Carburetor Upgrading the Smart Way

When it comes to improving an engine's performance, one of the first things enthusiasts look at is the carburetor. There are many choices between swap meets and speed shops. Getting the right set up for you takes some work. It's very easy for neophytes to get carried away and put together a bad combination. This guide walks you through the ins and outs of carbs and helps you map a plan on improving your performance without losses elsewhere.

Upgrading your carb usually includes changing an intake manifold. It's pretty hard to change a carburetor without altering another part of the fuel system. You can either buy direct replacements for Rochester Quadrajets or a Demon which is a modernized version patterned after the Holley carbs. It would be worth your while to check out the carbs offered by Edelbrock Performer series. These were patterned after the old Carter AFB carbs from the 1960s but with many improvements.

Edelbrock's enhanced version of the AFB carb allows for better fuel calibration, very useful for dual four barrel applications.

Old AFB's used a cutoff directly outside the venturi which breaks up the gasoline erratically when air hits it. The mixture becomes uneven and causes fuel calibration to jump around. Starting in 1988, Edelbrock placed a booster nozzle closer to the center of venturi and made the hole through the bottom in the same direction as airflow. This makes calibrating the carb easier especially with dual quad systems. You also get full and timed vacuum ports for use with PCV valve or distributor advance timing. A good four barrel mixed with the right intake manifold will do wonders for delivering extra power.

The first thing you need to do is figure out what you want your car to do. Do you own a cruise night machine that could use a bit of passing power or is your engine a modern stroker build up requiring more power because of the increased capacity in the cylinders? You should consider your camshaft profile, rear axle ratio, transmission type and general weight to put together the best system for you.

Generally speaking, light cars can use more carburetion than heavy ones because the initial acceleration will be quicker, allowing you to reach the power band sooner than in a heavy car. This is especially true for manual transmissions equipped cars.

Your rear axle ratio plays a role in determining how soon you reach your torque curve. The numerically high ratios are gears with low number of gear teeth on the ring gear. A 4.56 :1 axle is considered a low gear ratio. A 3.00:1 by contrast, is considered a high gear ratio. Many automatic transmission cars have high gear ratios especially if equipped with air conditioning which could put the ratio around 2.92:1. You can improve acceleration by switching to a lower ring gear set.

Another factor is your choice of intake manifold. Most factory cars used dual plane manifolds made of cast iron. They have two plenum chambers that feed a bank of four cylinders using small runners for a better vacuum signal. By contrast, a single plane manifold uses one main plenum feeding all 8 cylinders. The runners tend to be large and somewhat shorter than on a dual plane manifold. The dual plane is designed for all around street use and offers strong low and mid range torque. The top end power is sacrificed in the interest of fuel economy. A single plane manifold offers strong mid and top range power at the cost of poor street drivability and terrible fuel mileage for daily driving. Those huge tunnels simply cannot flow gas efficiently at low speed. This brings us to the issue of vacuum signal and manifold density.

Stock cars tend to use cast iron intake manifolds and four barrel carbs with modest 5/16 fuel lines.

Manifold density is a measure of how much air is present in the manifold. Engines run better with tightly packed, cool air molecules instead of warm, loosely packed molecules. Aluminum's ability to run at cooler temperatures explains why it's a common feature in performance upgrades. Vacuum signal strength is important too. Every carb's fuel system relies on a vacuum source to draw out gas from the idle transfer ports, main and accelerator jets. As the throttle opens, you switch from one circuit to the other to supply the needed gas to be atomized by venturi boosters. Gasoline has higher inertia compared to air. It takes longer for gasoline to go around bends inside runners and air will get there first. Gasoline can lag behind and separate from air and pool on the floor of the intake manifold. This is why carbs often have accelerator pumps to direct gas down the throttle until the bigger mass of air/fuel delivered from your secondaries catches up.

A modified engine with headers and decent perf manifold will need richer jets to prevent detonation.

Let's look at how carbs are measured for performance. The common method is the cubic feet per minute. The higher the number, the more volume of air and gas the carb flows. A stock engine with tame camshaft, high axle gearing and maybe an automatic transmission won't require a high flowing carburetor. Let's say it's a stock 350 Chevy Nova. A Rochester Quadrajet is about the largest carb you'd find in such a car. It doesn't need more than 750 cfm with a 5/16" fuel line under those conditions because it's a light car with a 3.00:1rear axle ratio and mild hydraulic cam with cast iron exhaust manifolds, intake, ignition. Now let's change the picture using the same body and see what we end up with.

A strong race engine needs lots of fuel flow, this big block Chevy uses a Holley Dominator, single plane intake, 3/8 inch fuel lines and pressure regulator to ensure adequate flow.

Suppose it's a Nova SS 396 car from 1969 we're looking at supplying with fuel. That's a completely different car. An L78 396 was rated at 375 horsepower from the factory. It was created for drag racing and was built with serious components for power. The transmission is a four speed manual Muncie and the 12 bolt rear axle is packing a 3.91 ratio which is considered low ratio. The intake manifold is a cast aluminum dual plane unit and the gas lines are large 3/8 inch diameter connected to a high volume fuel pump. Notice how every item down the line was altered to meet the demands of the massive air pump that we call the 396 Nova? That's how the factory built a fuel system and the same method can be used when building your own street machine.

One compromise from the factory included three two barrels with progressive linkage. It gave two barrel performance around town, but opened up the two end units for full throttle.

Another factor related to performance is your car's camshaft profile. Cams come with a dizzying array of configurations designed to meet certain needs. Most stock cams are considered low duration, hydraulic or roller style units designed for fuel efficiency and delivering torque at commuter speeds. A change in cams can affect your carburetion. Generally speaking, long duration cams with overlap require more carburetion than short duration cams. These cams also require better breathing through the exhaust which is why headers are used. If your car runs headers, be sure to change the jetting of your carb so it doesn't run lean. Remember that the engine is an air pump and improvements in air flow also require improvements in fuel delivery and won't be off the mark.