[Barry_R]

First off - - Holley always employed "fluid dynamics" people - - many of the finest in the industry, along with some very sophisticated equipment. If they had a shortcoming it lied in the underestimation of the value of "garage engineering". Holley always developed things instead of inventing them.

Signal is a term applied to the vacuum generated by the restriction a booster/venturi combination causes in an air stream. Want more signal - add more restriction (assuming that the main venturi and engine air demand are held constant). Downleg style boosters will have a stronger signal because they hang further into the tight spot of the venturi. Annular boosters will have a stronger signal yet because they are physically larger.

A stronger signal carb can be desirable because it will be more responsive to jet and bleed changes, and the main circuit "turn on" point can be more easily controlled. In the right hands, this might let you run a bigger carb for more high RPM power without the commonly quoted downsides.

Fuel management. A fine atomized spray from the boosters is considered optimal. Downleg boosters do a decent job 'cuz they have good lower edge placement that helps shear the fuel droplets in the airstream. Annulars work even better 'cuz they couple the increased signal with multiple fuel entry points around the periphery of the booster rather than having a single point of fluid entry. Why wouldn't everything have annular boosters then? Two reasons - they cost more than downlegs, which cost more than straights, and they Do restrict airflow.

Power valves can be responsible for nearly 1/2 of a carbs WOT fuel flow, and allow a leaner and much smoother part throttle section. If for some reason you want to delete a power valve, you measure the diameter of the power valve channel restrictions, calculate the area, and add that area to that of the existing main jet. I can agree with no secondary power valve on a quick car - - but would be a rare case where any street driven vehicle should not have one in the primary side.

How do you pick then? In heavy cars, or with automatics running a bigger than "normal" carb with annular boosters is almost always better. In something light like a Cobra, the downlegs are almost always OK. I'm not fond of straights.

I can get back to my normal habit of lurking now....