Aftermarket Parts - Are Premium Parts Worth The Price?

Spacers come in three basic configurations. We didn't test the open-plenum spacer on our tall, single-plane manifold, but open-plenum spacers can be worth power on shorter single-plane intakes. The four-hole spacer also worked much better on our engine than we anticipated. Do yourself a favor and look closely at how well the four-hole spacer performed. It was almost as effective for less than half the cost of the tapered spacer.

Four-Hole Vs. Tapered Carb Spaces
Sometimes you can find big power in the unlikeliest places. While most enthusiasts are preoccupied with high-fashion power parts like superchargers and nitrous systems, there is reliable power to be found in subtle changes to the induction system. Such is often the case with carburetor spacers. The trick, to quote Kenny Rogers, is to know when to hold 'em and know when to fold 'em. While there are catalog pages full of different designs, certain applications work better than others. Let's run through three of the most common designs, and then we'll bolt them on a typical street engine.

The most basic of all carb spacers is the open-chamber design. Here, the idea is to increase plenum volume of the intake manifold while also increasing the distance between the bottom of the carburetor and the manifold floor. The latter gives the incoming air and fuel charge a greater distance to make the change from vertical to horizontal flow path entering the intake port. These spacers are intended to be used on 5,000-rpm-and-up applications where the incoming air from the carburetor has lots of inertia. When you consider that fuel weighs more than air, the fuel will have more trouble making the change in direction at the bottom of the manifold where it transitions into the intake port. Additional carb height helps the fuel make that directional change. Open plenum spacers are generally used on single-plane intake manifolds although they occasionally do show power gains with dual-plane manifolds. These spacers can be made out of aluminum or plastic, which also helps as a heat insulator.

Wilson Spacer Comparison
'The insulating spacers are called "phenolic" spacers, referring to the type of thermostatic plastics used to make them.

The second design is appropriately called a four-hole spacer. These are generally intended to extend the length of the carb outlet and tend to improve the signal strength to the carb during part-throttle applications. Our test is only a wide-open-throttle test, so not all of the advantages of a four-hole spacer can be realized. However, if the combination includes a long-duration cam, it's possible to increase midrange torque with a four-hole spacer since it will improve the signal strength to the carburetor. Often this kind of spacer will affect the fuel curve by using more fuel. Sometimes this is an advantage, but if not, the four-hole spacer may require tuning the carburetor a bit leaner to take full advantage of the spacer's effect.

The final spacer type is the tapered four-hole from Wilson Manifolds. It is designed to actually increase airflow through the carburetor based on years of experimentation with NASCAR restrictor-plate manifold designs. Prior experience with these spacer plates on single-plane-manifold street engines tend to show horsepower gains from 5,500 rpm and up.

The sign says it all. We abused this motor for three solid days, and it never whimpered until we spun up to 7,000 rpm. It's a testament not only to quality parts, but an excellent job of blueprinting and assembly by the guys at JMS.

We've included fuel flow numbers with the horsepower and torque numbers since the addition of any kind of spacer not only affects airflow, but fuel flow out of the carburetor as well. This is seen in different conditions with all three spacers. Since we were already running a very tall Holley single-plane, we didn't test the open plenum spacer since it probably would not have made much of a difference in power. For a baseline, we ran our Dart-headed 383, equipped with the larger Dart 227 CNC heads and the bottom-of-the-page Douglas Glad-approved XR292 roller cam with no spacer underneath the Holley 750 HP carburetor.

We assumed we wouldn't see much of an improvement with the four-hole spacer in this particular application, but upon closer inspection of the entire power curve we noticed that the four-hole actually performed very well, picking up a small amount of power throughout except at the very top of the power band. The four-hole spacer actually mirrored the tapered spacer very well right up until 6,700 rpm, where it fell off. The tapered spacer also did well throughout the power curve, but delivered a stunning 23hp increase at 6,900 rpm, mashing the needle from 548 to a shocking 571. Unfortunately, when we tried to back up this run, the engine suffered from valve float right at 6,900, but the curve right up to that peak horsepower point exactly duplicated the big-power run. We're convinced that if the valvesprings had survived for two more pulls, we would have had solid backup on this effort. But don't worry; you're going to see this engine again very soon as we shoot for even taller peak horsepower numbers. You will also note the absence of average fuel numbers for Test 2. This is because we suspect an error in the fuel delivery numbers below 5,500 rpm, so we did not include them.

While this 23hp gain is almost twice what we've experienced before with other small-block combinations, it's not unrealistic. But what got our attention after looking at the overall numbers more closely was how much added power even the simple four-hole spacer produced. We may have to look into this phenomenon more closely in the future. There's power in those spacers, matey.

Spacer Test
The baseline run is with no spacer between the Holley 750 Street HP carburetor and the Holley single-plane manifold.

The second test is the same engine combination but with a Wilson 1-inch-tall, four-hole carb spacer.

The final test uses the same engine combination with a Wilson 1-inch-tall tapered spacer. The final difference column is the power increase compared to the baseline. Also note that the fuel flow numbers don't seem to make sense on Test 2 until the very top. We experienced intermittent difficulties with the fuel flow meters on several tests during our dyno flog. We included them here as recorded but the numbers are far larger than they should be.