I have been using mechanical secondary double pumper carburetors successfully on my Pantera cars. Conversely, I have struggled with vacuum secondary carburetors because it can be unpredictable as to when the secondaries engage if they engage at all. You have to use a test (zip tie or twist tie on the shaft) to tell if they actually open. With mechanical secondaries you are assured that they open and operate predictably. The stock configuration from Holley is for the mechanical secondaries to start engaging half way to full throttle and you can actually feel them opening in the gas pedal from resistance of the secondary return springs. Several of my friends have switched to mechanical secondaries after driving my cars and feeling the responsiveness. The engines in my cars have the typical Pantera performance upgrades including intake manifold, cam, lifters, roller rockers, valve springs, valves, headers, and ignition. BTW, may favorite is the hydraulic roller cam.