[ha_pe_1968]

Hi Richard!

Yep.. the valve opens on pressure....you will find lots of infos on the QFT or Holley website. But lot's of mysteries around...some says the right one is half the vacuum....others says 0.7-0.8 x vacuum at idle is the right one.
However mostly 6.5 is delivered with the carburator...for a heavy streetfriendly car it will go more in 8.0 direction for something with a more aggresive cam it will go more in 4.5 direction.

Greetings
Peter