"We all know size matters. But as opposed to many things in life, here bigger is definitely not better. The most common error we've seen on street cars and a few race cars is too big a carburetor. But if the engine is an air pump, why not get the biggest carburetor, to avoid having a restriction there? Without getting too technical, big bores mean less velocity and less acceleration of air/fuel mixture. That may be a little difficult to accept, one would think that the less restriction, the more velocity and acceleration. Though it’s not exactly the same principle, think of garden hose where the water is flowing at a certain rate. If you then squeeze the end of the hose, then you can get it to spray much further. At the top end of the rpm band, too small a carburetor will be a restriction and limit power. Small but adequate and non-restrictive passages builds air/fuel speed and thereby throttle response. Too small a carburetor may lose some power in the high rpm band, but will work excellent in the 0-5000 rpm band. But while too big a carburetor will work well in the 5-7000 rpm range, it’ll be sluggish everywhere else, and especially the throttle response will suffer. Do you want a car like that?
So what‘s the ideal size? Fortunately there is a simple formula. But with such a formula available and accepted by everybody, how come so many people still buy the wrong size carburetor? Unfortunately the result of the formula is usually much lower than what people tell you. So if your friends or competition runs 750 or 850 cfm carburetors, do you dare buying a 600 cfm? More must be better, right? No. This, like camshaft duration, is an area where you’ll be thankful that you trusted the formula and not the hype. The formula works. If the thermometer shows 30 degrees and your friend tells you it feels more like 40 degrees to him, which do you trust?
The formula is:
Engine size x Engine rpm x Engine VE / 3456
-Engine size is in cubic inches (cid). This is the easy one.
-Engine rpm is the max rpm that the carburetor should be able so support. Don’t go overboard on this one. You may wish to run a 7500 rpm screamer, but unless you strengthen the engine internals like crank, rods and pistons to be able to withstand it, you’ll only try it once. And honestly, when we accelerate full throttle, do we wait until max rpm to shift into next gear? Rarely, because it doesn’t make the car faster (see How to win, "4"), and most of us have so much money and time invested in that engine we want to keep it alive. So if you add up the total amount of time your engine has been above 5500 rpm, it’ll be a few seconds only.
-Engine VE is Volumetric Efficiency, a number telling how effective an air pump the engine is. It takes a dyno to measure your engine’s Volumetric Efficiency, but here are some ground rules. For stock smog engines VE would be around 0.8, for a well built performance engine it would be around 0.9. An engine with forced induction could be just above 1.0.
Example: A well tuned 351 cid engine that will see max 6000 rpm would need the following size carburetor:
351 x 6000 x 0.9 / 3456
equal to 548 cfm(!) So a 600 would be a great and fully adequate choice. Still many engines have 750 cfm or 850 cfm carburetors on them, enabling them to theoretically go into rpms that they never do. So it’s a waste of money buying the big carburetor. But even worse, the oversized carburetor makes the car less fun to drive below 5000 rpm because the carburetor is ruining drivability and throttle response."
Sorry...