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Carbs: How Big Is Too Big?

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Selecting the right carb is not an exact science. No…it’s more akin to nature’s complex tapestry of life, an intricate mosaic of complex variables combining in myriad ways to produce often unexpected results. There is logic and order behind the mystery, but the patterns are not the cut-and-dried solutions of engineering textbooks; instead, they’re derived from the school of real-world experience as learned through trial and experimentation by a leading carburetor expert like Barry Grant, owner and founder of Barry Grant Inc., parent company of Demon Carburetion.

“Formulas Are Useless!”

Grant doesn’t believe in formulas—and for good reason. Textbooks tell us you can accurately select the proper carburetor size based on a relatively straightforward formula (see the formula at the right) that takes into account engine displacement, max rpm, and volumetric efficiency (VE).

Plugging some actual numbers into the equation, what does it recommend for a 350 engine turning 6,000 rpm at 100 percent VE (see the second formula)?

607.6 cfm?! Virtually no one—whether the original equipment manufacturer, aftermarket tuner, or racer—actually installs such a small carb on a high-performance 350. In the real world, everyone knows these engines make more power with larger carbs. Yet dyno-tests show the formula is an accurate reflection of an engine’s airflow needs. The equation breaks down as a realistic carburetor size selection tool because carburetor flow ratings (in cfm, or cubic feet/minute) are taken at an arbitrary vacuum drop—3.0-inches Hg for two-barrels; 1.5-inches Hg for four-barrels—and there’s no guarantee that, at max-rpm wide-open throttle (WOT), any given engine actually sees the theoretical vacuum drop for which the carburetor is rated!


Pulling Vacuum Sucks

On the other hand, if a carb really does pull 1.5-inch-Hg (or more) vacuum at WOT, it has become a restriction. The carb is actually too small to let the engine realize its maximum power potential. That’s because the greater the pressure drop (the higher the vacuum reading) across or through the carburetor, the lower the air density is inside the intake manifold and combustion chamber. Racers like to see no greater than 0.5 to 0.75-inch-Hg vacuum on the top end. But race cars have high-stall converters and steep rearend gears, and most of them are lighter than the average street car. Racers don’t mind recalibrating the carb on the spot for changing track conditions. They are unconcerned about low-end driveability. That’s why all-out racers, when not restricted by the rules, run huge carburetors—the bigger the carburetor, the lower the pressure drop across it at any given airflow.

Driven to Compromise

It’s a different story for street cars and dual-purpose street/strip pack- ages. For these combos, the limiting factor in carburetor size selection is on the low-end side of the fuel curve. Will the carb meter correctly at lower airflows? Will it have good part-throttle driveability in the engine’s normal operating range? How will it drive in the winter, desert, and mountains?

As in life, so it is with carbs: Compromises are called for—a fine balance. Not too big because you’ll lose driveability. Not too small or the carb becomes a major bottleneck. For most hot, dual-purpose cars, pulling about 1.0-inch-Hg manifold vacuum at WOT, max rpm on the dyno isn’t far off. But there are no guarantees. It’s impossible to know for sure how a carb will perform in a vehicle based on how it did on an engine dyno. Every combo behaves differently; what works in a featherweight car with a manual trans and 5-series rear-gears ain’t gonna cut it in a 3,700-pound ride with a mild converter and 3.23:1 rear gears. As Grant puts it, “You’ve got to do lots of empirical work and go through lots of trial and error. It really takes thousands of carbs before you get an intuitive feel for what’ll work on a given application.”

Grant Us the Knowledge

Fortunately, you don’t have to wait until you’re old and gray to find the answer—you can take advantage of Grant’s gray hair instead. His guidelines are based on years of experience. While they apply specifically to Demon carbs, the general principles are applicable to other carburetor makes as well. The basic parameters revolve around engine load versus engine size as expressed in cubic inches. Compression ratio, vehicle weight, distributor mechanical advance curve, cam duration as it factors into actual running (as opposed to static) compression, trans and converter, and final drive ratio all factor into carb selection.


