Determining the Best Performance Carburetor For Your Application
Yes Virginia, People Still Use Carburetors
Yes, it is true, major automobile manufacturers have not built a new vehicle with a carburetor in over 30 years. When it comes to OE production vehicles fuel efficiency and emissions reductions mandate the precise fuel metering of electronic fuel injection. That said, the carburetor is far from dead. Even with NASCAR and NHRA classes moving to EFI, there are still thousands of sportsman racers that hit the track every Saturday night with good old fashioned four-barrel sitting on top of their race engine.
We cannot count out the countless tens of thousands of hot rod and muscle car enthusiasts that still run a carburetor on their rides as well. Be it to keep the period look, or perhaps the simplicity of it all compared to retrofitting EFI to their vehicles and the ensuring supporting components, wiring, and complexity. Whatever the reason, when it comes to the aftermarket, carburetors are certainly not going anywhere anytime soon and are still an extremely popular way of providing your engine the proper fuel it needs to propel you around the track or down the road to your favorite cruise night.
How Does a Carburetor Work?
There have been complete books written on the subject, so suffice to say, we are a little pressed for room here in our buyer’s guide. If you really want to learn all there is about how a Holley carb, Edelbrock carburetor, or Demon carburetor work, along with modifying them and rebuilding them then be sure to check out our vast collection of carburetor technical manuals we carry. However, for the purposes of our buyer’s guide a brief review of how a carburetor delivers fuel to your engine is warranted.
You may have heard that automotive engines are nothing more than air pumps. This is very true. Another common phrase is “all you need is air, fuel, and spark for an engine to run.” This too is true (without getting into gritty details). So, if an engine pumps air and you have an ignition system fitted to it, you obviously need some way to meter the fuel into the engine for it to run properly, if at all. That is where the carburetor comes in. While only one piece of the combustion puzzle, it is a very visible one right there on the top of the engine. Easy to access and mess with carburetors are often blamed for a lot of issues when it is in fact something else going on.
The main principal of how a carburetor works is something called the venturi effect. As the engine’s piston is pulled down in the cylinder bore it creates a vacuum, pulling air into the engine. This air must pass through the main bores of the carburetor sitting on top of the engine and it creates a high-velocity low-pressure area. This venturi effect pulls fuel into the engine with the air, mixing it to be burned in the combustion chamber. This mixing is called the air fuel ratio (AFR) and the best efficiency is found at 14.7:1 ratio for unleaded street fuel (racing fuels have different ratios). That is 14.7 parts of air to one part of fuel. While this is the best ratio for efficiency, it is not the best for peak torque generation, which is why if you have an AFR gauge installed on your car you would see the ratio change depending upon the load on the engine.
Additionally, the carburetor is used to control engine speed via a throttle linkage or cable, which is connected to the throttle plate(s) in the base of the carburetor. Varying the angle of the throttle plate(s) changes how much air (and fuel) enters the engine, thus allowing the engine to make more power.
Should I Buy a Used or New Carburetor?
The one concern with used carburetors is certainly the age of the unit and how long has it been sitting “dry” on a shelf. Unless it was recently removed from a running engine (similar in specs to your own engine) the carburetor is most likely going to need a full rebuild. Wear that is not easy to fix, such as throttle shaft leaks, base plate warpage, and permanent modifications to carburetor passages, are all things that will have you buying new hard parts and not just your basic carburetor rebuild kit. For that reason alone, unless you have seen the carburetor in use and it is a known good piece, we recommend shying away from the unknown and going with a new carburetor specific to your application and needs by reading further.
What Carburetor Features are important?
A feature you may consider important to you may not be for someone else. Much of this is going to be relevant to the intended use of your carburetion system. A perfect example is a street driven carburetor versus a carburetor used strictly for racing purposes on a dedicated track vehicle. Your street car will be much more enjoyable with a carburetor that utilizes and electric choke system and a full height air horn/choke plate for cold starts along with vacuum secondaries like the Holley 650-cfm unit seen here, whereas in a racing application the choke system is essentially a restriction that racers remove, including milling of the air horn height (or simply buying a carburetor built without these parts) and mechanical secondaries are the norm.
Many features though are desirable no matter the engine the carburetor is installed upon or its intended usage. Common features you will find on performance carburetors today are intended to aid in performance and tuning. Features like four-corner idle control circuits, center hung float bowls with external float adjustments, replaceable/upgradable boosters (or at the least dog leg boosters), vacuum secondary control, non-stick reusable gaskets, and more are now commonly found on new performance carburetors. Some older carburetors can be updated to add these items, depending upon the brand and model, but often it is not worth the investment compared to simply buying a new carburetor with these items standard.
How Do You Determine the Proper Carburetor CFM Required?
If a 650-cfm 4-barrel carburetor works well, an 850-cfm 4-barrel carburetor will be even better, right? This is far from the usual case. Carburetor sizing has long been a “bigger is better” fight that even back in the old factory muscle car days the factory was even guilty of going a bit generous on carb sizing. When you factor in your engine’s displacement, volumetric efficiency, intended rpm use, and fuel requirements, quite often you will be surprised at what all the automotive math out there says is optimum for carburetor cfm needs!
The key to obtaining an accurate cfm rating (besides the basic math we are about to drop on you) is being realistic about your engine’s volumetric efficiency (VE). The VE of an engine is the rating of how efficient said engine is at moving air (remember, engines are just air pumps). Knowing your engine’s VE is not critical, but if it has been calculated by the engine builder then all the better for more accurate calculation results. Otherwise, it is a good general rule of thumb to accept stock engines to have a 70-75 percent VE rating, while performance engines will usually be in the 80-85 percent efficient. Stroker engines and other ultra-high-performance builds will often see 85-95 percent VE. So be realistic with your engine’s VE and you will end up with a more accurate cfm rating.
