An engine needs more air flow capability to make more power but only needs the proper fuel mixture to maintain good fuel economy. Carburetor upgrades are similar to ignition upgrades in that you should first consider trying an OEM solution first. Often, manufacturers would equip engines with a variety of carburetors. This will involve some homework on your part in understanding the carburetion. You will need to find a suitable donor car which will involve some more homework. An excellent source of information are on-line discussion forums for your vehicle. Chances are that someone has already done this upgrade and would be happy to share his experience with you.

A carburetor upgrade will likely involve a new intake manifold, throttle and transmission linkages, fuel lines, and air cleaners. In some ways this upgrade is trickier than an ignition upgrade because under hood clearances must be considered. You don't want to install a slick-looking carburetor only to find that you can't close the hood. Going the OEM route generally minimizes the amount of fabrication you need to do to make the upgrade work.

An excellent upgrade for a 1960s era American vehicle with V8 engines is to change a two barrel carburetor (2bbl) to a 4 barrel (4bbl). A barrel is another name for a venturi, which is what the carburetor uses to mix the fuel with the air. OEMs used the 2bbl because they were cheaper to produce than the 4bbl and they could charge a premium to upgrade to the 4bbl. The 4bbl did not necessarily cause the engine to use more fuel than the 2bbl unless you wanted it to. This is because most 4bbls were progressive (or staged) meaning that the primary (front) barrels were mainly used under cruising conditions. The secondary barrels were closed until the primaries almost fully open and then both would fully open at the same time. Interestingly, the size of the primary barrels of a 4bbl are typically smaller that the barrels of a 2bbl.

Similarly, for a six cylinder an excellent upgrade is to go from typically a 1bbl carburetor to a 2bbl or even a 4bbl carburetor. Six cylinder engines were typically economy options for older American vehicles. A side benefit of having a six cylinder engine is that these cars would handle better than V8-equipped cars because a better front to back weight balance. To keep costs down, OEMs equipped their cars with cheap restrictive induction systems. A simple carburetor upgrade makes a world of difference in improved drivability while maintaining fuel economy and handling.

Once you have identified the parts you need, you will only need to visit to the local junkyard if you're lucky. You might need to once again turn to the discussion forum for help in locating the parts you need. It's also a good idea to learn about carburetion. Excellent books I have read about the subject are:

Title Author Publisher
Carter Carburetors Dave Emanuel SA Design Books
Holley Carburetors & Manifolds Mike Urich & Bill Fisher H.P. Books
Rochester Carburetors Doug Roe H.P. Books

Often, only the aftermarket performance parts manufacturers have parts that you will need to upgrade your carburetion system. Offenhauser (no web site available), for example, produces intake manifolds for a wide variety of vehicles, often for vehicle applications considered obscure today. Clifford Performance produces a lot of performance parts for older American six cylinder vehicles. These manufacturers still make cool-looking performance intake manifolds. The design of the intake manifold you should use depends upon the intended use of the car and this decision should not be made lightly. A selection based on looks alone could result in a car with poor throttle response.

Modern aftermarket carburetors are a good alternative to OEM because they can be easily calibrated to the engine in your vehicle. Some manufacturers to consider are Weber, Edelbrock, and Holley. OEMs sometimes used aftermarket suppliers to supply carburetors for their standard production engines so making jetting changes could be quite easy. Edelbrock now makes the Carter AFB and AVS so metering rod and jet part numbers are different and you will need to verify the parts you need. General Motors made a sophisticated 4bbl known as the Quadrajet. People are sometimes quick to replace them with Holleys simply because they don't understand them and don't want to take the time to do so.

The formula for determining the air required by a 4-stroke engine is:

CFM = (engine size in cubic inches) x (maximum RPM) / 3456

Let assume that we are going to upgrade a 225 CID engine is going to run at a maximum of 5000 RPM. With 100% volumetric efficiency (VE), we will theoretically need 325 CFM but, since there are pressure drops in the induction system, we will only need 244 CFM with a VE of 75% and 276 CFM at 85%. Two barrel carbs are rated at 3.0 inches of mercury (@ 3.0" Hg) pressure drop while four barrel carbs are rated at 1.5 inches.

The relationship between the air flow rating in CFM between the two pressure drops is:

(Flow @ 3.0" Hg) = 1.414 x (Flow @ 1.5" Hg)

The original Carter BBS 1bbl carb was rated around 200 CFM (~ 141 CFM @ 1.5" Hg). The two barrel BBD carb that came with the Super Six package was rated around 285 CFM (~ 202 CFM @ 1.5" Hg). A summary of the flow ratings of the carbs people have used or have talked about using on slant six cars:

Carburetor CFM Flow Rating @ 1.5" Hg CFM Flow Rating @ 3.0" Hg
Carter BBS ~ 141 ~ 200
Carter BBD (/6 model) ~ 202 ~ 285
Edelbrock 1403 500

(no info about primary flow yet)

707

(no info about primary flow yet)

Holley 1920 99 - 156 140 - 220
Holley 1920 (/6 model) 131 185
Holley 1945 120 170
Holley 2300 Series 156 - 442 220 - 625
Holley 7448 248 350
Holley 4412 354 500
Holley 2280 184 - 240 260 - 340
Holley 2210 184 - 240 260 - 340
Holley 2245 212 - 226 300 - 320
Holley 2305 248 & 354 350 & 500
Holley 5200 163 230
Holley 0-8007 390
195 (primary side only)
551
276 (primary side only)
Holley 0-1850 600
300 (primary side only)
848
424 (primary side only)
Rochester Dualjet 161 227
Rochester Quadrajet 750
161 (primary side only)
1061
227 (primary side only)

