Which carburetor to use

A well-designed single-plane manifold, such as this Parker Funnel Web for the Ford small-block, can have great airflow capability compared to a dual-plane intake. This is a port runner schematic for a production Chevy small-block with dual-plane intake. The stock flow numbers are the ones posted in the port openings and then modified on the outside of the runners. I used this stock-port factory intake on my dirt race car. Although rules mandated no porting, I found an extra 20 ft-lbs and 20 hp.

See Chapter 12 for more details. There are two things to note on this intake manifold. Second, it has an exhaust heat crossover arrow. Neither factor is good for output.

Many manifolds, such as this Weiand Street Warrior for a Ford Windsor V-8, are compatible with all stock factory equipment so they are a direct replacement. The downside is that they give up flow potential over a high-rise intake design. If your intended carb is too small for the engine as this is for the inch big-block , a street single-plane makes better results than a dual-plane intake. The Weiand intake was designed to be compatible with all OE installations.

The manifold carb pad was virtually at stock height so making the manifold taller to get a more favorable runner shape was not incorporated in this design. Its air-flow, though, compared to some of the taller, raised-pad manifolds, was down by a measurable margin. The result was that the cubes this engine had could be satisfied in the low and mid ranges but not at the top end. Because the manifold became the prime restriction, the HP Street carb produced as good an output as a all the way to about 4, rpm.

It was only between 4, and 6, that the showed any benefit. Even then it only improved by about 5 hp! The point to note here is that if the manifold is not really strong on flow the need for a higher flowing carb is largely negated. High-Performance Dual-Planes. By the time the new millennia started some serious steps had been taken among intake manifold manufacturers to design and produce a new class of high-performance dual-plane intake manifold.

This was a category that bridged the gap between the typical dual-plane and the one that retained a dual-plane layout but featured a raised carb pad and runner shapes that maximized airflow to the cylinders.

They were also probably the first volume-production manifolds to be designed using computational fluid dynamics CFD. This illustration shows how dual-plane intake runners evolved from a very inefficient shape to the current high-efficiency designs.

Forms, such as these, are typically used to manufacture modern high-rise dual-plane intakes. Given the right carburetion, this type of intake can show extremely good performance increases throughout the entire RPM range. This graph shows why a modern, high-tech, high-flow, dual-plane needs much more carb CFM than an older and significantly less efficient design.

Look at the average flow loss Columns 1 in yellow of the three sample intakes. You see that the current Performer-style intake is far more efficient, so it reduces head flow by much less than the stock intake. Shown in Columns 3 red is what happens to the flow when a carb is installed on the intake: The intake flow is reduced by a smaller amount on the inefficient stock intake. This is why current high-efficiency dual-plane intakes run best with more carburetion than might be expected.

A high- performance, dual-plane intake cannot deliver much increase on an otherwise stock smog- gear-laden engine such as this Chevy The blue lines represent the stock factory manifold. In terms of carb CFM, these intakes require a serious amount of consideration when it comes to choosing a carb.

In turn, this means that an engine so equipped is far more sensitive to carb capacity. On a single-plane intake, all cylinders see all four barrels of the carb to draw on. But consider this: Given a dual-plane intake with efficiently flowing runners, the carb flow seen by any one cylinder of the engine is half of what it is on a single-plane intake. This means the cfm carb that worked so well with a good single-plane manifold looks more like a to cfm carb.

With such intake manifolds the required carb capacity can go way over what you might ordinarily expect see Figure 6. Some high-performance dual-plane intakes have the divider between the plenums cut away to form a communicating passage between them see Figures 6. The purpose of the cutout is to allow any one cylinder to see more than just the two barrels of the carb immediately over the plenum. This has the effect of improving output at the top end. The drawback is usually reduced low-speed torque and idle to low-speed cruise vacuum.

This cutout brings about various consequences. In effect, it turns a dual-plane manifold into a single-plane manifold with much longer but more tortuous ports. In other words, the cutout turns a potentially good dual-plane into a substandard single-plane.

