Four-Barrel-Carburetion Tech Tips - Carbin' Out

As the driver steps on the gas, the throttle blades open and the carb transitions to the main metering system. With the engine pulling in more air, the draw is sufficient enough to pull fuel from the bowls and into the boosters. As the throttle is opened further, there is a reduction is manifold pressure (less vacuum) and airflow increases. To prevent a lean condition, the carburetor has an accelerator pump, which squirts a predetermined amount of fuel directly into the venturis through a set of squirters.

In addition, many carburetors use a power valve or power enrichment system to supply additional fuel during wide-open throttle conditions. Professionals say a good starting point for tuning the power valve is to have one that is rated to open at about 1-1/2 to 2 inches of vacuum below the vacuum at idle. The power valve is equivalent to about six to eight steps up in jet size, and some racers block off the power valve and simply run larger jets when the engine is run almost exclusively at WOT. As you have probably figured out, a larger venturi will equal more flow; that's why larger-displacement engines require larger carbs.

Racers are most often concerned with airflow capability. Airflow is usually measured in cfm (cubic feet per minute), and this rating dictates how much air can pass through the carb. The carb must not cause a restriction (by being too small), but it can't be too big either or the velocity of the incoming airflow will suffer, as we discussed before.

Experienced engine builders and tuners will tell you the first step in finding maximum performance is to purchase the correct size carburetor for your application. Common sizes are 600, 650, 750, 780, and 850. Most street/strip engines will run best with a 600- to 750-cfm unit, however, if you've researched the latest and greatest, you know there are many different-style carburetors on the market-and the size alone might not tell the whole story.

If good street manners are important, give driveability precedence over all-out performance. If your car must be used in all weather conditions, as well as at the track, make certain the carb has a choke and consider vacuum secondaries. A vacuum secondary carburetor relies on the pressure drop in the manifold to open the secondary butterflies rather than a manual linkage and will generally work better on street cars, especially those with automatic transmissions.

"That's because the secondary butterflies open upon demand from the engine," says MM&FF's tech Q&A guru, "Yo" Ken Miele. "Vacuum secondary carbs are easy to tune because all you have to do is swap a spring to make the secondaries open quicker or slower. This can also help increase fuel economy, even on a high-performance engine."

It's easy to be overly concerned with peak horsepower, which is normally measured at peak rpm. But smart builders will be gunning for average power over the entire useable powerband. For instance, let's take a hypothetical look at two different cars, in which we'll bolt the same engine. Car "A" is a 2,900-pound Mustang LX that's backed by a T5 stick and 4.30 gears. The engine is a 450hp 347-inch small-block with 10.0:1 compression. This combo will be used on the track only, so low-rpm power is not a big concern, and therefore, it's likely to run best with a mechanical secondary, 750-cfm carb that provides great power from 4,500 to 7,000 rpm.

But, if we put that engine in a street-driven, 3,600-pound GT with an automatic, a street converter, and 3.55s, it would probably be a dog off the line. In this case, you might opt for a smaller carb to provide better low-rpm throttle response and midrange power, because that is where the engine will operate. Of course, either owner would want to hear that his engine is making the most horsepower, but dyno numbers and driveability don't always go hand in hand. In this case, the smaller carb is more likely to provide snappier throttle response and better midrange power, albeit, at a slight cost to upper-rpm horsepower. This is just one simple example, but it should make you think about the type of driving you will be doing so you can make the best possible choice.

The most inexpensive route is to select an out-of-the-box carburetor from popular manufacturers like Demon, Edelbrock, or Holley. Each manufacturer offers OE-type replacement units that bolt on directly and feature provision for vacuum-assisted brakes, emission controls, and electric or mechanical choke.

For high-performance and race applications, consider a modified or blueprinted carburetor. The manufacturers listed above, along with many aftermarket companies such as Pro Systems and Quick Fuel Technology, offer custom, race-ready carburetors that will generally increase performance over an out-of-the-box unit. There are also plenty of expert tuners, most of which specialize in one area or a specific class.

Time To Tune
In order for your engine to sing those sweet tunes, the carb must be tuned to perfection. After selecting your new unit, bolt it to the intake, attach the linkage, install a throttle return spring, connect the fuel line, and you'll be ready to run. Always check and recheck throttle actuation (for smooth operation and for WOT)-you don't want that baby sticking wide open. It's not smart to rely solely on the carburetor's throttle spring as a return (and most racing sanctions require a second spring, too). Next, be certain the gas pedal actuates the throttle blades in a smooth fashion, from fully closed to fully open. Flooring the pedal should give you wide-open throttle. Naturally, check this with the engine off. Some applications might require a throttle stop to prevent the butterflies from going over-center where they can jam in the open position.