Dual Plane vs Single Plane Manifold with TBI

No F.I. experience here, but it would seem that the A/F ratio would be more consistent with a TBI when using a single plane intake. With a Holley 750 DP carb, I have experienced different jetting requirements between plenums on a dual plane by 2 jet numbers. The deeper plenum ran leaner than the shallower plenum. I happen to notice one day, when I had all the plugs in a holder, that the plug coloration coincided with the plenums. Car ran several hundredths faster when the "square" jetting was adjusted accordingly.

I can't be sure if that applies to all dual planes. However, also with carburation, it appears that in most cases when an adapter is used, it has a negative affect on airflow. Spacers, on the other hand, can be a useful tuning aid.