914 Sub Woofer Installation
Bee Jay Jones

I grew up in the 70s, and my favorite music is “old school” 70s music, with a heavy bass line.  I upgraded the sound system in my 914 with a CD changer, 6 ˝” separate speakers in the doors, 4 x 6” JBL plate separates in the original grilled kick panel location, and a huge amp to drive it all.  I was happy for a while, but I really missed the deep bass that cars with trunk-mounted woofers enjoy.  I figured that this was just a handicap of driving a 914.

Well, while I was stripping my car’s interior for the trip to the paint shop, I “discovered” a foam footrest under the passenger’s carpet (Photos 1 & 2).  Immediately upon removing this foam, I thought of making a sub woofer enclosure using this foam as a pattern.  My good friend Gill came over, and we built a 914 subwoofer. 

Before you start construction, read the speaker manufacturer’s recommendation for the desired volume of the speaker enclosure.  The volume of the stock foam footrest is about 390 cubic inches, which is a bit smaller than I desired.  The manufacturer of my speaker recommends a minimum 518 cu in (0.3 cu ft) enclosure for best sound with a 6 ˝ in speaker, and 691 cu in (0.4 cu ft) for the 8 in speaker.  You can increase the volume of the speaker enclosure by gluing a slab of Styrofoam to the front of the foam foot rest before you start fiberglassing it.  You will have to hand-fit the edges of the added Styrofoam to the car’s interior contours.  A Styrofoam slab 1 inch thick will increase volume by about 190 cu in, ideal for the 6 ˝-in speaker.  A slab 2 inches thick will add roughly 380 cu in, just what you need for the 8 in speaker.  Whatever you add to the enclosure will directly reduce the passenger’s leg room, but the enclosure will resonate at the proper frequency.

1. The first step is to fiberglass the foam footrest using woven fiberglass cloth and epoxy (Photo 3).  Wear disposable latex gloves when working with epoxy to make cleanup easier and to avoid an allergic reaction.  We used 9 ounce/square yard woven fiberglass cloth, and epoxy from Bob Smith Industries, Atascadero, CA (www.bsiadhesives.com).  Gill used the extra slow (2-hour) curing epoxy because he works at glacial speed.  Don’t use polyester resin; it will dissolve the Styrofoam footrest. 
   1. Use a minimum of four layers of fiberglass cloth.  Cut the four pieces to rough size, and drape the first piece over the footrest from the back.  You will have to slit the cloth and overlap the corners to get it to conform to the irregular shape of the footrest. 
   2. Mix the epoxy resin and hardener in 1 or 2-ounce batches, and spread it over the cloth with a plastic squeegee.  Start from the center of the back of the footrest and work outward.  Spread the epoxy thinly over the fiberglass; the cloth needs to be wet and stuck to the foam, but you should not have any puddles of resin.  The weave of the cloth should be clearly visible.  More resin than this adds weight and cost, but no strength. 
   3. Wrap the cloth around the front of the footrest, covering at least the outer two inches of the front face.  You can use masking tape to keep the fiberglass from springing away from the front face. 
   4. Drape successive layers of cloth one at a time over the lower layers, and squeegee on more resin to wet the cloth.  The resin on each layer should be getting sticky by the time you add the next layer, which makes it much easier to hold the cloth in place.  You will need much less resin for the second through fourth layers than you needed for the first layer.  There should be no white (dry) cloth visible, and no air bubbles between the layers. Work out air bubbles with the squeegee. 
   5. We used a total of 13 ounces of epoxy for this project.  If you need much more than this, you are probably slopping it on too thick (Photo 4). 
   6. The fiberglass on the edge of the footrest’s face must be flat so the High Density Fiberboard (HDF) can be glued to it (Photo 5).  Assure the face is flat by laying a piece of Saran Wrap on a flat surface (like the HDF board), and laying the footrest, face down, onto the Saran Wrap.  The extra slow epoxy will feel dry in eight hours, but it takes a full 24 hours to cure at room temperature.

2. Once the resin had cured for 24 hours, I hogged out most of the foam.  To remove all the foam remnants from the corners, you can pour a cup of lacquer thinner or acetone into the enclosure and slosh it around.  This will dissolve the remaining foam.  Obviously, this should be done outdoors by someone who doesn’t smoke.  Pour the thinner out of the enclosure into a can, and use it to clean paintbrushes.  Don’t pour it down a drain.  Trim off the rough or dry edges of the fiberglass on the front face (Photo 6).  Leave 1 ˝ to 2” all around to glue the HDF to.

3. Then I cut out a piece of  3/4-in thick High Density Fiberboard (HDF) to match the front of the hollowed footrest casing.

4. I cut a hole for the 6 ˝” Kicker bass speaker in the fiberboard (Photo 7).

5. Glue the fiberboard to the fiberglass enclosure using silicone glue. (Photos 8 & 9)

6. After the silicone cures, fill the footrest with water, using a measuring cup.  This will allowed you to measure the volume of the woofer and check for leaks.  Seal any leaks with silicon or epoxy, because the woofer needs an airtight enclosure to work properly.  To convert liquid measure to volume, 1 gallon = 231 cubic inches.

7. Drain the water, dry the enclosure, install the speaker and wire it.  I purchased a speaker wire quick disconnect at my local automotive stereo shop. (Photo 10)

8. Cover the whole thing with felt.  I used spray contact cement. (Photo 11)

9. You need a separate amplifier with a low pass filter to drive the woofer.

10. For protection, I covered my speaker with a heavy duty speaker grill (Photo 12), and I keep my Porsche floor mats over the woofer.  The sound is not affected at all.  The single speaker works well, because low frequency sound tends to be transmitted and received from all directions, as opposed to higher frequency sound which the human ear perceives to be very directional.

11. Drive the car with the top off at around 85 mph, and crank up the stereo.  You should hear and feel the full range of music.  I like War and Earth, Wind, and Fire, and Gill likes Iron Butterfly and Dvorak.  One man’s music is another mans noise.  But, the 914 passed the top-off test with flying colors.

The woofer fits perfectly in the passenger foot well, and can be removed rather easily.  Oh, by the way, the speaker sounds much better (translation: louder) if I use the box as a woofer rather than strictly as a sub-woofer.  I'm sending the speaker frequencies from the 2500hz range on down.  A true sub-woofer would never see anything above 500hz. 

If I had it to do over again, I would add the 2 inch Styrofoam slab to the face of the foot rest, and make the much more affordable 8” woofer speaker fit.  The passenger may lose a bit of legroom, but what a foot massage! 

Bee Jay

Front view of the foam footrest block.

Laying on the fiberglass.  Position it dry, then squeegee on the resin.  Numerous cuts and patches will be necessary to cover the irregular contours of the foam block.

Back of the footrest after the fiberglass is applied, but not trimmed.  Note the weave of the fiberglass cloth is clearly visible if you don’t apply excessive resin.

Front of the fiberglassed footrest, before the opening is trimmed.

After trimming the opening.

Medium Density Fiberboard front face is cut to size.

MDF front face is glued to the fiberglass shell with RTV.

Smear RTV into the joint to assure an airtight enclosure.

Speaker and wiring installed for a test fit.

Felt is glued on with contact cement.

Figure 13

Figure 14

Finished speaker enclosure is test fit into the car before the carpet kit is installed.  Sturdy grill protects the speaker from the passenger’s feet.