Car Craft
Click here to find out more!

Aftermarket Parts - Are Premium Parts Worth The Price?

We Test A Pile Of Speed Parts To Discover

Photography by

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.

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.

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.

Parts List
DESCRIPTION PN SOURCE PRICE
Wilson 1-inch open-plenum spacer 000011 Wilson Manifolds $37.58
Wilson 1-inch four-hole spacer 011111 Wilson Manifolds 56.88
Wilson 1-inch tapered spacer 004110 Wilson Manifolds 120.58

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.

TEST 1 TEST 2 TEST 2 DIFFERENCE
No Spacer Wilson Four-Hole Wilson Tapered          
RPM Fuel TQ HP Fuel TQ HP Fuel TQ HP TQ HP
2,500 368 175 80 361 172 130 364 173 79 - 4 -2
2,700 353 181 82 348 179 134 350 180 79 -3 -1
2,900 347 213 89 368 217 148 370 218 89 +9 +5
3,300 383 241 96 395 248 164 395 248 99 +12 +7
3,500 410 273 109 419 279 176 419 279 109 +9 +6
3,700 429 302 125 437 308 183 436 307 125 +7 +5
3,900 443 329 135 447 332 193 448 333 134 +5 +4
4,100 457 357 144 455 355 194 459 358 143 +2 +1
4,300 468 383 157 467 382 184 471 386 151 +3 +3
4,500 475 407 165 481 412 177 480 411 160 +5 +4
4,700 484 433 171 489 438 178 487 436 171 +3 +3
4,900 489 456 177 495 462 184 493 460 180 +4 +4
5,100 491 477 186 497 483 192 496 482 185 +5 +5
5,300 490 495 197 494 498 200 494 499 194 +4 +4
5,500 484 507 206 487 510 208 489 512 206 +5 +5
5,700 477 518 216 479 520 220 481 522 219 +4 +4
5,900 470 528 227 470 528 236 472 530 229 +2 +2
6,100 462 536 234 460 535 252 463 538 236 +1 +2
6,300 452 542 246 454 545 264 454 545 249 +2 +2
6,500 442 547 259 443 548 284 447 553 263 +5 +6
6,700 430 548 270 427 545 296 442 563 272 +12 +15
6,900 407 535 283 415 546 279 434 571 284 +27 +36
Peak 491 548 283 497 548 --- 496 571 284 +27 +36
Avg 438.3 402.9 173.8 444.1 405.6 --- 444.1 408.8 173.4 +5.8 +5.9
SOURCES
Flowmaster
8-00/-544-4761
www.flowmastermufflers.com
Trans-Dapt (Hamburger)
Cerritos
CA
Holley Automotive Racing Products (ARP)
531 Spectrum Circle
Oxnard
CA  93030
805-278-7223
Dart Machinery
353 Oliver St.
Troy
MI  48084
248-362-1188
www.dartheads.com
Johnson Machine Service (JMS)
724 E. Huntington Dr.
Monrovia
CA  91016
Federal-Mogul (Fel-Pro)
2-48/-354-7700
federal-mogul.com/en
Edelbrock
Dept. 5.0
2700 California St.
Torrance
CA  90503
310-781-2222
www.edelbrock.com
COMP Cams Hedman Hedders
16410 Manning Way
Cerritos
CA  90701
310-921-0404
www.hedman.com
Summit Racing Equipment
P.O. Box 909
Akron
OH  44309
Holley (Lunati)
Bowling Green
KY
Wilson Manifolds
4700 NE 11th Ave.
Ft. Lauderdale
FL  33334
954-771-6216
www.wilsonmanifolds.com
Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!
0 comments
Car Craft