Choosing A Carburetor For Your High Performance Engine. Which One Is The Right One?

Choosing a carburetor for your high-performance engines is not an exact science. 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, transmission and converter stall speed, and rearend gear ratio all factor into choosing a carburetor.

Selecting a larger carburetor can produce more power on a dynamometer, but under normal operating conditions can result in slower acceleration and lower efficiency of fuel atomization. Select the smaller carburetor, especially if you’re undecided about sizes. The smaller diameter venturii increases the velocity of the air-fuel mixture. As a consequence, it usually provides better acceleration and proves to be more efficient.


To quote Barry Grant: “The most common error is choosing a carburetor that is too big. Big carburetors make headlines in magazine stories, but usually don’t provide the best drivability. Choosing a carburetor that's too big may idle poorly, hesitate at part-throttle, and become lean at wide open throttle.”

Here are some very simple guidelines to go by when choosing a carburetor for your high-performance engine, courtesy of Barry Grant of Demon Carburetors.

All things being equal, a bigger engine requires a bigger 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 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 rearend gears require a smaller carb

• Low compression requires a smaller carb

You also may wonder: When choosing a carburetor, which style should I use, mechanical secondary or vacuum secondary, and why? Well, generally, choosing a carburetor with mechanical secondaries is preferred on vehicles with manual transmissions and on automatic transmissions with 3000+ RPM stall-speed converters. For automatics with less than 3000-RPM stall-speed converters, the vacuum-secondary carburetor is usually the better choice.

Double pump, mechanical secondary carburetors initially depend only on the accelerator pumps to provide adequate fuel until enough air flow can be established to begin pulling in the main system. The larger the carburetor the higher the air flow required to accomplish this. If the carburetor is too large, the pump shot will be consumed before the main system starts. The result is a “bog” or a “sag”. So, if you going with mechanical secondaries, take great care in choosing a carburetor that is not too big!

Another important factor influencing you in choosing carburetor is intake manifold design. By terms of definition there are two basic configurations for V-8 intake manifolds: single and dual plane. The dual plane, 180° designs feature a multilevel plenum design that essentially separates adjacent intake pulses by alternating planes, while the single plane, 360° models feed all cylinders through a single plenum. The plenum is the large chamber underneath the carburetor mounting pad(s), while the runners (or ports) direct the intake charge to each individual cylinder.

With its divided plenum, a typical dual-plane intake provides a stronger signal to the carb than a single-plane manifold. A dual-plane intake can usually use a larger carb than a single-plane manifold. As a result dual plane manifolds are preferred for street applications (especially vehicles equipped with automatic transmissions) because of a stronger individual carburetor “signal” that provides improved bottom-end performance. By virtue of a less restrictive, more direct design the single plane is superior for top-end performance.

The main point to remember here is that there is no magic answer to choosing a carburetor. Your carburetor must be an integrated member of a complete engine, chassis, and drivetrain combination working harmoniously together to deliver the desired performance both on the street and at the strip.

Barry Grant offers this advice for track tuning your carburetor. Cooler weather calls for larger jets than warm summer weather, as do changes in altitude. As density altitude improves, an engine may need larger jets, and as density altitude deteriorates, smaller size jets may be required. Generally, for every 1,250- to 1,500-foot increase in density altitude, jet size should be reduced by one number. However, some motors aren’t as susceptible to density altitude changes as others. A nitrous-powered car, for example, is far less likely to be affected by density altitude variations than a supercharged car. Furthermore, when using a density altitude gauge to determine jet changes, be sure to keep an accurate record of the changes. Since improvements can only be made through trial and error, it is imperative to keep detailed records of jet changes and how they affect ETs.

Similar to changing jets, carb spacers can be used as an effective tuning device. Since a motor reacts differently to varying heights and configurations of spacers, trial and error is often the best method of selecting the proper spacer. While the effect of a carb spacer is barely noticeable on some engines, others will show significant gains or losses. Generally, an open spacer will increase top-end performance, whereas a four-hole spacer improves low-end torque. An open spacer has the effect of increasing plenum volume, while the four-hole spacer gives the effect of a longer runner.

Open Spacers. Usually an open spacer; that is, one with a large square-shaped hole beneath the carburetor, will decrease throttle response and dampen acceleration. It can also raise the torque and power-band within the RPM range, which is accomplished by increasing the plenum area. Adding an open spacer can be helpful in overcoming traction problems during acceleration or corner exit. Furthermore, an open spacer can also help compensate for an undersized component: a carburetor, camshaft, intake manifold, etc.

4-Hole Spacers. Generally, a 4-hole spacer; that is, one with four holes that align with the four barrels of the carburetor will increase throttle response and acceleration. It can also lower the torque and power-bands within the RPM range. This is accomplished by keeping the columns of air and fuel flowing longer, which increases the air velocity. Adding a 4-hole spacer can be an effective and practical solution for a vehicle with sluggish throttle response or lethargic corner exit speeds. In addition, a 4-hole spacer can also help compensate for an oversized component: a carburetor, camshaft, intake manifold, etc.

Combination Spacers. A combination spacer is distinguished by being part four-holed and part open and can provide the best of both worlds. Choosing a combination spacer increases throttle response and acceleration; it can also increase or broaden the torque and power-band. The four-hole surface of the spacer interfaces with the base of the carburetor.

Spacer Tuning. Since each spacer will react differently to each engine combination, there is neither a right nor a wrong type. Spacers are valuable tuning tools. They’re especially helpful when dialing-in a new combination or when tuning a racecar for varying track conditions. Swapping a spacer is quick and simple, and the change can have significant influence on the drivability of a racecar or streetcar. Possessing spacers of several different types and thicknesses is practical and always a sound investment.

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