This got me thinking.  For years I've just ascribed to the old adage that vacuum secondaries were for automatics and mechanical secondaries were for manuals.  So a bit of research turned up some interesting info.  Vacuum secondaries are best for most street applications due to their adaptability.  They also tend to be more forgiving with carburetor sizing, because if the engine only needs say 650cfm and you put a 750 on it the secondaries will simply not fully open, in effect only "using" 650cfm of the carbs available airflow. 

Mechanical secondaries hit sooner and harder, and in a performance application have advantages as a result.  They are harder to get dialed in though, and carb sizing is more important.  BUT, with low vacuum from a radical cam there's no guess whether or not the secondaries will work, or how well. 

In summation, in a heavy car (3,500lbs or more) with an automatic with a low stall speed and/or a stick with a numerically low rear gear (under 3.50) and an engine with less that 9.5: 1 compression & associated cam you are better off with vacuum secondaries.  In a light car with a high stall and/or high rear gear with a stick or auto with an engine with 10:1 or greater compression & a nasty cam you are better off with mechanical secondaries. 

Vacuum secondaries also tend to get better fuel economy, if anyone cares.  I mean, to me, mileage is one of those things to muse about, but if your primary bragging right is your car's fuel mileage it means your car sucks and you secretly hate it. 

My son works at a machine shop and we just finished drilling and installing the secondary power valve and accelerator pump on a 600 Summit adding the squirters to the secondary booster etc. I have been curious as to what advantage the mechanicals would be as I felt I was dropping a little on my shifts as the secondary has to close and reopen with a slight delay as I let off. I like the throttle response with the annular boosters but they only produce that carb in a 750 which I feel is to big. All the castings are there to complete the needed circuits as they must use the same master castings for the body. Not sure how to post pics here?

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Nice photos, 1 too many. A beautiful red '67. Very nice.
Wish I could post pics. My '68 is just about the same color. I have black interior. I used to have slot aluminium wheels with Daytona sport 60s and 70s. I'm on Crown Vic wheels and hub caps. I like um OK.  Looks almost original. Still working on posting pictures.

1toomanyprojects wrote:

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