Should I go double pumper or vacuum secondary carb for my 408 SB. It is a pretty hot setup and I think it would have more snap with a manual seconday carb. With the 750 Vac no real snap, 850 vac better still not the snap.

What would I loose by running manual sec?

700 R4, 2800 stall, 3.73 gears, 10:1, .530 lift hyd roller, Dart Pro 1 215cc heads, RPM Air Gap,

My other car was the same until I put a 50cc accelerator pump up front. Right sec springs and such. It is a Street Avenger 870 on my 489. Runs great now.

Typically the rule of thumb is vac. secondaries go better with auto trannies and heavy cars. But I'll let someone else who really knows confirm that.

It is no light weight and does have an auto.

I have tried a 850dp, 750 vacumn sec and a hp950 on my 405 at the strip. The hp 950 was 4 tenths quicker than the 750 and 1 tenth quicker than the 850. The hp950 uses a 750 body that is smoothed to flow better. My car has a 2500 stall and 3.73 gears, the 850 seemed a little to big for good off the line responce. With more converter I would think it would work well. If I were you I would try to borrow a 750 double pumper and see how your car responds.