I checked the minimum intake vacuum at hard acceleration when I made a pass at the "Strip" (it is a closed road, nearly 2 miles long). So the minimum is 1.47 in HG. What I know is, if the vacuum is between 1-1.5 at this test, the carb is too small. Now the facts. - 1955 chevrolet 150, weight with me and full tank = 3527 pounds. - Muncie 4 speed manual trans (1967 Version) - 3.55 Posi Trac rear end (1960), with Cal Tracs on leaf spring - car has new suspension at standard hight, with Ridetech RQ Shocks - front sway bar - PU bushings everywhere - 355 cui Sbc, 10.0:1 compression, 1.6 ex / 2.02 in valves polished, Aluminum Head, flat tapped cam 229 in / 230 ex @.050, Edelbrock Performer RPM intake (I will switch soon to a Air Gap RPM) - Carb is a Holley 4160 / 1850-14 600cfm, main jets 64, blue acceleration pump cam, .37 nozzle, vac, 6.5 power valve, secondary with Quick Fuel adjustable diagraphm, manual choke - AFR at idle = 12.8-13.2, AFR at WOT = 12.3, at some faster cruising speeds (between 50-60 mph) the AFR is 15.8 sometimes 16.1 - when I accelerate in traffic the off idle is terrible, it leans out at AFR = 19.5-20.1, the engine struggles for a second or two, I tried to compensate it with the blue acc pump cam and the .37 nozzle but no luck - vacuum at idle is 13.3 in HG at 850 rpm I also tried a 600cfm double pumper for one day, the feeling was that this carb performed better. The acceleration was more direct and straight forward, the idle was better too. But I don't know if this carb had changes from his standard jetting etc. I have no numbers on AFR or vacuum. I didn't installed the O2 sensor at this time and forgot to take the vacuum gauge with me. Quick Fuel carbs seems to be popular today, I looked at the recommendation, the 650 Street Q came out. But the carb has no choke and I need one. So I will not buy it. After a research for a carb with choke the Speed Demon 650 came my way. But I don't know if I would buy a vac secondary or double pumper version. I think the I will take one with annular boosters for better streetablity. The car drives more on the street (80%), ok a bit road racing and some 1/8 and 1/4 mile events. We have many Highways with no speed limit, so I can race there too. What you think?