The boosters that are used in a carburetor are always a compromise, some produce a stronger signal, some allow slightly more airflow and some don't do either of these things. All of them have their place too.
From my understanding in a 4150 style Holley the down leg style booster esspecially one that's machined to have a small groove around the ID is the overall best compromise. The reasons are: It produces a good signal with a minimal loss of flow. It produces good mixture distribution and it delivers the fuel to the engine with good atomization. But most importantly, it produces a signal that allows the fuel curve to be controlled and taylored to the engine very effectively using the emulsion and air bleed arrangement of the Holley carburetor. The annular style booster tends to sometimes be too much of a good thing but only under certain conditions. For some reason it can be very in tune and very out of tune I'm not sure why.
The situation changes somewhat with the Dominator style carburetors. In those the annular style booster insert seems to be the best compromise in many instances.
In the Engine Masters' Challenge I never used less than a Competition Carburetion built billet annular insert equiped 1050 Dominator even when pulling the engine down to 2500 on 408cid. It's a dyno test not the real world and it was the best for our combo..
This is a GREAT topic.
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