I am new to this site, not new to Mopars or performance, but new to problems created by turbo boost conditions. I am currently running a 225 slant six with a Buick Gran Am Turbo (Garrett), and blowing through a 390 vacuum secondary holley. Sensing that the secondaries might not get enough timed vacuum to open under boost conditions, I employed the trusty screw in the secondary acuator cam slot to force them open. Of course this only works well when I feather the throttle for the first 15 feet or so before burying the pedal (2.7 gears and auto). Question: has anyone actually figured out a way (not just in theory - no blufffing now) to put boost pressure on the bottom side (shaft side) of the secondary diaphram to correctly boost reference the opening of the secondaries? There is such a large area to try to seal up (the rectangular opening to the housing and then the shaft that has to have freedom to move up and down), that I have decided to try to see if anyone else has tried this before I attempt some radial measures: e.g. : using surgical tubing and zip ties around the opening flange and the diaphram rod - or - reversing the secondary cam and plumbing boost pressure into the vacuum area.

Has anyone out there tried to solve this problem other than to make the 390 into a "mechanical" secondary carb?

Try the "carbed" section on http://www.turbomustangs.com . This has been done. It is not easy. The perfered solution is to use a mechanical secondary.

This area is probably the best information on carbs and turbos outside of Dick Datson's Yahoo mailing lists. Some of the more knowledgable people that Dick has alienated hangout there.

Kesteb,

Thanks for the tip about the turbomustang site. However, it still deals mainly with the double pumper (mechanical secondary Holley type). I presently have my carb functioning as a mechanical carb, I would still like to know how anyone made a vacuum secondary carb operate under boost pressure.

Thanks again for the help,

Turbocuda

In the book turbochargers(?) by hugh mc innes or something, he wrote about turning the vacum thing upside down, and using boost pressure to open the secondaries.
Does your carb use a secondary pump? I think the screw in the linkage is only halfway there. You can get a kit to install nozzles from the primary side, thru small tubes to the secondary side.
But the carb should work as it is.
It has to have a pressure drop in the venturis to pull the fuel, doesn´t it.
So, there has to be "vacum" in the manifold, even if it is above atmospheric...
Please can someone correct me if I am wrong?

Do a search on the turbomustangs site. There are a least 2 individuals that have made it work. In my opinion, by the time you figure out how to make it work, you would be way ahead using a double pumper.