Few parts are more synonymous with a classic American hot rod than the carburetor. Ironically, few parts are more misunderstood than the venerable carb, which still came equipped on some vehicle models as late as the 1990s.
The carburetor, which is essentially an air pump that feeds your engine with air and fuel, remains a popular choice for many gearheads who get their automotive parts through Summit Racing. According to our expert technical staff, it also remains a recurrent source of tech questions. That’s why we’ve put together a Summit Racing Quick Flicks video dedicated to the basics of the carburetor, including:
- What is CFM?
- What is the difference between mechanical and vacuum secondaries
- How to choose the right CFM size for your vehicle
- Formula for selecting the right carburetor CFM
Find the perfect carb for your ride—the search starts with this installment of Summit Racing Quick Flicks:
Hi, I’m Mike and on this installment of Summit Racing Quick Flicks we are going to talk about selecting the right carburetor for your application. Before we can determine what size carburetor is right for an engine application, it is important to understand what a carburetor does.
A carburetor is an air valve that is going to control the amount of air and fuel entering the engine. The positioning of the butterflies in the carburetor dictates how much air and fuel is entering the engine which in turn is going to go ahead and control the RPM of that engine combination. In turn this is also going to have a drastic effect on the engine’s efficiency as well as the amount of power that is capable of producing. It is important that all this is correct for your engine combination. This way we don’t have a situation where the engine either under performs, gets poor fuel economy, or fouling spark plugs Basically, it gives you the best running engine possible.
So what is CFM rating? CFM stands for cubic feet per minute. When you hear CFM, you automatically assume this is relating to fuel supply. Well, that is only part of the equation. When we look at the CFM rating of a carburetor, we are talking about cubic feet per minute of air and fuel that is passing through the carburetor. The higher the number gets relates to the ability of this carburetor to supply more air and fuel to that engine type.
There are three factors that come into play when selecting the correct CFM carburetor for an engine. The three things we are going to look at are: engine cubic-inch displacement, max RPM of the engine, and we are going to look at how the vehicle is going to be used— whether it’s going to be used for a street application or a drag race application. All three of these combined are going to give us the end result as far as what size CFM carburetor is right for our specific situation. Now, what you will typically notice is that the street carburetor selection—even though the engine combination may be the same in cubic inch displacement and in max RPM—the carburetor sizing will be different between if it was going to be used on a drag race vehicle in comparison to a street vehicle.
Because in a street application the carburetor only needs to run at roughly 80 to 85 volume efficiency which basically means that the carburetor does not have to be able to get the engine to its maximum RPM consistently which in turn is going to give us better throttle response and better fuel mileage. Whereas in a race application were going to run at 110 volume efficiency which means that we are going to go ahead and exceed the CFM needs of the engine by giving it a slightly larger carburetor than it needs. This in turn gives us the full potential of the engine but on the back side of that, what that does hurt is fuel economy as well as low-end throttle response in some specific applications.
The formula for selecting the proper CFM carburetor for an application is as follows: Cubic-inch displacement multiplied by Engine Max RPM multiplied by Volume Efficiency divided by 3546 equals the recommended CFM rating. This can change depending on vehicle application because the volume efficiency number will change in this formula. For example, if we had a 302 cubic-inch engine that had a max RPM of 6000, and it was a street application, which means we would use .85 for a volume efficiency number, this would give us a CFM rating of 445.65 in comparison to a race car with the same engine but instead of using 110 percent of volume efficiency this would give us a CFM rating of 576.73.
A vacuum secondary carburetor is commonly used in street vehicle applications and it is also common to vehicles that have an automatic transmission. The reason for this is the secondaries will be opened off of engine load. You will notice as I open up the throttle completely that the secondaries do not open at all when the carburetor is not actually installed on the engine. That is because the secondaries are going to be actuated by engine load or engine vacuum as you can see here as I push this lever on the side of the carburetor. The reason why this is so desirable for a street car is because it is typically more responsive and is going to supply better fuel mileage as an end result. In comparison, a mechanical secondary carburetor is common to race applications and vehicles with manual transmissions. The reason for this is that the secondary butterflies on the carburetor are connected to the primary butterflies via linkage set up, and as throttle position changes, the butterflies are directly related to one another and open via a mechanical linkage set up which is on the side of the carburetor here. This makes this type of carburetor more consistent, especially for race applications and a manual transmission application is also valuable due to the fact that a manual transmission vehicle typically exerts a different type of load on the engine in comparison to an automatic transmission vehicle.