The general impression of carburetor spacers is that they're down-to-earth, power-building devices. But that might not be completely true. Sure, spacers can be used to increase the power level of a Chevy engine. More importantly, however, they can be used as a sophisticated tuning aid. Spacers often provide an increase in top-end power, but at the same time they can reduce bottom-end power (as well as mid-range torque). The short version is that power gain comes from an increase in plenum volume, but that's not the complete picture.
Using a single-plane intake manifold as an example, you have to first consider how it functions. Generally speaking, a single-plane intake includes a large centrally located plenum that has reasonably straight runners leading from the plenum to the port entries in the cylinder head. In a single-plane configuration, there is a large common plenum under the carburetor. This "common" plenum allows each runner and cylinder intake port combination to draw from all four of the carburetor venturis at wide open throttle. As the partially vaporized air-fuel mixture leaves the base of the carburetor venturis, it forms four individual "mixture streams." When each of the cylinders places a demand on the plenum chamber, these mixture streams (or in some cases, portions of the streams) physically bend in the direction of demanding runner-port entry. The mixture streams combine to form a single "mixture river" which flows into the runner, eventually feeding the cylinder that is making the demand.
One of the best features of a single-plane manifold configuration is that it allows each runner to withdraw a larger volume of air-fuel mixture during the available induction time span. Unfortunately, life isn't always simple--and neither are intake manifolds. As each cylinder withdraws a charge from the plenum, the mixture streams are forced to change direction constantly.
Creating more havoc inside the manifold are pressure pulses that travel backward from the cylinder into the manifold runner and eventually into the plenum. And some engine combinations have more of this reverse pressure pulsation than others. These constant directional changes in the plenum along with pressure pulses can create a healthy amount of turbulence inside the plenum.
Some single-plane intake manifolds are designed with a very short turn radius coming out of the bottom of the carb venturis into the respective entries of the intake manifold runners. When the carburetor is moved up (most often with a spacer), the velocity of the intake charge is reduced, which in turn allows the previous mixture streams to make the bend around the corner (or short side radius) easily. In certain applications, a 1/2-inch spacer will work, but in other cases, the manifold design dictates a larger spacer.
It doesn't take a rocket scientist to figure out that the addition of a spacer effectively increases the distance between the carburetor and the floor of the plenum. Because of this added distance, the carburetor signal is weakened. And when the signal is weakened, a larger jet (or jets) in the carburetor will be required. Carburetor spacers designed with four separate holes tend to recapture the velocity of the mixture stream that has been lost when an open carburetor spacer is installed. In simple terms, more exit velocity in the mixture stream creates a stronger carburetor signal than that found with an open spacer. Generally speaking, the jet size still will have to be increased when a four-hole spacer is used, but not as much as with an open spacer.
How much spacer should you use on your Chevy? As a rule of thumb, single-plane intake manifolds seem to respond best with larger spacers (2 inches in height and larger). On the other hand, most dual-plane intake manifolds work best with open spacers with a height of between 5/8-inch and 1 1/2 inches.
It should come as no surprise that the market is filled with dozens of spacer styles and configurations. Some spacers are manufactured with special insulating materials. These spacers decrease the amount of heat that is transferred from the intake manifold to the carburetor throttle plate and main body. This reduces the fuel temperature inside the carburetor. Naturally, the result is a denser fuel charge to the manifold, which in turn creates more horsepower.
So how can spacers be used to improve performance? Here's a theoretical example: You have a Chevy that hooks. It works well and everything is fine until you're up against conditions where the traction becomes limited and the weather is conducive to building power. Pretend your car still hooks up to the pavement but on the 1 to 2 gear change, it turns the tires heavily (in this case, assume that the engine rpm is brought down to the torque peak on the gear change). The performance is simply gone. Now what? It's a tough situation to "tune" out. None of the normal tricks work because they kill the way your car launches.
Sound like a familiar situation? This is where a spacer can come in handy. Add a spacer (or increase the spacer height) and increase the jet size by a couple of numbers. The launch characteristics will remain almost the same, but the increased plenum volume helps to shift the torque peak (and peak horsepower) upward. Because of this, the engine isn't dragged into the meat of its torque band during the gear change and it doesn't turn the tires. The result? Your Chevy is still quick, even when traction is limited and power is abundant.
That's but one area where spacer tuning can work, and there are dozens of other tuning applications. The point is, a spacer is a very inexpensive way to move the power curve around to suit the surroundings. Are spacers right for you? It definitely depends upon your application, but if you don't tune with them, you could be missing out on some performance that is very easy to get.
