Taken from the Knowledge Base & NHRA
Racing Technology
Tuning with carburetor spacers
by Wayne Scraba
The art and science of tuning headers
Most people think of carburetor spacers as simple horsepower-increasing devices. But that might not be completely true. Certainly, in some cases they can be used to increase engine power levels, but, more important, 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. Typically, power gain comes from increased plenum volume, but that's not the complete picture.
The manifold dilemma
Using a single-plane intake manifold as an example, you must first realize how a single-plane intake manifold functions. Generally, 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 this configuration, a large common plenum is under the carburetor. According to the experts, this common plenum allows each runner and cylinder intake port combination to draw from all four carburetor venturii at wide-open throttle. As the partially vaporized air/fuel mixture leaves the base of the carburetor venturii, it forms as 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.
Moroso Performance Products points out that the beauty 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.
The cure
Moroso notes that some single-plane intake manifolds are designed with a very short-turn radius coming out the bottom of the carb venturii 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 short, 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 gets 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, the jet size will still 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? As a rule of thumb, single-plane intake manifolds seem to respond best with larger spacers — two 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.
The market is filled with dozens of spacer styles and configurations. Some spacers are manufactured with insulating materials. These spacers decrease the amount of heat 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.
Spacer materials
Believe it or not, plywood can be an ideal material for constructing carburetor spacers. The thin layers of laminated wood, bonded with resin, form a natural heat sink. For example, a wooden Moroso spacer features top-quality hardwood plywood with a sheet of phenolic resin-impregnated paper bonded to both sides. This creates a durable product that can be used in drag racing, circle track competition, or on the street.
Plywood spacers are easy to modify to suit a given manifold for optimum performance. On the other hand, phenolic spacers are constructed from an advanced material similar to plastic that can reduce heat conductivity 10 times greater than aluminum for a much denser air/fuel charge. Obviously, aluminum spacers are still widely available. The advantage in aluminum is that it can be easily modified for a given application. The disadvantage is that in some cases, an aluminum casting can be porous. Because of this, several manufacturers now offer billet-aluminum spacers CNC-machined from 6061-T6 material.
So how can spacers be used to improve performance? The folks from Moroso provide this theoretical example: You have a car that hooks. It works well, turning the tires slightly during the rollout. Everything is fine until you're up against a track that's greasy after the launch pad or provides conditions that resemble a mine shaft. Typically, the car still hooks reasonably well, but on the 1-2 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 e.t. goes away, and so does the consistency.
Now what? It's a tough situation to "tune" out. None of the normal tricks work because they kill the launch. That's where a spacer can be used. Add a spacer or increase the spacer height and increase the jet size by a couple of numbers. The launch 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? A quicker e.t. and a bunch more consistency.
That's but one area where spacer tuning can work; however, there are dozens of other tuning applications, especially those where you have to tune to the engine, the car, the track, or the atmospheric conditions.
One final item to consider when buying or testing carburetor spacers is hood clearance. If the carburetor air horn is moved too close to the hood, then the airflow and fuel metering can become restricted. If the airflow is restricted, you simply won't be able to take advantage of the spacer.
Are spacers for you? That depends on your application, but if you don't tune with them, you could be missing out on some easy e.t.
