While I do not have a big block engine,I have a 350 hp 351-W and have tried about six carbs in the last 10 years,here is what I have found........
started out with a Holley 600 vac. sec. carb,ran good gave good gas mileage and performance,then I wanted "a little more", so I tried a 715 cfm marine carb,vac. sec.,could not tell any difference,but it was an old carb and may have needed more tuning than I did to it at the time......
Tried a Holley 750 vac. sec.,car ran o-k,performance was not as good as the 600,tested both on the drag strip same day..........gas mileage was down also.......
Then a buddy gave me a "wore out" 20 year old Holley 600 double pumper,I rebuilt it myself,put it on my car and track tested it,a lot better performance on the bottom end and better all around......been on my car ever since......
Everyone told me my gas mileage would suffer going from a vac. sec. to a double pumper,well, I drive my car to work at least once every two or three weeks (250 to 350 miles per day),lost all of 1 mpg with the double pumper,granted if I keep my right foot in it,gas mileage will definetly go down,but if I drive the same I only lose about 1 mpg and the performance difference is VERY noticable as well as track proven on my car........was also told the double pumpers are "more finicky" for city driving than a vac. sec. carb,to me, I find NO difference in city driving and prefer the double pumper,cause if I need to step on it,the throttle responce is immediate.........
I do not care for the Edelbrock carbs mainly cause I can not work on them and can now take apart and rebuild a Holley with my eyes practically closed.......
Holley's formula for carb size vs. motor is this,multiply your engines cubic inches by the max rpms your enigne turns and then divide that number by 3456,this will give the cfm your engine needs at it's max rpms assuming your engine is 100% effecient.......
427 cu in x 6000 rpms divided by 3456 = 741.31 cfm assuming you max your engine out at 6000 rpms,I would think a 750 to be the minimum and an 850 to be about right, and I would go with a double pumper..........
Just my opinion on the subject and that and 75 cents might get you a cup of coffee in downtown Marksville,La., on saturday morning...
David
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