The key to Qjet CFM in the secondary air valve (the flapper on top), by adjusting the limiting tab (either adding or removing material - some folks add an adjustment screw) you can set the amount the air valve opens. So for a V6 (4.3) the air valve will open to say 65%, now on a 454 HP rat motor or a 500 CI caddy the air valve open to 90 degs.
Yes there are 2 size primary bores, for our inlines use the smaller one.
