Why smaller? Thats easy. We're talking carbs here, not EFI. Carbs need vacuum to function corectlyand that's what pulls the fuel from the bowls into the venturis. The fuel pump only supplies the bowls with fuel, it does not pump fuel directly into the engine like on an EFI motor. The bigger the carb, the less vacuum signal it'll see under the venturis, and that's not good at lower rpms when the secondarys open too soon (as when using a mechanical secondary carb) Now, a bigger carb WILL work, but it'll do it at better at higher rpms . A smaller carb is what you want for a street vehicle that rarely sees the upper rpms. A smaller carb will see a stronger vacuum signal, making the venturis work better in atomizing the fuel. The only thing a small carb will do is at some point in the upper rpms, limit the engines ability to make power, at that point it becomes a restriction. And what rpm it does that at is anyone's guess, but I can tell you it won't be anywhere near where you think it will, it'll be somewhere above what you normally will use it on the street. ONLY on a dedicated race motor would you want the biggest carb the engine will need. My Ranger's roller 302 has a Holley 570 carb on it. It pulls to 6000 rpms, where the cam and heads and valvetrain limit the power. My 331 has three Holley 250's on it, it pulls to 7000 rpms, where the cam heads and intake limit the power. Both have excellant street manners in the lower rpms (off idle to redline) The 331's center carb, although rated at 250 cfms, is enough carb to feed the engine at 4500 rpms,which is about 90 mph. At that point, it's actually flowing about 365 cfms. The carb cfm rating is an arbitrary number, what it actually will flow is dependent on the engine it's bolted to. So just becasue you think a smaller carb will choke the motor in the upper rpms, that's not always the case. That is why when it comes to carbs, smaller is always better choice than larger.
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