Carb rotation and plenum division

[SoCar72]

This question comes from looking way down the road for future upgrades involving the CI Alum head and manifold on my small six. A few questions come to mind concerning carb orientation and plenum configuration.

It seems to be the popular choice is to have the carb (2bbl or 4bbl) turned so that the throttle blades are parallel to the crank, as with the Offy intake on my 300 F-250 and 390 Holley.

1. The CI intake is single plenum. With a non-progessive 2bbl (Holley or Autolite), has any benefit been found with the carb turned one way or the other? With 6 runners feeding from 1 plenum and 2 venturis flowing, orientation seems to be a toss up.

2. With a 4bbl, parallel seems to be the logical choice to keep venturi to runner distance as balanced as possible between the front 3 and rear 3 runner. Has any advantage been found with the primaries close to or far from the head?

3. With either carb oriented parallel to the crank, has any advantage been found by dividing the front 3 runners from the rear 3?

Thanks for your input.

 

[StrangeRanger]

I would expect a divider with a balance notch in it to equalize the pressure waves would give some increase to low end performance without costing anything on top end. Whether the gains would be significant or not I have no idea. Someone, I think it was Mustang Geezer, played with this idea a while ago but I don't remember the results.

 

[80broncoman]

I can tell you on my 300 it really made it better down low (from idle to 1500)
And it was amazing at how much easyier it was to adjust the idle mix screws on mine with the divider installed.
do yourself a favor, cut two notches in the intake so you can tinker with different height dividers.
Do Not make the divider out of plexiglass. BTDT did NOT like what happened.

 

[SoCar72]

Excellent. Down low is where I'm looking anyway.

I'll give it a shot on my F-250's 300. I've never really liked, at least with this Offy intake, how the primaries are separated from the secondaries but yet all 6 runners feed from common plenums (each runner is split horizontally with separated P & S sub-runners and plenums). I didn't really question, figured Offy had a reason for it.

I can imagine what the heat and gas did to the plexi divider. Thanks for the heads up.

 

[80broncoman]

....I can imagine what the heat and gas did to the plexi divider. Thanks for the heads up.

It was lexan actully. and the heat and gas never bothered it. It was the pulses of the air that broke up the lexan.
My lexan was IIRC about .125 thick.

 

[SoCar72]

REALLY?!?!?!

Wow, I would not have expected the vacuum pulses would be that strong! Considering the engine fires alternately between the front 3 and rear 3 cylinders, it makes sense.

Thanks for the input.