Picking the Best Carburetor
To get the most from an engine you need to have the right carburetor. With so many possibilities how do you know which is the right one. There are many things to consider but the most important is, what will you use the car for? If it is your daily driver you will get the best performance from a small vacuum secondary carb. This is where most guys screw up. They think, I'll get more horsepower from a big double pumper. You may get more horses from a big ass carb but horsepower does NOT equal performance. If it's your daily driver then get a carb with vacuum secondaries. This will give you the best throttle response and gas mileage. If you race your daily driver on the weekends then get a bigger vacuum secondary carb. You will lose a bit of the bottom end in exchange for more power but still get decent gas mileage. If the only street use your car sees is to and from the track then go ahead and get the double pumper.
Picking the right size is when it gets fun. You may have seen that old equation <CFM = CID * RPM * VE * 1/3456> Where cid is the engine size, rpm is the maximum rpm, and ve is the volumetric efficiency. Volumetric efficiency is how full the cylinders get. A stock motor will have about 70% ve, a street/strip motor will be around 85% and a full race motor can be as high as 100% or more. Say you built a 302 with a cam, aluminum intake, and headers which you plan to shift at 6,000 rpm. <302 * 6000 * 0.85 * 1/3456 = 445 cfm> Your probably thinking that 445 cfm is way too small. That's because this equation only tells you how much air the motor will pull not the size carburetor needed. To find the right size carb you first need to understand how carburetors are rated. Four barrel carbs are rated at 1.5"Hg where one and two barrel carbs are rated at 3"Hg. This means if you pull 600 cfm through a 600 cfm 4v carb at full throttle you will have 1.5" of manifold vacuum. If you have manifold vacuum it means the carb is a restriction. The more vacuum, the more restrictive the carb is. Now back to the 302. If you did use a 445 cfm carb you would have 1.5"Hg at 6,000 rpm. This would be acceptable for a daily driver where you were more concerned with gas mileage than performance. A street/strip car will perform much better with 1"Hg or less. To find out what four barrel carb is needed to attain 1" when pulling 445 cfm you can use this equation. <CARB = CFM / squareroot( desired vacuum / 1.5 )> So <445 / squareroot( 1 / 1.5 ) = 545cfm> That sounds better doesn't it. You can use this equation for one and two barrel carbs also, just change the 1.5 to 3.
With vacuum secondaries you can pick a carb rated higher than you need. This is because the secondaries, when setup properly, will only open when needed. You could put a vacuum secondary 750 on our theoretical 302 and it would probably work fine, though a 600 or 650 would be better. Mechanical secondary carbs are more sensitive because when you stab the gas it is always wide open. Picking one that is too big will drastically hurt performance. For our 302 you would want a 550 or 600 cfm double pumper.
Picking the right carb isn't an exact science. All the equations and stuff will only give you an idea of where to start. Every engine is different. Even the same engine in two different cars will require different carbs. It also comes down to personal preference. How much economy and drivability are you willing to give up for more power? To find the right carb you need to play around with different setups to find the one you like best.
If you have any questions or comments please e-mail me at mrriggs@gofastforless.com.