The flow ratings at the two pressure drops are simply a guide to help people to match a carburetor to their engine. A small carburetor will more closely result in a 3.0" Hg pressure drop while a large carburetor will more closely result in a 1.5" Hg pressure drop. A carburetor that has a lower pressure drop results in an engine with a higher volumetric efficiency. The important thing to remember is that the carburetor must provide the engine with as close to the optimum fuel mixture for the operating condition as possible. If you are using a staged carburetor, the maximum airflow through the primary (or primaries) should be less than the maximum required by engine. If the primary airflow is greater, the secondary airflow will be minimal. If the primary airflow is too large, small changes in throttle position result in large changes in flow, which results in poor drivability. Also, a too-large of primary barrel size results in the carburetor cruising with the idle circuits because there is not enough airflow to start fuel flow in the main fuel circuits.

A wide variety of carburetors can be used on a 225 CID engine. The conventional 4bbl upgrade for this engine is the 390 CFM Holley #0-8007 and the 500 CFM Edelbrock #1403. These carbs run on their primary barrels under cruising conditions. However, as air flow increases (and vacuum decreases), the secondaries open. It is possible to shut secondary airflow off completely in Holley vacuum secondary carburetors with a stiff-enough secondary diaphragm spring. The Edelbrock AFB-style carburetors rely on a counter-weighted air valve to control flow through the secondary barrels. The properly sized 4bbl carburetors should theoretically provide better fuel mixture control and better drivability compared with an equivalent (and relatively large) 2bbl carburetor.

For those of you with a Holley carburetor and need to tune it, you can find tuning parts at Holley Carburetor Components and the Holley Carburetor Tech Info will give you a basic understanding of the operation of a Holley carburetor.

Another carburetor that people often overlook is the Rochester QuadraJet. Most QuadraJets are rated for 750 CM which is far more than the what a 225 CID engine needs. However, since a Rochester DualJet is the primary half of a QuadraJet, we can see that the primary flow rating of the QuadraJet is 227 CFM @ 3.0" Hg. Using the four barrel pressure drop, the QuadraJet primary flow rating would then be 161 CFM @ 1.5" Hg. This is less than the flow rating of the Carter BBD carb (a popular performance upgrade originally supplied by Chrysler) and less than the rating of the conventional Holley 390 CFM performance upgrade. Therefore, for normal operation of the slant six, the QuadraJet is not oversized. The flow through the large secondary barrels are controlled by a spring-loaded air valve which also serves as the pressure drop to provide the air flow signal for metering the correct amount of fuel. As long as the secondary barrels can begin mixing fuel properly when the airflow is sufficient, there will be a smooth transition and the engine will get as much fuel mixture as it requires. A Carter ThermoQuad carburetor would perform similarly but parts are much harder to find for it.

In upgrading their induction systems, especially with in-line six cylinder engines, people often install both a 4bbl carburetor and headers. The problem they run into with this upgrade is that the intake manifold no longer has a source of heat. Gasoline requires heat to evaporate and this evaporation can greatly cool the airstream, often enough to cause condensation on intake manifolds. Heat is necessary to help keep the fuel mixed in with the air stream. At low loads, the high vacuum manifold vacuum keeps the fuel vaporized, which results in good fuel mixture distribution among the cylinders. However, when the load is high like at wide open throttle (WOT), the low manifold vacuum cannot vaporize the fuel and much of it puddles on the manifold floor causing the fuel mixture to become lean and the engine to loose power. The car manufacturers have always equipped their carbureted engines with intake manifold heat for this very purpose. Only all-out race, propane-fuelled, and multiport fuel-injected engines can really benefit from cold intake manifolds.

For street engines, make sure that the manifold heat control system or heat riser system is working properly. Clifford Performance offers water-heated intake manifolds, with the heater being an additional item to be purchased from them. Be sure to use their intake heater. Offenhauser makes intake manifolds that are designed to use the OEM exhaust manifold. For in-line 6-cylinder engines, you can find an water-heated intake manifold heater kits here:

In case anyone is wondering, when I was thinking about a performance upgrade for my car several years ago, I also considered the Super Six route (using the Carter BBD 2bbl), the Holley 500 CFM 2bbl, and Holley 390 4bbl. There was no Proline Fuel offering dual 2bbl Webers and Doug Dutra hadn’t yet made the reproduction Hyper Pack intake manifold. I decided in the end that the 625 CFM Carter AFB was what I was going to try because the primary barrels had approximately the same flow capacity as the BBD and the air valves on the secondary barrels would allow more air if required. Setting up the metering rods and jets seemed to be more straight forward that the jets on the Holley 390. It also helped that someone had a surplus AFB lying around that I could have for free. After thoroughly reading the Direct Connection Slant Six Racing Manual, I was concerned about fuel distribution with the 4bbl carb. I decided to machine a new baseplate to mount the carb sideways with the throttle shafts parallel to the engine. From what I can tell from the spark plugs, my fuel distribution seems to be symmetrical and none of my cylinders run rich or lean. I have also reduced the bog I used to have on left turns by raising float level. The AFB also gave me better gas mileage than the original Carter BBS 1bbl carb.