OK so I'm the only guy that thinks that a 750 mech. secondary doublepumper works very well on a 345hp 302 crate motor. Well it does! I didn't think it would until I tried it . I was going back and forth between a 650 and 700, 714cfm carbs were used on high per 289's in the mid 60's, before I discussed this with a guy who I know that runs a 7 second prostock cammaro and has been a dealer of superperformance cobras. He went so far as to say he would give me my money back and exchange it with any other carb I wanted if I didn't like it.
The basic logic is this. With a light car like ours, 2300lb or so, the vac. sig. that is produced in the plenum of the intake maifold is not depleted as much as a heavier car over the same lenght of time it takes to increase rpms when you matt it. As the light car is accelerated, quickly going from closed to fully open butterflys, there is a rapid enough increase in rpms and thus maintains a correct flow of air in the carb and no venturi stall with associated "bog". In a heavy car, 3500lbs or so, there is a delay caused by overcoming the slower acceration because of the increased weight and the low pressure area in the intake manifold returns to atmosphere before revs increase enough to compensate. Heavy Drag cars overcome this with very low final drive ratios such as 4.11 or as low as 5.0 to 1 and very high rpm launchs. Most of our cars are 3.8 or higher and will not tolerate 4000rpm launches. Mine has a 3.44 diff and works very well when I put my foot into it from about 1200rpms. No "bog" just goooooo. Of course in a road race car engine revs are mostly up in the rev range and the problem of venturi stall is not likely to be a problem.
The plenum area of the intake manifold is also important and that is why I used a RPM preformer eldebrock. It has a larger plenum area than a performer type and thus holds a larger volume of low pressure air for a longer period of time again preventing venturi stall. The advance curve is also very important. I run mine with 33-35 degrees of total advance all in by 2400rpms.
The car runs very good thruout the rpm range and really comes on like a huge 2 stroke above 3500 rpms all the way to the self imposed low rev limmit of 6000rpm with a MSD 6AL spark box. I have no reason to believe this motor wouldn't make good power up to 6700 or so where the hyd lifters would begin to limit further useful revs.
As a note, the Trans Am cars of the 60-70's used 750-1100 holleys,the 1100 was a 3 barrel! or in later years TWO 500-600 cfm 4 barrels. They were limmited to 305 cubic inches pushrod V8's and made something like 550hp or more. These were 1 hour road races with much like the same type of use we would see during a track day. Also the 8 barrel FI systems being used on some of our 40's must flow at least 750 cfm.
CFM ratings alone are not the sole thing to consider when selection of a carb is the question. This is very much a question of ballancing the proper cam grind, intake manifold, spark curve advance, etc. with a correctly set up carb. But I will tell you that these 345 hp 302 crate motors from Ford WILL WORK VERY WELL with a 750 cfm holley and a RPM performer manifold.
As a final word my friend told me that of all the holleys he has dynoed on 302's the 750 mech.secondarys doublepumper was the closest to perfect from the box.
The down side is I'm sure I'm using more fuel. I hope I have added to the discussion, and hopefully made a little sense, without ruffleing too many feathers, but I won't be changing my 750 anytime soon.