That factor may not be good as, in part, it indicates that if the cutout were necessary then maybe you should have chosen a streetable single-plane intake. Also, if the cut-out helped top-end output, it is a sure sign that the carb is too small for the application. There is most certainly a delicate balance here. My thoughts are that it is better to use a slightly bigger CFM carb without a cutout in the intake than a slightly smaller carb with a plenum cutout in the manifold.

The relatively basic test engine has a set of Gil Mink—ported World Products Sportsman iron heads with a The cam is one of my hot street-spec hydraulic flat-tappet grinds. The point, however, is that if the typically recommended carb had been used, the output peaks would have been ft-lbs of torque and hp. A couple of points to note to validate the results here are that the intake had no plenum cutout and the torque curves of all three carbs were virtually identical up to 4, rpm. This Weiand high-rise air-gap-style intake does not utilize a plenum cutout, which is commonly at the point indicated by the upper arrow.

The bulge indicated by the lower arrow is an attempt to equalize the plenum volume seen by each pair of carb barrels. With no cutout, this manifold is far more sensitive to carb CFM.

This particular high-performance intake features an inter-plenum cutout. This slightly compromises idle quality and reduces torque at the bottom of the RPM range. However, it does make the intake a little less sensitive to carb CFM. If you need to keep overall costs down without compromising quality, a high-flow dual-plane intake with a plenum cutout shown used with a vacuum secondary Holley works well.

In such instances, a bigger carb on, say, a shows less of a gain over the Dual-plane, air-gap intakes, such as this one for the Ford small-block, really work well. I had a Holley body, a big-butterfly base plate, and the goal to build as high an airflow carb as possible without getting into anything too drastic. I dressed up the boosters and venturis and then reworked the butterflies and shafts. These modifications delivered cfm. This worked well on one of my inch, dual-plane intake, big-block Chevys.

To be able to make any kind of top-end output on a carb called for some serious attention to the size of the inter-plenum cutout. By progressively enlarging the cutout, the engine produced 35 hp more than having no cutout.

There are times when a cutout for carb barrel sharing between the cylinders becomes essential, but again, it is because the carb for the application is just too small.

A good example is the use of a carb on a dual-plane intake that has to feed a inch or more engine. However, at the end of the day, the carb so often used is woefully short of adequate CFM capacity. For these intakes to make a decent top-end output, their plenum cutouts extend to the very limits of what the wall between the two plenums allow.

Let me remind you that if the plenum is cut out, it is progressively turning a dual-plane intake into a single-plane intake, but without the flow advantages of a single-plane. If you are in a position to perform tests, my advice concerning dual-plane intakes is to use as large a carb as possible first.

If that does not satisfy the top-end output needs, start slotting the divider. It helps if you have access to a dyno. Much of the popularity of the single-plane, single 4-barrel carb is the fact that it works well for the money spent. However, it might just leave you to wonder if and by how much a pair of Holleys on a tunnel ram would better a single 4-barrel setup. It is very often claimed by many who should know better that a tunnel-ram-style intake is for the race track only.

If you consider nothing other than installation hassles and the inevitably large hood scope, that viewpoint is largely correct. Also, mileage was better than might have been expected considering the strong bias toward performance. For this test, the carbs used a four- corner idle setup. This Chevy big-block intake manifold is PN This Chevy ci big-block sported two tricked-out Holley carbs. Idle was a smooth rpm. This graph shows what you can expect in the way of additional output by utilizing a pair of mildly modified cfm 4-barrel Holleys versus a single 1,cfm unit.

There seems to be a renewed interest in the three 2-barrel performance street intakes that were popular during the s and s. The only one that might be difficult would be whether the carburetor should use vacuum or mechanical secondaries. This refers to how the second set of throttle blades are actuated. The primary side is controlled by the throttle linkage. A mechanical secondary carburetor uses a mechanical link that literally opens the secondary throttle blades as the throttle linkage is pushed towards WOT.

Vacuum-operated secondaries work a little differently. This system uses air velocity through the primary throttle bores to open the secondaries only after there is sufficient airflow from the engine. This may not seem like an advantage, but in fact, at low engine speeds, slamming open four large throttle blades can result in low air velocity and sluggish initial performance.

