Here's your thought for the day: "After the air and fuel leave the carburetor, if anything happens to that fuel to cause it to fall out of suspension, there are no tricks to getting it back," says intake guru Keith Wilson of Wilson Manifolds. "That fuel will not make power; it's lost energy."
You don't even like to think about how much money you spent getting your carburetor built. The intake runners on the cylinder heads are ported to achieve optimum flow, and the intake manifold has been massaged to match the intake ports, but how much attention was paid to the spacer that ties it all together? Most rule books allow a spacer that's no more than 2 inches tall, but is 2 inches the best size for your engine at your track? Also, what type works best?
Fortunately, there are ways to get a good handle on the size and type of carb spacer that should work best in your application without spending weeks on the dyno. To get that information, we went to Wilson, one of the established experts in the business. For years, Wilson Manifolds has worked with the top teams in Nextel Cup racing. Last year, the company supplied the intakes for Tony Stewart's championship-winning Chevy team, so that's a sign that the organization is on top of its game. In addition, Wilson Manifolds develops spacers for every level of stock car racing you can think of: from two-barrel carburetors, to Quadrajets, to the unlimited dirt Late Model four-barrels. Over 75 different carburetor spacers of every description are offered.
When it comes to circle track racing, there are four main variations on the venerable spacer. In addition to these variations, you can tune with the spacer size. Your rules may limit what is legal, but you still have options when it comes to what size you can run. Unless you are using an intake with an unusually tall plenum, a high-performance race engine will almost always benefit from a spacer of some sort.
An open spacer is the most...
An open spacer is the most basic design. Consider it an easy way to add controlled amounts of plenum volume to the intake to tune the engine's powerband.
Consider the open carburetor spacer your baseline. An open spacer is basically an extension of the neck of the intake manifold. Its two main functions are to increase the intake's plenum volume and to increase the distance between the bottom of the carburetor and the roof of the intake manifold runners.
Increasing plenum volume is important because it can be used as an engine tuning tool. "If the motor combination-the cylinder head, manifold, and carburetor-is pretty much in tune," explains Wilson, "the general guideline is that the more plenum volume you have, the more top-end power you are going to make while taking away a little bottom end. Reducing the plenum volume basically does the opposite. It gives the engine more bottom-end torque while taking away some of its ability to make power at the high-rpm range."
Wilson says many teams will use different carb spacers of different heights as a tuning tool-especially on dirt tracks, where the conditions can change dramatically and quickly. If the track conditions are slick, you can add a taller spacer to increase plenum volume. By reducing low-end power it will help keep the car from breaking traction on turn exit, and the increase in high-rpm power will assist the driver from the flag stand through turn entry, where the extra power can be used effectively.
Wilson says that real differences can be felt from the driver's seat with spacer height changes that are as small as fractions of an inch. For that reason, he produces open spacers between 1 and 2 inches in height in 1/8-inch increments to allow racers to tune their engine's powerband to the demands of the track on a particular night.
A tapered spacer is the most...
A tapered spacer is the most advanced design.
Increasing the distance between the bottom of the carburetor and the plenum floor also has advantages-and it works no matter what type of spacer you use. For any carburetor to work at its best, you need a strong vacuum signal from the combustion chambers to create good airflow through the venturis and ensure good mixture quality at the boosters. In a street engine, the distance that you can move the carburetor from the combustion chambers while still getting a strong signal is quite limited. In a more efficient race engine that has more efficient airflow through the ports and runs at a much higher rpm, the signal is typically much stronger and travels a greater distance. This allows you to move the carburetor farther away from the chambers with the use of a carburetor spacer and gives the air/fuel mixture a chance to reduce its velocity before having to make the turn to get into the intake runners. When this turn is too sharp, the fuel (which has a greater mass than the air in which it is suspended) tends to fall out of suspension and collect into larger, unburnable droplets. By using a carburetor spacer to reduce the speed and angle at which the air/fuel charge must make the turn into the runners, you can keep more fuel in suspension all the way to the combustion chambers.
They increase the distance between the carburetor and the intake runner without excessively increasing plenum volume. A four-hole spacer uses holes that are the same diameter or slightly larger than the carburetor's venturis with no taper or angle to the walls. Generally, they help to build low- and mid-range torque and are especially useful on small or slick tracks, where a racer may spend a great portion of the lap at part throttle.
"The throttle blades in the carburetor give so much direction to the air/fuel flow at part throttle, you have to do something to help it out," explains Wilson. "And most racers are at part throttle way more than they would ever believe. I've been to tests at places like Atlanta, Charlotte, and Michigan with Nextel Cup teams-tracks that are real big and people would think are mostly wide open. But the telemetry shows that the drivers spend much of the lap at part throttle, more than even most of them could believe. It's the same situation on smaller or slick tracks.