All things being equal, a bigger engine requires a larger carb. But for any given engine size:

• Higher rpm requires a bigger carb

• Higher horsepower requires a bigger carb

• Higher compression ratios require a bigger carb

• More distributor mechanical advance requires a bigger carb

• A manual-trans car can use a larger carb than an automatic-trans car

• Steeper (higher numerical) rearend gears tolerate a bigger carb

• Lighter cars can use a bigger carb

• Heavy cars need a smaller carb

• Too large a cam for the application requires a smaller carb

• With an automatic-trans car, too low a torque-converter stall-speed for the application requires a smaller carb

• Mild (lower numerical) rearend gears require a smaller carb

• Low compression requires a smaller carb

Specific recommendations for Demon carb selection can be found in the carburetor selection chart (see the "Demon Carburetor Selection Guide" sidebar.


Manifold Destiny

Another important factor influencing carb selection is intake manifold design. With its divided plenum, a typical dual-plane intake provides a stronger signal to the carb than a single-plane manifold. Grant says a 350 Chevy with a dual-plane intake can usually use a 750 carb; if it’s running a single-plane, a 650 carb will probably get the car down the track quicker. One way to crutch a single-plane/large-carb combo suffering from insufficient signal is to use a four-hole carburetor spacer.

Although not recommended, some people just have to run a tunnel-ram on the street. That’s possible with careful tuning and proper carb and manifold selection. Grant reports that two 650s work well, provided you use a manifold with long runners to maintain low- and midrange torque. “Long” in this case means at least 6-8 inches. The larger the plenum volume, the less the signal, and the smaller the carb(s) needs to be.

Secondary Opinions

If you are reluctant to trade top-end performance for all-around driveability, there are several cheats that will allow a larger-than-normal (for the given application and combo) carb to do a satisfactory driveability job. One important method is secondary actuation.

Normally a mild converter, weak rearend gears, and/or a heavy vehicle will call for a smaller carb to retain decent low-end performance. But we know the smaller carb restricts power upstairs. One possible solution: Run the larger carb, but with vacuum (instead of mechanical) secondaries. Vacuum secondaries won’t open until the engine needs the extra airflow. Assuming the vacuum secondaries are properly tuned with the appropriate-tension spring, the engine won’t bog even if you punch the throttle wide-open at low speed. But when there’s sufficient primary airflow to allow the diaphragm to open the secondaries, the engine is ready to accept the extra capacity. Grant says vacuum-secondary carbs work particularly well if running under 3.55:1 gears, the car weighs over 3,500 pounds, or if you have 8.5:1 or less compression.

For just about any other combo, Grant recommends staying with the twin-squirter, mechanical-secondary carb. “If the engine withstands a mechanically actuated secondary without bogging, the rpm will rise quicker and performance will be better than with a vacuum secondary,” says Grant. Note that twin-squirter carbs are much more finicky about proper sizing than the more forgiving vacuum-secondary models, which is why there are more different twin-squirter sizes available in comparison to vacuum-secondary cfm ratings.


Booster Shots

Yet another solution to the big carb for power versus smaller carb for driveability dilemma is to use a different-design booster venturi. Demon offers two different styles on some carbs: downleg boosters and annular boosters. Annular boosters are more responsive than downleg boosters. They pick up the vacuum signal quicker and start flowing sooner. While annular boosters offer more bottom-end driveability, they run richer on the top end and may siphon fuel out of the float bowls. The annular booster also decreases the carb’s ultimate airflow potential because it has a larger physical presence within the venturi. For example, the Race Demon 950-cfm carb with annular boosters has the same venturi size as the 1,050-cfm Demon with downleg boosters.

Generally, annular boosters work best on 850-or-larger-cfm carbs to restore bottom-end performance on small-displacement engines or heavy vehicles. On lightweight cars and/or large-cubic-inch engines where bottom-end response isn’t a problem, you’re usually better off with a downleg-style booster.

Integration Now!

Hopefully, the preceding has given you some fuel for thought on selecting the proper-size carburetor. The point to remember is that there is no magic answer; your fuel-metering device must be an integrated member of a total engine, vehicle, and overall drivetrain combo working harmoniously to deliver performance under all desired conditions. Of course, selecting the proper-size carb is just the beginning in the same sense that choosing the right golf club for the next shot is only one small (but necessary) ingredient of getting the ball into the cup.

Whether it’s golf or carburetors, technique and skill are nearly as important as proper equipment selection. So next month we’ll cover the other half of the carburetor equation. Now that you have the right carb in hand, how do you tune and modify it so it can realize its full potential?



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