[Tommy]

As no one else is rushing to answer the OP's question, I'll add this. My understanding is that a properly tuned vacuum Holley can run just as strong as a properly tuned mechanical Holley at full throttle. What little difference there is occurs during the transition from opening the primaries to opening the secondaries. On the mechanical, the transition is as fast as the throttle can move, so its up to the operator to set up the accelerator pump to dump additional fuel in accordance with the way the driver moves the throttle. On the vacuum Holley, the operator uses the throttle to tell the carb he wants the secondaries open, but they don't open until the vacuum signal is right. This helps assure the fuel-air mixture from the secondaries is right. The result is that the mechanical Holley may be very slightly quicker during the transition to full throttle, but only if the driver does it well. ... I've never noticed the difference.