Tuning with Carburetor Spacers
By Sam Moore
Carburetor spacers are valuable tuning aids for both performance cars
and racing cars alike. Positioned between the carburetor and the intake
manifold, a spacer will alter the relationship between these two components.
Consequently, the spacer can usefully raise or lower the position of the torque
and power-band within the rpm range. They can also be used to correct
inefficiencies in a mismatched combination. Spacers are available in several
different forms: 4-hole, open, and combination;
they are also made from different materials and in
different thicknesses.
4-Hole Spacers. Generally, a 4-hole
spacer; that is, one with four holes that align with
the four barrels of the carburetor will increase
throttle response and acceleration. It can also
lower the torque and power-bands within
the RPM range. This is accomplished by
keeping the columns of air and fuel flowing
longer, which increases the air velocity.
Adding a 4-hole spacer can be an effective
and practical solution for a vehicle with sluggish throttle response or lethargic
corner exit speeds. In addition, a 4-hole spacer can also help compensate for an
oversized component: a carburetor, camshaft,
intake manifold, etc.
Open Spacers. Usually an open
spacer; that is, one with a large square-shaped
hole beneath the carburetor, will decrease
throttle response and dampen acceleration. It
can also raise the torque and power-band
within the RPM range, which is accomplished
by increasing the
plenum area.
Adding an open
spacer can be
helpful in overcoming traction problems during
acceleration or corner exit. Furthermore, an open
spacer can also help compensate for an undersized
component: a carburetor, camshaft, intake manifold,
etc.
Combination Spacers. A
combination spacer is distinguished by
being part four-holed and part open and
By lowering the torque and power bands
within the rpm range, the 4-hole Phenolic
spacer quickens throttle response.
The open spacer is used to slow
the throttle response, making the
racecar easier to handle on slick
surfaces.
The combination spacer increases
throttle response and acceleration and
tends to broaden the torque and power
band.
can provide the best of both worlds. Choosing a combination spacer increases
throttle response and acceleration; it can also increase or broaden the torque
and power-band. The four-hole surface of the spacer interfaces with the base of
the carburetor.
Plenum Dividers. In an open plenum intake manifold, a plenum divider
partitions the plenum from side to side. The divider
helps prevent unwanted side to side fuel movement in
high G-force oval-track or road-race applications. On
certain engines, it is common to have lean cylinders
due to fuel surge; for example, a small-block Chevy
oval-track engine running on methanol can run lean
on cylinders 3 and 5, while cylinders 4 and 6 are rich.
By using a plenum divider, equal fuel distribution can
be restored to the each cylinder bank.
Spacer thickness. Normally, a thicker spacer
increases the effect; that is, if a ½”-thick spacer
improves matters, a 2”-thick spacer will further
increase the effect. Varying the thickness of a spacer
affects engine performance.
Spacer Material. There are many different types of materials used for the
manufacture of spacers, and all have their advantages and disadvantages.
Wood, for example, is a poor conductor of heat and therefore thermally efficient.
Unfortunately, it can absorb fuel, which is not safe. Plastic or composite spacers
are also thermally efficient, but they are not as strong as phenolic or aluminum
and can be laborious to modify. Phenolic fiber spacers are popular and resistant
to the transfer of heat, but they, too, can be hard to modify. In contrast,
aluminum spacers have poor resistance to heat transfer, but they can be ported
or modified easily.
Spacer Tuning. Since each spacer will react differently to each engine
combination, there is neither a right nor a wrong type. Spacers are valuable
tuning tools. They’re especially helpful when dialing-in a new combination or
when tuning a racecar for varying track conditions. Swapping a spacer is quick
and simple, and the change can have significant influence on the drivability of a
racecar or streetcar. Possessing spacers of several different types and
thicknesses is practical and always a sound investment.
