Vaccuum or Mechanical?

[69chargerboy]

I'm going to by a Barry Grant Speed Demon 750 carb and I was wondering whats the big diffrence between vaccuum and mechanical secondaries? Does it really matter? This is going on my mildly built 440 HP engine with tti headers, tti exhaust, FBO ignition. Thanks.

[Ghoste]

It does matter but either can certainly work well for you.  As a generalization, the vacuum secondary because it opens based on a vacuum signal from the carb is a more forgiving setup and works better for the street.  It's load based so the engine theoretically will only be getting more fuel/air as it tell the carb it wants it.  You can tailor this load signal somewhat to suit your needs and your vehicle combination by various means and it isn't perfect but it is damned close and as I said, pretty forgiving.
The mechanical secondary, again as a generalization, tends to be more of a strictly high performance piece.  Maybe not race only but is happier in that environment.  The mechanical comes in based on the throttle position.  You can tailor it to the car but at some point in the throttle travel, it will be coming open regardless of the engine load requirements.  This is great for racing, and less so for the street.
There is certainly more to it than this but typically a car that sees mostly street duty and is a mild build will be better off with a vacuum secondary.  If you will be using it hard and frequently then the mechanical is the way to go.  Transmission plays a factor too with automatics often being happier with vacuum and four speeds liking the mechanical.  There are lots of exceptions to all of it though with individual wants and needs being the ultimate deciding factors.

[69chargerboy]

Ghoste, Thanks for the info. That helped out a lot. I really appreciate it.