pressurized air.
-So, it isn't until 3000-ish RPM, when the secondaries open up, that the air in the primaries is swirling/tumbling enough to keep the fuel off the walls and pull it nicely down the runner.
Conclusions:
1. With dual runner manifold, you NEED to have the secondaries shut off at lower RPM's.
2. Pretty plastic 3L upper WITH big oval lower (with siamesed intake head ports) could be a righteous option for the turbo.
