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Author Topic:   holley 650 2bbl and 750 4 bbl
posted August 01, 2004 08:54 PM
At one track on fridays we have to use our 650 2 bbl and at the saturday night track were allowed a 750 4 bbl if we want. Has anyone went from a slightly modified 2 bbl to a 750 four? If so what kind of difference did it make for you out on the track ? Did the 100 or so cfm's make a big difference for you ? A c&s 2 bbl isnt in our budget right now and changing back and forth between the two carbs every week can be a pain.

posted August 02, 2004 10:46 AM
i have changed from a smaller 2bbl to the large c&s 2bbl (gained about 120cfm) and on the dyno i picked up about 35hp from top to bottom. on the track all things being the same, switching carbs gained me about 300rpm. it also pulled much better.



posted August 02, 2004 11:01 AM
2 bbl and 4bbl carbs are rated differently. 2 bbl are rated at 3" pressure drop and 4bbl at 1.5". They may sound close in flow rating but the 750 4bbl i would guess will flow almost twice as much as your 650 2bbl carb. Take the 390 4bbl vs. 500 2bbl for instance. Even though it sounds backwards the 390 will easily outflow the 500.

KP could do a better job of explaining this.

posted August 02, 2004 06:41 PM
It's interesting to me why they would test 2 and 4 bbls different. Can anyone explain why?

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