Carburetion

Please note that this page is still under construction!

Few subjects will cause as much dissention among engine builders as carburetor selection. The correct size to be used for a given application is as controversial as the choice of manufacturer. The carburetion of this page will present a general discussion of the Y Block’s needs. Next, a number of carburetion options will be presented. Following this, the carburetion section will close with a brief discussion of how to tune your carburetor for street use, emphasizing performance and economy.

Because there are so many choices and opinions on the subject of carburetion, a full discussion of every viewpoint will not be attempted. Instead, some important points will be presented, however the bulk of the technical information will focus on what I consider to be a very good all-around carburetion solution for the Y Block that seeks sharp street performance, but retains the ability to give a gallon of fuel some mercy.

Because the Y Block is a small engine, relative to the many V8 powerplants that are common to the street scene, quad (4 venturi) staged carburetors are recommended for most applications. While large 2v carburetors can supply the needs of mild street engines (examples here would be the 500 cfm Holley 2300 model and the 425 cfm Autolite 2100 model), they will sacrifice the low end crispness offered by the same size 4v carburetor, which, because of the progressively staged primary and secondary venturis, meters fuel more accurately at lower air flow levels.

Carburetor Sizing

One of the most misunderstood concepts known to the hotrodder today is how to size a carburetor for a given application. In reality, this process is somewhat hit and miss. There are formulas that will calculate how many Cubic Feet per Minute (cfm) are displaced by a given engine at a given RPM, and there are formulas that even factor in the volumetric efficiency of the engine (volumetric efficiency is the ratio between the theoretical amount of air/fuel mixture an engine should take in, based on displacement, and the amount it actually does take in.)

Take those formulas and throw them away!

What is really needed is not to match the carburetor’s airflow to a certain formula, but how to get a carburetor that is large enough to provide good top end power, but is also efficient enough to meter fuel correctly when there is very little airflow. The compromise here is that a small venturi provides the strong signal to the metering circuit that is needed for crisp low speed response, but a large venturi is needed for the least restriction and maximum top end power.

It is clear that the CFM rating cannot measure either of these issues directly. First of all, what a carburetor flows at a 1.5” of H2O (or 3.0”, in the case of a 2v carburetor) has nothing to do with the carburetor’s ability to provide crisp low speed performance. Likewise, all you need to do is check out your vacuum gage to see that, when your gas pedal is mashed to the floor, there is a lot less than 1.5” of H2O of vacuum in your manifold!

Furthermore, note that the whole carburetor is rated for a specific CFM flow, and this doesn’t take into account that the flow capacity of a 4v carburetor is divided at low speeds, due to the secondaries being closed. Nor does the flow rating account for
“spreadbore” carburetors, where the primaries are much smaller than the secondaries.

One way to get both low speed and top end performance out of a carburetor is to design it with very efficient venturis; ones that impose only small restrictions at the top end, but provide strong signals for proper low speed operation.

4v Carburetors

		Holley model 4150/4160 This 4v carburetor is the best known and in many ways, the most “Ford” of carburetors. The company was built on supplying carburetors to Henry for his Model T. Holley also supplied the carburetors for the later Flathead V8s, often known as the “Ford 94”. The Model 4150 first made its appearance on the 1957 312 Y Block. The 4160 is a modified version, with a secondary metering system that is cheaper to produce. Items such as jets, power valves, and secondary tuning springs are much more available for these units than for any other. Easy to tune and offering excellent performance, smaller 4150s merit first consideration for Y Block performance.

		Holley model 4010 This Holley carburetor is very similar to the Autolite 4100, discussed below. It differs chiefly in its use of the model 4150 vacuum diaphragm mechanism for controlling the secondaries. Holley parts are used for tuning. Uses the same efficient annular booster venturis as the 4100. A good choice, some polished models were available that would nicely dress up a Y Block.

		Holley model 2140 Holley’s first 4v carb was derived from the Model 2000 2v design. The secondaries were added behind the primaries, which are centered in the airhorn as in the 2v. The secondary jets hang from the bowl cover into the fuel by means of tube extentions, and the long booster venturies convey the venturi signal initiating fuel flow. Secondaries can be mechanical or vacuum controlled.

		Holley model 4000 An improved Model 2140, with conventional booster venturies in the secondaries. A pair of special down tubes conducts fuel from the bowl to the throttle body (several designs were used for these tubes). The high, small float bowl was once a favorite of off-roaders, due to its ability to work at extreme angles. It looks different, is of high quality (brass bushed throttle shafts, for instance) and one or two can really set off a Y: they truly look like they belong! Make sure that the secondary closing linkage is correctly adjusted. The spark control valve used for the Load O Matic distributor can be plugged with a Holley power valve block off plug. Use a tubing connection between the spark control ports of both carb of a 2x4 installation to synchronize the secondary opening of both units.





		Autolite 4100 A Ford design that looks similar to a Holley, this carb is a very underrated choice. Coming in sizes useful for a Y, the 4100 uses Holley jets and power valves. The annular discharge booster venturis are very efficient. A great choice for an all-Ford ride, the 4100 is simple and reliable. 3 sizes were made: a 480 cfm unit uses 1.08”venturis, a 600 cfm unit uses 1.12” venturies, and a rare, 1958-only version came with 1.19” venturis for 650 to 670 cfm. Check the carb base for venturi size numbers.

		Carter AFB A very simple and reliable carb that offers a lot of performance, the AFB is easy to maintain. Jets, metering rods, and power circuit springs are available for performance tuning. For those who don’t want to use a Holley, this carb is a fine choice.

		Carter AVS Very similar to the AFB, except that the control of secondaries is accomplished by a spring-loaded flap, rather than a counterweighted butterflies. A transition between the AFB and the later spreadbore Thermoquad, was used on Mopars.

		Carter WCFB To many older hotrodders, the WCFB is the original performance 4v carburetor. The accelerator pump linkage also controls primary metering rods. Secondaries were controlled by a counterweighted shaft or vacuum diaphragm. Often the carb of choice for small base dual quad manifolds.

		Rochester 4G