Selecting the correct carburetor for your engine is a tricky task and can define how well the entire engine functions, so just randomly picking one on a whim is not the best idea. Fortunately, there are companies like AED Performance that not only make great carburetors, but are able to assist customers in sizing and selecting the correct one for their engine.
Many factors go into how a carburetor will function with an engine, so looking at the application of the engine is the first place to start. If the car will see street or strip action, the cubic inches, cylinder head sizing, compression ratio, camshaft type, headers, intake, and RPM range are all important to consider when looking at what carburetor to use. There is a very basic formula that can be used to get a solid baseline of what size carburetor based on the CFM’s needed.
CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456
John Dickey and Jeff Harris from AED Performance have helped many customers with their carburetor selection needs for over 30 years and understand sizing goes well beyond this basic formula.
“You could use the time honored formula which just figures carburetor sizing based on max power RPM, but there’s much more to consider when doing this correctly. It appears the bigger the numbers that come from the formula, the larger the carburetor requirement, however, that’s where experience adds some divergence to the baseline parameters,” Dickey says.
There are five different areas that need to be examined when you’re trying to size a carburetor correctly for a specific engine and car combination. The first element is the size of the cylinder heads you will be using on the car, more specifically the overall size of the intake runners. Typically if you’re using a set of heads that have a large, unrestricted intake runner, you will need to use a smaller carburetor compared to smaller heads. In turn the smaller the heads, the larger the carburetor requirement.
“You already have a choke point on smaller heads that control port velocity, usually at the pushrod area, so you don’t want another restriction on the intake side. A good example is a 440 Mopar engine; it has a small-block Chevy-sized cylinder head and requires much more carburetor than makes sense to produce results. Many small-block Fords, Pontiacs, and Oldsmobile engines also fall into this category,” Harris says.
The second factor to look at is the RPM range the engine will be operating in. If the RPM band is wider for the engine it could require less carburetor. If you think that the maximum RPM number is the most important, you would be wrong, according to Dickey.
“The minimum RPM is the converter flash speed, RPM where stick shifted cars pull down to after launch, or on gear changes. Ideally, your bottom RPM number needs to be above max torque RPM since engines don’t like to accelerate from below max torque. You can make all the power in the world at max power RPM, but if it won’t accelerate from your bottom RPM number it won’t run. Having a carburetor sized correctly where it will still accelerate from that bottom RPM number without being detrimental to max power is the best of all worlds.”
The weight of the car is the third item that affects the size of a carburetor that you will use. If you think about it, the heavier the car, the greater the load it will put on the engine, which will also change how the converter hits. Dickey explains how that really ties into the final carburetor selection you will need to make.
“A heavier car will experience additional time in the lower power band increasing the load factor. Increased carburetor signal with reduced sizing or custom boosters is beneficial in most cases when carrying additional weight.”
The fourth factor that needs to be looked at, especially in cars that see both street and strip action is the type of intake manifold being used. Because in applications like this you will see both dual plane and open plenum intakes, sizing the carburetor correctly can become tricky. “Dual plane intakes basically split the carburetor up, increasing carburetor signal. Everything else being constant, the dual plane intake requires additional carburetor sizing. Depending upon application it can be anywhere from 20 to 60 cfm over an open plenum intake,” Dickey says.
The final factor that plays into sizing the carburetor you will be using is the fuel type the engine will feed on. For the most part, the fuel doesn’t play a big role in the size of the carburetor until you start looking at fuels like methanol. “The additional torque generated with methanol and the additional air that’s displaced by this fuel is 2.1 times as much as gasoline, and that requires some additional carburetor sizing,” Dickey explains.
There are other variables that can weigh on the choice of carburetor size you choose, but these are some great baseline items you must consider. Make sure you check out AED’s website for additional information on how to get the correct sizing for your carburetor and see what induction solutions they offer.