I'm curious because I'm thinking about putting a Comp cams XE-274 cam in my 350 engine, maybe even something a bit more radical. But also I think I'm going to replace the carb as well. As I understand, vacuum secondary carbs are good for autos with vacuum and mechanical secondary carbs are for manuals and/or autos where the engine has little vacuum due to radical cam. I need to know this because I plan to order my carb and I need to know what will work best. BTW it will most likely be a Demon or a Holley. Thanks.

The secondary operation of a vacuum sec carb does not depend on intake manifold vacuum.From the diaphram pot there is a small channel that connects to the primary side of the carb....As air flow in the primaries is sufficient to pull a signal ( vacuum ) through this channel and is enough to overcome the secondary spring then the secondaries will start to open....You can tune the secondries with different color coded springs that Holley sells. Generally the lighter the spring the quicker the secondaries will kick in and the heavier springs will delay it.... Contrary to popular belief a vacuum secondary carb can provide outstanding performance.... Some will say that they wont open the secondaries all the way.... Taylored correctly that is not true as they certinally can open all the way...It's just a matter of understanding how they work and tuning them to your application.... Assuming correct carb selection.

If you look at many of the NHRA Super Stockers you will find many of the capable of running in the 9's with 780 Vacuum secondary Holley carbs.