A vacuum secondary carburetor maintains high air velocity through primary venturis until the engine needs additional airflow. Then the secondaries can open, making it completely tunable. So for the street, a vacuum secondary carburetor can improve drivability and throttle response. One of the newer additions to the Holley starting lineup is the Brawler carb that offers some performance-enhancing additions even to the smaller cfm version carburetors like this electric choke cfm version.

These are also dual inlet carbs with either mechanical or vacuum secondary actuation and billet aluminum metering blocks. If choosing a street-driven carburetor for the first time, it should be equipped with an electric choke. This mechanism will automatically apply the primary side choke blade in cold weather, producing a richer mixture combined with a fast idle when cold to prevent stalling.

Once warmed up, the electric choke will pull the choke blade open for proper warm-weather metering. All you have to do is add a switched volt wire to the choke housing.

Mechanical secondary carbs use a simple link to connect the primary and secondary throttle shafts and can be identified by accelerator pump nozzles on both the primary and secondary sides. This simple program will supply you with a large list of potential fuel mixers that would best fit your needs.

We did exactly that and inputted a mildly-modified, normally-aspirated not supercharged ci small-block that would run on gasoline along with vacuum secondaries and an electric choke.

With this data, the Holley program instantly came up with the appropriate recommendations for Holley, Quick Fuel, Brawler, and Street Demon carburetors ranging in sizes from cfm to cfm. Among these carburetors are many with only slight variations in options and features.

But the beauty of the Holley line is the incredible array of options. For example, the basic Holley that can trace its lineage back to the early s is made from a very durable zinc material with a zinc dichromate finish.

But there are also aluminum versions that might be of interest due to their more highly polished presentation and color options. For those more interested in performance, Holley offers carbs like the Brawler that includes interesting amenities like responsive, down-leg boosters, billet aluminum metering blocks, and screw-in air bleeds that can make tuning much easier.

These carburetors utilize a jet and metering rod combination that some tuners feel can offer part-throttle tuning opportunities. Among the other prospects is a choice between a mechanical or electric choke. For the daily-driven street car, the electric choke option is really the way to go. These carburetors use a round, black plastic cover that contains a bi-metallic spring that when connected to switched bolt power, allows the spring to close the choke valve over the primary side of the car when the weather is cold that also increases the idle speed to a fast idle.

As the engine begins to warm, the electric heating element expands the bi-metallic spring which pulls the choke blade to its off position. The system is self-contained and easy to adjust if necessary. Vacuum secondary carburetors are easily identified by their large diaphragm housing hooked to the secondary throttle linkage. Most entry level Holleys employ a pair of idle mixture screws on the primary metering block. However, some carbs like the Quick Fuel SS series offer four idle mixture screws that offer some advantages in stabilizing idle mixture.

The Street Demon uses a one-piece secondary throttle blade called the Goggle Valve Secondary GVS that uses a single, uniquely-shaped throttle blade that delivers over twice the airflow of the primary side.

Another consideration is single versus dual inlet configurations. For smaller cfm carburetors, the single fuel inlet connection makes installation easy. But if you desire a more aggressive looking carburetor, Holley offers multiple adaptations like the Quick Fuel SS series that uses dual inlet bowls that require a dual inlet fuel line.

The SS series carbs also offer other small technical enhancements that might make them attractive for the more performance-oriented enthusiast. The right idea would be to match the carburetor to the engine's breathing ability or volumetric potential. The engine's volumetric efficiency is a measurable value and with a correctly matched carburetor, you will improve the performance of your engine. If you try this formula yourself, be honest with how much rpm the engine will see. A vacuum secondary carburetor is usually most fuel efficient when it's used on street driven cars that have automatic transmissions.

Vacuum secondary carburetors have one accelerator pump and work off sensing the engine load, which progressively open the secondary butterflies as rpm increases. A mechanical secondary carburetor uses mechanical linkage to open the secondary butterflies and most also have dual accelerator pumps a. Mechanical secondary carbs are best used on applications with more radical camshafts, high rpm racing situations, and manual transmissions.

Will a bigger carb make more power? Recently the Engine Masters show on the Motor Trend On Demand site did a comprehensive test to see just what differing sized carbs would produce. Here is what they learned. A better than average HP street performance engine.