Most of us have heard the expression, “one size fits all” and that can usually be taken with a grain of salt. While that might work out okay for a knit cap or socks, it doesn’t work very well at all when it comes to automotive performance parts, specifically carburetors. We’re not referring to bolt patterns, though, we’re talking about cubic feet per minute, better known as CFM.
In the video above from Jegs Performance, it shows us the various types of carburetors available, from two- to four-barrel configurations, and helps to shed some light on how to choose the proper carburetor for your specific vehicle application. The video also provides a formula to help determine what the proper carburetor size is for your engine based on cfm rates, utilizing a couple of parameters from your engine. The formula clearly shows that going bigger isn’t always better – in some instances, it could be worse.
When choosing a carburetor, the first thing that needs to be determined is what flange/bolt pattern the intake has. There are a few different flange patterns for the two- and four-barrel carburetors, and the layout of the carburetor plays into this as well. For this application, we’ll stay strictly with single carburetor applications. The second thing to determine is how you’ll be using the car: racing it at the track every weekend, or taking the car to a local car show occasionally. Still, even if you race your car often, a bigger carburetor doesn’t promise bigger performance numbers.
Most people looking for a performance upgrade for their two-barrel application tend to purchase a four-barrel carburetor. While there are certain adapters to mount a four-barrel carburetor to a two-barrel-style intake, it’s always recommended to get the proper intake manifold for your application instead of using those adapters.
The three common patterns for a four-barrel intake manifold are the square-bore (4150-style), the spread bore (used for older quadrajet or Ford intakes), and a 4500 pattern (which is often used for the bigger, racing carburetors like the Holley Dominator). Some intakes will have a dual pattern for more than one type of carburetor, while others will be dedicated to the 4150 or 4500 style intake.
Upgrading To A Four Barrel Intake
Many times we come across a used intake manifold, or we get one from a friend, and it’s almost too good a deal to pass it up. But if you’re looking to upgrade to a four-barrel carburetor, it is recommended to determine the proper cfm prior to installing an intake manifold. A good deal on an intake could turn out to be a bad investment if you can’t bolt the proper carburetor to it.
For example, if you’ve got a stock, SBC and the power is only about 350 horsepower that revs to about 6,500 rpm. Using the formula that Jegs provides, with an estimated 75 percent volumetric efficiency would leave result in a smaller four-barrel carburetor of about 500 cfm. This would mean that a racing carburetor at 850 cfm with the 4500 flange pattern is probably not going to work for your application. Too much carburetor for your engine will lessen the performance aspect and reduce economy on top of that.
Other choices for your carburetor includes what type of secondaries, whether they are vacuum or manually operated, and finally whether or not you want a choke, either manual or electric. With your formula figured out, you’re ready to hit the Jegs Performance website and then you have even more options to choose from: finish being one of them. But that’s primarily cosmetic, and has little to do with performance, so we’ll skip the math lessons for this one!