Dave, we've done quite a bit of dyno testing with single plane intakes and spacers/adapters topping them off with q-jet carburetors. The open spacers are best, and it helps to blend them into the plenum area of the square flange intake.
There will be a small HP loss, most of which is simply from the narrower plenum on the square flange intakes and the fact that the q-jet is pretty wide across the rear throttle plates. This slight missmatch will cost some top end power, but not really very much.
We see the same thing on dual plane spread bore intakes when they are topped off with square flange carburetors, as the spread bore carbs line up better with the wider plenums and make more power at high rpm's.
Keep in mind, were are not talking about a lot of power with a good spacer/adapter. If you want to loose a lot of power, bolt a 600cfm Holley on a factory spread bore iron intake with a 4 hole adapter. We've been watching folks do this for over 30 years!......Cliff
