I just finished rebuilding my top end on my engine. Right now I have an Air-Gap intake with a Holley 750 on top of a 1 inch riser. Should I keep the riser on, or would it be more beneficial for me to remove it? Right now the low end is a little poor, and I want all around performance.

So-Cal Jack

Well first of all,how big of a motor are you running? if its just a mainly stock 350,with some bolt on's,the carb spacer isn't needed,the main reason for running a carb spacer is to increase the atomized feul/air mixture in the plenum,this is mainly needed when u have a motor with a big enuff cam,to support higher rpm horsepower,if this is mainly a street car with a mild motor,than its not needed,other wise im not sure what your running or what your trying to do with it. -Dan-

I would try it first without, and tune for best performance. Then after establishing a base line on the performance, add it to see if it helps or hurts. A spacer increases plenum volume and isolates the throttle bores from the reflected induction pulses in the plenum area. A four bore isolates the throttle bores more than an open spacer so depending upon your booster type (annular, or dog leg) the vacuum signal is enhanced (amplified) more with a four hole than an open.


Big Dave

Some perform better with one and others without. But then we could get into weather 4 hole or open is better and 1" or 2".... comes down to the combo and what it likes/wants.

No two set-ups are alike so there is no definitive yes or no to this question. Cam, heads, intake, headers, carb, even the ignition curve have an effect on whether you will gain or loose power with a spacer. There are also variations in spacers in height (half inch, one inch, two inch, or four inch), material (phenolic plastic, pot metal, aluminum, wood), and shape (open or contoured to match throttle bores).

Big Dave

Thanks guys for the information. I have a 400SB, ported and polished 76cc 1.94 heads, 280h cam, 750 Holley 3310, Air Gap intake. I just tried removing the spacer, and now all of my cables are offset by a few inches.

I'll have to wait and see how it performs once I get the correct bracket.

So-Cal Jack

One time I used a 4 hole 1/2" phenolic spacer to isolate the carb from the heat from the intake, and made the 4 hole into 2 oblong holes, left and right, on a Performer RPM intake. I wanted only to isolate the carb, and a 1/2" did that for me.

Jack, that was my set-up and then I bought aftermarker heads (72cc chambers and 200cc intake ports). Car is 7mph faster and 1.0 ET faster :D

Times below. Old times were a 13.2 @ 103 best.

I finally got my carb on "without" the riser. It seems to run a little better around town now. I'll keep it this way for a while, until I get bored and want some more power. Then I'll be able to go back to the old setup. It's good to have options available.

So-Cal Jack

depending on what you are looking for out of the car ,you can easily make some changes,and have a soild 11 sec car on pump gas with a 406,what piston and rod combo do you have in the motor ?

My motor has stock pistons, steel crank, and not too sure about the rods.

We may be able to shed a little more light on the whole spacer issue.

Carburetor Spacers:

Carburetors spacers can be a very useful tuning aid when working on your streetcar, or racecar. A spacer can be used to move the torque and power-band where it is more usable in your application, or they can be used to help work out inefficiencies in your combination.

4-Hole Spacers. As a rule of thumb a 4-hole designed spacer (4 individual holes one under each barrel of your carburetor) will increase your throttle response, and acceleration. They can also move the torque and power-band down in the RPM range. This is accomplished by keeping the air and fuel flowing in more of a column, which increases the air velocity. This can be a perfect addition if your vehicles throttle response is not as good as you’d like, or you’re getting passed when you pick up the throttle coming off of the corning. A 4-hole spacer can also help make up for something in the intake tract being larger than optimal (too large of a carburetor, cam, intake, etc.)

Open Spacers. As a rule of thumb an open designed spacer (1 big hole underneath your carburetor) will decrease your throttle response, and acceleration. They can also move the torque and power-band up in the RPM range. This is accomplished by increasing the plenum area, which will help in the higher RPM’s. This can be a perfect addition if your vehicle has traction problems when accelerating, or coming off of the corner. A 4-hole spacer can also help make up for something in the intake tract being smaller than optimal (too small of a carburetor, cam, intake, etc.)

Combination Spacers. A combination spacer (half 4-hole, and half open) can give you the best of both worlds. Increasing your throttle response, and acceleration over not using a spacer, and increasing or broadening the torque and power-band.

Plenum Dividers. A Plenum divider does as the name implies divides the plenum in an open plenum intake manifold from side to side. These are generally used to help prevent fuel slosh from side to side in high G load Oval-Track, or Road-Race applications. It is common on certain engines to have lean cylinders do to fuel slosh. A SBC oval track engine running on methanol can run lean on cylinders 3 & 5 while cylinders 4 & 6 will run rich. A plenum divider can help eliminate this.

Spacer thickness. Varying the thickness of your spacer will affect how it affects your engine. Normally the thicker the spacer the more of an affect if will have on your combination. Meaning if a ½” thick spacer helps you a little a 2” thick spacer can give you more of the same affect.

Spacer Material. There are many different types of materials used for manufacturing spacers. They all have pro’s and con’s. Wood for example is a great material as far as thermo efficiency, but can wick fuel, which is not safe. Plastic, or composite spacers are also very good at not transferring heat, but are not as strong, and can be harder to modify. Generally Phenolic fiber, or Aluminum is preferred. Phenolic is very good at not transferring heat, but can be hard to modify. Aluminum is not as good at heat dissipation, but can be ported or modified easily to work on specific applications.

Spacer Tuning. Since each spacer will react differently on each combination there is not a right or wrong type. Spacers are a great tool to have to help dial in a new combination, or tune your racecar for varying track conditions. Swapping out a spacer is a very simple change that can have great impact on the drivability of your streetcar, or racecar. Having a couple types, and thickness of spacers around is always a good investment.