The Holley 390 in theory seems to be just about right for a six motor in terms of 4 bbl carbs but the reality is that they require a bit of tinkering to get them right because they were not really designed for the use that we put them. Its not terrible work, but it takes a little time to get the accelerator pump and pump shot right to get rid of any stumble, the power valve usually has to be changed out, some jetting changes. But once done, they work pretty well.
Be careful comparing cfm values between the 2 bbls and 4 bbls. They are measured at 1.5 inches of depression for the 4 bbls and 3 inches for the 2 bbls which in direct comparison means that the equivalent advertised flow rating for a 2 bbl is about 35% or so smaller in direct comparison to the advertised flow rating of a 4 bbl. So a 350 2bbl is roughly in the 240 cfm range using the 4 bbl conversion.
283 chevy motors in the late 50s used a carter WCFB. Those carbs were rated anywhere between 400 and 500 cfm. The most streetable 2X4 set ups (at least in my opinion) all used dual carters. The best 2X4 intake set up I ever had running on the street was way back in the late 70s on a 327. It was an offy medium rise intake and two carter 500 cfm carbs. The Carter's were easier to get running right for a street driven car then were the hollys. Little more fuss to get the top off but the rod and jet system for the Carters is pretty good.
I have never run a carter single 4 on any of my motors only because I love multiple carbs. But if I was, I would definitely consider one of the small carter's, like above, for a mild street driven 250-292 motor. Its in the right flow range, both carter and edlebrock sell the carbs and the parts to adjust them. I think they are a better call then the Holley 390.
Last edited by Dragsix; 12/21/2020 3:25 PM.
