Spaced Out: Do Carburetor Spacers Really Work?”

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The common consensus out there is that a carburetor spacer is a simple engine-tuning device, useable for only a select few applications, and they don’t produce much extra power either.  Short and simple, however that might not be completely true.  Certainly in many cases they can be used to change engine power characteristics. But they can also be used to find measurable horsepower (more on that later).  Here’s the bare basics of how a spacer functions:

The addition of a spacer effectively increases the distance between the carburetor and the floor of the intake manifold plenum.  In many cases, this straightens out the intake mixture flow pattern.  Because of this added distance, the carburetor signal is weakened.  And when the signal is weakened larger jets in the carburetor will be required.  Carburetor spacers designed with four separate holes tend to recapture the velocity of the mixture stream, which has been 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 speaking, 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 (in the order of 2-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.

When it comes to materials for spacers, plywood is a pretty good choice. Thin layers of laminated wood, bonded together with resin, form a natural heat sink.  For example, a wooden spacer features top quality hardwood plywood with a sheet of phenolic resin-impregnated paper bonded to both sides. As expected, plywood spacers are easy to modify to suit a given manifold for optimum performance.  On the other hand, pure phenolic spacers are constructed from an advanced composite material (similar to plastic), which can reduce heat conductivity ten 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. 

Do they work? For the most part, the answer is yes.  Certain engine combinations respond more favorably than others, particularly to traditional spacers. But there’s a wrinkle in the world of carb spacers and airflow. It’s called a “Super Sucker” and it’s manufactured by “HVH” (High Velocity Heads).   Like others, it’s made from an advanced composite material (you can also get them in aluminum).  And like others, most are 1-inch tall.  And yes, the top side (carburetor mount side) looks like any other four-hole spacer. But the bottom (intake manifold side) is far different than anything seen before (see the accompanying photos).

Ok. It’s different, but does it work?  A number of years ago, I tested one with remarkable results. My pal, the late John Heida volunteered to bolt one onto a small block Chevy he had strapped to his dyno. Both John and I weren’t expecting much.  In this case, we were wrong:

The engine we used for our test was mild 355 cubic inch small block Chevy complete with lightly ported small valve heads.  It was an efficient piece. Using a standard correction of 29.92 inches Hg., 60-degree F dry air, the tests were done in a conventional A-B-A format (for the sake of simplicity, the final back up test isn’t included – it duplicated the first test almost exactly). When we ran the  baseline test, the horsepower peak was 432.4 at 6,300 RPM.  The engine torque peak of 402.8 foot-pounds occurred at 5,000 RPM.  In truth, this is a pretty nice street engine with good manners.  For test number two, nothing was changed, except for the addition of a four-hole HVH “Super Sucker” carb spacer:

The little Chevy saw a power peak of 447.3 HP at 6,400 RPM, coupled with a torque peak of 413.2 foot-pounds at 5,000 RPM.  Bottom line? The HVH Super Sucker added 14.9 HP and 10.4 foot-pounds of torque.  It’s actually a pretty serious power maker. In theory, the two-inch tall model should have produced even more power. Common consensus regarding minimal power gains with a spacer just went out the window.  What makes this thing tick?  We presented that same question to High Velocity Heads: 

“These spacers work, obviously, by helping the fuel distribution.  If you look at the bottom of the spacer, you’ll see that the shape is different.  The reason is we wanted to improve the feeding of the outside runners on a single four-barrel manifold.  The design picks up the air speed and gives it a specific direction.  Finally, the plastic composite material insulates the carburetor.”

HVH makes use of a Haas CNC mill to carve out their spacers from solid composite stock.  Included in their product mix are spacers for 4150 Holley carbs (1-inch, 2-inch, four hole and two hole), 4500 Holley carburetors (1-inch tall, 2-inch tall four-hole spacers), 4412 series Holley carburetors (1-inch tall open, 2-inch tall two hole). In case you’re wondering, these spacers aren’t inexpensive (they can cost $139 or more), but in our book, that’s still cheap power.

For more, check out the accompanying photos:

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The folks from HVH offer a number of different spacer configurations, obviously for varied engine applications. The four-hole setup on the left is for a Holley Dominator while the spacer on the right is a two-hole model engineered for a 4150 Holley carb mounted on a dual plane intake.

On the topside, this HVH four-hole spacer appears almost identical to any other aftermarket model. On the bottom side, though, it’s a different story:

When flipped over, you can immediately pick out the differences. As pointed out in the text, the idea is to pick up the air speed, to better feed the outside runners of the intake and to give the air speed direction.

These spacers aren’t cast. They’re CNC-machined plastic. Note the intricate design. Plastic is relatively easy to machine, but more important, it insulates the carburetor from the hot intake manifold.