the biggest problem here is were talking about carbs in general and you keep coming up with a Q-JET. a holly and a Q-JET is two dif carb all together. with a holly if it says 600 CFM then that's what it means, the front venture and rear venture stays the same, you can re jet it up or down a few # and it will work. On a Q-JET the primary venture stays the same, but on the rear there is some verbal venture built into it. the small Q-JET will flow from low 500 cfm up to 750 cfm if you adj. the secondary butterfly properly and change the jets and the metering rods. they came from the factory set up for a stock what every motor it came on. there was about 50 dif. part # for Q-JET because every motor had dif jets and metering rod in them. if you have a Q-JET that came off of a 327, and put it on a 400 motor you would have to loosen up the secondary butterflies and put bigger jets and smaller metering rods for it to be 100%. it would take about 3 dif. hollies to cover that ground. it still come down to you cant just bolt any carb on a motor and its gonna run, its got to be close on the right size venture and jet size.