David Vizard wrote it up in his book Super tuning carburetors, remember your drawing your vacuum through 8 venturis so the pressure drop is far less, so they flow far less. Its a good read. I ran 2 390s for about 2 minutes until I popped the 1/4 inch custom top. Then I ran a single 650 Demon on a 3/8 plate and it ran great. Probably could have ran a 750 on the 340 but it was a street car, no track time.