"If you hold a carburetor and open it to 3/4 throttle, you will see that the blades drive everything to the center of the manifold. So your center four runners-especially on Fords because of the runner layout-get a majority of everything, especially the wet fuel. What happens is they run rich and the outside cylinders go lean.

"To make a motor accelerate best, you want to start where all eight cylinders get the same mix. When you are pulling some cylinders rich and some lean, the motor won't accelerate as quickly as if all eight are the same. So a four-hole helps with that. It straightens out the air/fuel flow at part throttle and sends it straight down out of all four holes, or two if you are running a two-barrel."

The disadvantage of a four-hole spacer, however, is that the exit of the holes can potentially cause a separation of the air/fuel mixture. Because the exit of the hole in the spacer is so close to the roof line of the runners, the turn must be sharper than an open spacer of the same height. A softened edge can help reduce the separation, but the rules often don't allow it.

"When the air/fuel mixture comes off that sharp edge at the exit of the hole, that's where the mixture is forced to turn hard to get into the runners and the fuel separates from the air," Wilson says. "You spend your whole life trying to get the fuel and air mixed on any engine into vapor form. You can buy a $1,000 carburetor that mixes the air and fuel perfectly, but then when you turn it off that sharp edge at the bottom of the spacer it goes wet again. And once it goes wet, there is no getting it back.

Finally, Wilson Manifolds offers this tip: If you are racing in a class that requires a 390 carb, which is typically a choke point for the engine, you can actually pick up airflow by using a four-hole spacer with the holes slightly larger than the carburetor venturis. By doing this, you are creating a low-pressure area just underneath the carburetor that helps pull additional air through the venturis. If tapered spacers aren't allowed, this may be a worthwhile option.

A tapered spacer is really a four-hole spacer with softened exit edges taken to an extreme. It begins as a four-hole spacer, but the holes open up in carefully engineered variable radii until it is essentially an open spacer at the base. The way the holes open up creates four venturis that can significantly increase any carburetor's maximum airflow. The result is better air/fuel distribution and more horsepower than just about any other spacer of equivalent height. Wilson Manifolds claims it has seen improvements in terms of airflow through the carburetor by as much as 110 cfm in its tests with the addition of a tapered spacer.

"The tapered spacer also improves performance at part throttle," Wilson says. "Everybody dynos at full throttle, and too few pay attention to what's going on at part throttle. It does help pick up airflow at full throttle, but a tapered spacer-seeing as it starts as a four-hole spacer-also helps with the throttle blades directing the air/fuel mixture at part throttle. The spacer also opens up into an open spacer, so it gives the air and fuel time to slow down and make the turn into the runners without separating.

"If there is a disadvantage with a tapered spacer," he continues, "it is if you are getting reversion. Nowadays, with cylinder heads and camming being so much better than they used to be, reversion is rarely a problem. But if you have an engine with a lot of overlap and you have reversion, a four-hole spacer actually funnels that back up the carburetor, which will hurt you. In this situation, you want to use a combo spacer. But that's the only time a tapered spacer won't run better than anything else."

Wilson says a combo spacer is essentially a compromise. A combo spacer is created for situations in which a tapered spacer isn't allowed. It is essentially half of a four-hole spacer mated to half of an open spacer. Unlike a tapered spacer, there is basically no transition between the two. The four-hole half serves to help straighten out the direction of the air/fuel flow coming off the throttle blades at part throttle, while the open half gives the air/fuel stream a little more room and space to make the turn into the runners. What's the advantage of a combo spacer? Wilson says if you are currently running a tapered spacer and notice your engine is suffering from power-robbing reversion, a combo spacer can help eliminate it without an expensive cam change.

If your rule book says nothing about what type of spacer you can use, consider yourself lucky because all the options are open to you. Even if you are limited to one or two types, understanding how spacers affect an engine's performance will help you choose what size you need and can help you get the tuning advantage you seek.

Carburetor spacers constructed from phenolic material to shield the carburetor from heat have been around for years, but Comp Cams recently introduced one of the most unique spacers designed to protect the carb we've ever seen.

Known simply as the vibration absorber carburetor spacer, this piece uses billet aluminum sandwiched around a shock-resistant composite material to shield the carb from the engine's vibrations and heat. Comp claims it prevents upper-level rpm lean out and irregular fuel atomization by preventing the harmful harmonics produced by the engine from making its way to the carb. It is available for 4150-, 4500-, and 2300-style carbs in alcohol and gas versions. For more information on this unique piece, contact Comp Cams at 800/999-0853 or www.compcams.com.

Wilson Manifolds
4700 NE 11th Ave.
Ft. Lauderdale
FL  33334