With your engine’s VE in mind you can then input a few additional figures into this cfm calculator to determine the optimum carburetor cfm for your engine’s displacement and VE.
CFM Calculation
- Divide the engine’s displacement by 2
- Multiply the engine’s maximum rpm by the result from Step 1
- Divide the answer from Step 2 by 1,728
- Multiply the answer from Step 3 by the engine’s VE percentage The answer will be the cfm requirement for the engine’s displacement.
For example, let us calculate the cfm for a 383 small block Chevy stroker engine that will see a maximum of 6,000 rpm and has a VE of 95 percent:
- 383/2=191.5
- 6,000x191.5=1,149,000
- 1,149,000/1,728=664.93
- 664.93x.95=631.68
Our result is 631.68 cfm for our 383 stroker build. Using a 650-cfm four-barrel carburetor will be perfect to feed this engine.
What Carburetor Will Fit My Engine?
There are hundreds of carburetor combinations available to the consumer and the reason is because there are so many induction setups out there. Knowing what you are using for an intake manifold, be it a stock production piece, or an aftermarket performance manifold, will quite often determine your carburetion choices. Besides the physical fitment requirements, you must also consider application. Our 383 small block Chevy stroker we discussed previously could be a lumpy street engine that is going to see cruise nights and an on occasional red-light rip, or it could be sitting in a drag car that is going to see max rpm run after run. Popular crate racing series engines makes the decision a little easier with ready-to-run crate engine carburetor offerings.
Once you have completed your cfm calculations and know the carburetor cfm value you will need to determine whether your intake manifold requires a spread-bore (seen here to the left) or square-bore carburetor base first. This is the primary decision as to proper fitment. Most aftermarket intakes use the standard Holley 4160/4150 square-bore mounting flange, but there are spread-bore manifolds available, plus many GM muscle cars used factory spread-bore intake manifolds as well.
In some instances, you will find both the Holley carburetor and Edelbrock carburetor with dual-pattern base plates that allow mounting to either intake manifold bolt pattern. If you are not sure of what these bore size terms mean our previously mentioned link on carburetor literature will help provide the proper carburetor education you seek; but in a nutshell a spread-bore carburetor has two different bore sizes between primary and secondary bores, whereas a square-bore carburetor features the same bore size for primary and secondary. Lastly, in a pinch we do offer carburetor flange adapters to allow you to mix-and-match bore types or even 2-barrel and 4-barrel carburetors to save the cost and effort of changing intake manifolds.
Following the bore size/baseplate decision will be deciding between vacuum and mechanical secondaries. As you will imagine, the secondary bore/throttle is opened either by a vacuum signal or a mechanical linkage depending upon carburetor model. Both are highly adjustable, the vacuum system with different spring rates (shown here) and the mechanical via linkage adjustments. Application of the secondary system is dependent upon the intended usage of the engine and needs of the driver. Vacuum secondaries are the preferred application for street use because they are only opened as far as the vacuum signal and spring rate will allow, creating a softer transition and only feeding the amount of fuel the engine truly needs. Vacuum secondaries can help when too large a carburetor is installed as well. Mechanical secondaries are often considered a must have for racing, especially oval track racing where you want instant full power coming out of the corner. You will find carburetors with progressive and 1:1 ratio mechanical secondary control, so choose wisely.
Float design is yet another aspect to consider. Most carburetors we offer are available in either a side-hung or center-hung float design. What is the difference? Literally it is the pivot location of the float. On a side hung float the pivot location is on the side of the primary (and secondary) fuel bowls. This is how many OE carburetors were designed and aftermarket side-hung float carburetors are great replacements for street rods and muscle cars that see daily driving, cruising, and the like.
Center-hung floats are more common in performance applications because they are not as sensitive to fuel slosh from hard acceleration, braking, and cornering. Most any carbureted race car will be fitted with a center-hung float four-barrel.
Center-hung fuel bowls are usually a little deeper, so in a dual four-barrel setup you might be forced to run side-hung float bowls for clearance as shown here on this big block Ford. Often your intake manifold manufacturer will spec the two carburetors to use, but we just wanted to mention this potential issue for those running dual fours.
In conjunction with the float type is the fuel feed design (mainly for Holley applications). Side-hung float bowls are fueled by a single inlet and a transfer tube that moves fuel from the primary fuel bowl at the front to the secondary bowl at the rear. A dual-feed fuel inlet is typical of the center-hung fuel bowls and requires a fuel log of some sort to thread into the fuel bowls to feed both bowls at the same time. While it is possible to convert from one fuel inlet style to the other, quite often you would be further ahead by simply upgrading your complete carburetor to the fuel inlet design your application needs.
Lastly, you will need to consider whether you need a choke system and what style to use. Obviously if you are fueling a racing engine a choke is most likely not even on your radar and we offer plenty of carburetors without a choke. However, for those enthusiasts looking to upgrade the carburetor on their street-driven ride, a choke system should be considered standard. Back in the day the choke was nothing more than a manual push-pull cable that the driver set from the dash when starting a cold engine, and yes, we offer manual choke carburetors as well. The choke system was updated over time into a hot-air choke. Hot exhaust was used to heat an air tube or spring that controlled the choke. Eventually this system was further updated to an electric choke system with a bi-metallic electrical heating element within the choke cap itself. Today, electric choke- equipped carburetors are most of the carburetors that we offer.
We know this buyer’s guide has been a long read and if you have made it to the end you should have enough information to begin down the path of choosing your next new carburetor for your project. Whether you are looking for a retro muscle car carburetor, something to fuel your Flathead project, or the right carburetor to put you in the winner’s circle Saturday night, Speedway Motors offers great carburetor brands like Holley, Demon, Quick Fuel, Edelbrock, and more.