When it comes to improving an engine’s performance, one of the first things enthusiasts look at is the carburetor. The aftermarket has so much available, the problem boils down to a matter of choosing what’ll work best for you. It’s easy to let your excitement carry you away and choose a set up that doesn’t meet your needs. This guide walks you through the stuff you need to know before upgrading. It’ll make choosing your ideal setup that much easier.

Upgrading your carb usually means changing your intake manifold as well. There are carbs available that are direct replacements for the Rochester Quadrajet such as a Holley 4360 or a Demon. It would be worth your while to investigate some of the performance oriented carbs being made such as the Edelbrock Performer series. In many instances, an intake manifold swap will be required to unleash your available power. Your intended usage will be the deciding factor towards what other parts you need to add. Indeed, there are entire induction packages available that work with each other to build serious power.

The first thing you need to do is determine what you want your car to do. Do you own a cruise night machine that could use some extra power for enjoyment? Perhaps you own a stroker engine capable of pulling a lot more fuel than is currently available. Here are some factors you should consider when putting together a new carb system. You need to consider your camshaft duration, axle ratio and vehicle weight to get the right package together.

Generally speaking, light vehicles can use more carburetion than heavy vehicles because the initial acceleration will be quicker, allowing you to reach the power band sooner than a heavy car. This is especially true if the car has a manual transmission as there is no delay involved with a torque converter reaching stall speed. 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. A 3.00:1 is considered a high gear. Many automatic transmission cars tend to have high numerical gears which curbs the performance potential. You may have to lower the ratio with a new gearset or by changing tire sizes to get a workable compromise.

Another factor is your choice of intake manifold. Most factory stock cars used dual plane manifolds. They have two plenum chambers that feed a bank of four cylinders using small runners for increased vacuum signal.

A single plane manifold uses one large plenum that feeds all the cylinders. The runners tend to be large and somewhat shorter than a dual plane system. The dual plane manifold is designed for all around street use with strong gains in low and mid range torque. The top end is sacrificed by most factory issue manifolds in the interest of fuel economy and cost.

For instance, the stock four barrel manifold in your 1970 Chevelle 350 would be greatly improved by switching to an aftermarket dual plane piece for at least two reasons. The first is an aftermarket manifold has better intake runners for improved low and midrange torque. The second is the aftermarket piece is cast in aluminum which reduces weight, and more Importantly, reduces heat. The single plane manifold can be used for the street without much penalty if your camshaft doesn’t have excessive overlap. The single plane intake brings out mid and top range power at the expense of a weaker bottom end. Now we look at some important details that affect carburetion; manifold density and vacuum signal strength.

Manifold density is a measure of how much air is present in the manifold. Engines run better with tightly packed and cool air molecules rather than warm, loosely packed air molecules. Aluminum manifolds run cooler than their cast iron counterpart as the metal dissipates heat much quicker than cast iron.

Vacuum signal strength is important too. Every carburetor’s fuel feed 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 off-idle to main, then accelerator jets to supply the needed gas to be atomized by venturi boosters. Another factor to remember is that gasoline has high inertia compared to air. It takes longer for gasoline to go around bends inside the runners and air will get there first. Fuel can lag behind and separate from the air at high speed and pool on the floor of the intake manifold. The use of An accelerator pump in carbs takes care of this problem by shooting gas down the throttle until the secondary venturi catches up.

Fuel drop out is a problem on large plenum manifolds used in low rpm applications such as street cars. It appears in tunnel ram setups before they’re warmed up and in larger race oriented single plane manifolds. Remember this if you’re bolting a 850 double pumper onto a Victor Jr into a daily driver 383 Charger. Here you have too much carburetion, innappropriate intake for street use and the auto tranny will play havoc with your fuel calibration because the secondaries are mechanical linkage dumping too much gas at once. Flooring this car would bog the engine. A Holley 650 with vacuum secondaries would be a better choice as the secondaries open up with a vacuum signal supplied by the primary venturi. This way the throttle gets a more accurate dose of extra gas based on engine load or full throttle.

Let’s look at a 383 stroker Chevrolet engine that was built by Ri-un Racing in Georgetown, Ontario, for use in a 1963 Corvette coupe. The owner wanted lots of power without problems in driveability. The Corvette weighs about 2,860 pounds, pretty light. The transmission is a four speed connected to a 4.11 rear axle. The engine has Edelbrock RPM Performer aluminum cylinder heads, lightweight JEG pistons, Eagle crankshaft, hot hydraulic camshaft, rockers and a host of other goodies inside.

The owner could have used a large single four-barrel carb on a dual plane manifold or a single plane manifold if he only wanted strong top end power. Rick Rymal chose a pair of small cfm Edelbrock Performer Series carburetors which are revised AFBs with new technology added by Edelbrock. The manifold is a dual plane Edelbrock design as well. The 500-cfm carbs use progressive linkage which means the pair act as one single four barrel running on the primaries until the throttle is opened at a predetermined point activating the secondaries. The dual plane manifold keeps his car streetable in city traffic with only a bit of sacrifice at the top end. Although the combined total cfm is 1000, the system works because the cam duration is long with decent lift and his reciprocating assembly is light, allowing fast, high revs. There is good vacuum signal through the rpm range with a dual plane manifold and no lag caused by a torque converter.

Manifold density is a measure of how much air is present in the manifold. Engines run better with tightly packed and cool air molecules rather than warm, loosely packed air molecules. Aluminum manifolds run cooler than their cast iron counterpart as the metal dissipates heat much quicker than cast iron.

The Edelbrock Performer carb is an example in which modern updates enhance a basic sound design. Old AFBs used a cutoff directly outside the venturi which breaks up the gasoline erratically when air hits it. The mixture was uneven and caused fuel calibration to jump around. The result was a less efficient delivery. Starting in 1988, Edelbrock placed the booster nozzle closer to the centre of the venturi and placed the hole through the bottom in the same direction as airflow. This makes calibrating the dual quad setup easier. You also get full and timed vacuum ports for use with PVC valve and power your accessories.

Stepping back and looking at the big picture, the carb choice was successful. It fulfilled Rick’s desire for vintage street machine looks and strong performance. Since it was such a light car with so much torque due to the stroke, Rick found he was busy shifting gears with a low cog out back. After a few weeks, he switched to a 3.73 and is much happier with the package. Remember with a light powerful car, you don’t need a really low gear. Now you can start carbing for the street on your own machine.

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