[Streetwize]

I actually prefer the vacuum secondary on my cobra and mine easily blows the tires off at will, it's just the vacuum secondary (mine is an 800 AVS style) is more predictable going into a corner in terms of laying down the power. a mechanical secondary can hit like a small nitrous shot going around a corner and can (and often will) unload the tires right at the time of maximum lateral g's....not ideal.

A well-tuned Vacuum secondary is great in a light car, if you think about what happens when the tires break loose you are reducing the load on the motor which also reduces the draw on the carb venturi.

I've been drag racing for 45 years I know holley double pumpers backwards and forwards, but I still prefer a Vac secondary on a street driven Cobra. A lot of people don't even realize the secondary shot in drag racing is more for helping launch the car under high-traction high-load conditions when the primary shot is already used up, once you're at WOT and the butterflies are open the motor just wants booster flow...the pump shots are already out of it. My drag car has 5300 stall and I leave off two step at 3600 rpm....so my rear pump shot works more like most people's front shot.

I suspect on your FE 428 you may want to look at your ignition curve either not having enough initial or not a fast enough curve below 2800-3200. after you optimize that you can start optimizing the carb jetting, pump cam and shooter and rear diapragm. I prefer a thumbwheel rather than changing springs and always optimize the primary barrels/shooter, pump cam before moving on to the secondary.