How to Build a Fiberglass Plenum

1. I am not a Fiberglass expert, and this was my very first project.  I am of sound mind and judgment, and believe this is doable by ALMOST any one, but it does take patience, as Rome wasn’t built in a day! It will take 4 to 5 days depending on what end results you want with regards to “All out Show Piece” that gets you the awards or a “Race Car cold air Box” that just needs to be functional.

2. Don’t be Stupid! Most all this stuff is Flammable, and not good to breath, work outdoors in a ventilated shop.

3. Read up ONLINE on how to build “Lost Foam” Fiberglass Molds. This can only help you understand the procedures better like I did before I started this Project. I read 10 to 15 different other projects that pertain to relevant subjects like my project!

4. Basic tools are needed:

   1. Fiberglass Roller to roll out Air Bubbles. This is Critical tool don’t start a project without one!

   2. A couple of 1” throw away brushes. Cut ½ the Bristles of one of them as this is used to wet out/press the Mat.

   3. Rubber Gloves. Lots of these, hell get a box for your shop always handy to have!

   4. Quart of Fiberglass Resin

   5. 3 Bags of Fiberglass Cloth. Usually comes at any parts store in bags of 8 sft!

   6. Gallon of Acetone, for Clean-up of Fiberglass Resin

   7. Florist Foam from any Craft Store I used the Cheap GREEN stuff.

   8. 1 Quart of Bondo.

   9. 1 Package of 2 Part Spot Putty.

Start with a good solid but COVERED Work area. A 2’x5’ table covered with Cardboard is a good start!

Next you will need some foam to work with. I started with 12” x 36” x 2” Inch Block and 2 Cones I found at Michael’s Craft Store. I also bought a can of the Adhesive they had to go with it.

 Next I ripped the Sheet in half “length ways” to make 2 x 6” pieces. Then I ripped 1 in half to create 2 x 18” x 6” x 2” Blocks. Careful to see that the 2 blocks were Long enough to cover the Length/Width/Height of the Plenum that I needed!

Using the Adhesive I bought I coated both halves of the 2 Blocks and used the Plenum Plate “not shown in photo” to spread out the pressure and a Gallon of Oil as a weight to help set the blocks together. I let them set a couple of hours to be sure it was set good!

Next I set the Band Saw up at 30 degrees and set a fence to rip the corners off the Block. You can also just sand the edges off with SAND Paper.

The Results after I ripped the Corners off and hand sanded and beveled the edges:

Next I took the 4” x 15” Cones over to the Band Saw and Cut the Cones down about 12” from the small end to form “Flat Spots” on the sides. This could be done with a Razor knife and sand Paper as well, but the SAW is so much quicker.

Now you have to decide if you want 1 inlet or 2, if you want both on one end like Ii did, or if you want to feed it air from both ends? It’s all up to you and what ever room you have to work with. In this case I chose to feed it from just one end.

This Particular unit will be mounted with both inlets facing the Drivers side of the Car with dual Cold AIR Ducts to the Front side of Radiator in the lower Spoiler Region.

 I also cut and Placed a couple of Blocks under the Plenum Cover to get it up off the Table and Allow the bottom portion to be worked with Fiberglass. I also added a piece of Plywood to the small blocks so it would stand up by itself.

Now Wax the Plug with Carnauba Wax! ONLY CARNUBA WAX! Other types of wax can interfere with the Resin Chemical reaction Process! The Wax helps the Tape to not stick to the inside of the Polyester Resin.  

Now cut your Cloth and Mat if you wish to use it into strips and sheet about the size of one surface area of the Plug above. For instance the Back Bone was 18” x 6” so I cut a few of those and laid them up there to see if it cover the little low areas on each side of the Inlets, you need some over lap to make the Joints between each sheet you use will be stronger! You need small thin strips to fill in the divets from the different areas of the foam meet. Get your Table Set up with all your Cut strips and resin and tools!

Old Plastic Coffee Can makes a Great Rein Bucket! I mixed up ½ of the Quart of Resin per the instructions on the can, and it was a 78 degree day, so I mixed in just a little less hardener than it called for, because the hotter the day is the faster it set’s up! I then laid up my first layer of Cloth and Resin. Painting the Plug first then laying the Cloth into it and wetting it out completely! To me Cloth works easier than mat, but this could just be a beginner’s thing I am not sure! I also bought some RED Pigment just because I like RED, bit you do not have to have this!

The Little Fiberglass Roller I used is a ¼” Aluminum kind you can just see it in the Picture above in the old Gojo Can that has Acetone in it to keep the Tools Clean. Also comes in handy to soften the Resin as it hardens to keep it from sticking to the roller as you work the air out! If the matt wants to stick up, never fear, as it hardens it will get sticker, just keep rolling it out as it hardens. I laid up 2 layers on the first half a quart of resin.

Then I mixed up the other half and added 2 more layers of Glass and Resin, making sure it was overlapped well with my cloth. As a final Sacrifice layer to help smooth out the Finish, I added more hardener than the resin called for, then I laid on a thick quick layer of Resin to give me something to sand off to help level the finish.

Next day after about 24hrs of cure time, I took 80 grit sand paper and hit the whole thing to smooth out the surfaces. Without going into the Fiberglass cover very much I then mixed up a small amount of regular Bondo you can buy at any parts store!  I then started filling the low areas in the Glass and working them down. Don’t mix up a bunch of Bondo, only 3 Table spoons at a time, because you want to concentrate on working the areas, not getting it all on before it hardens.

Ruff up Plug!

Bondo and Sand!

When you have the Plug semi like you want it, it’s time to hit it with some “Sand able” Primer that is compatible with your Spot Putty! Ii found some at the Dollar General Store for $2.59 a Can!

Spot Putty and Sand some more!  When you have it like you want it, go back and hit the whole thing with another Coat of Primer!

 Now the fun Part!   Take the Part outside to a ventilated place and Pour half a cup Lacquer Thinner into some holes you jobbed into the Foam at an Open Area! It doesn’t take that much to work it’s magic! The Green foam turns to some Awful looking Bile stuff! Good Gloves come in handy!

  Process starting to work!

 Pour some into the Cones!

 Once you have all the Green Foam out, remove all the Tape from the inside as well! This takes some doing to get it all out! As you will see, it makes a damn sticky Mess!

 Set the Part aside and let it Dry for 24 hrs!

 After it has dried use the Plenum Plate to Mark the Plenum Cover for the recess area. Then use a die Grinder to cut out the Area to fit the Plenum Plate.

Check your Fitment and sand the inside edges by hand to get all the little shards of Glass out! It is easier to cut the opening smaller and sand it to size than it is to cut it to big and have to fix your mistake! Take your time and measure twice and cut once!

This Plenum cover was designed to cover up a Staged injection System. The inlet and Outlet for the Staged Fuel rail actually will exit the Plenum Plate via a set of –6AN bulkheads on the TPS end of the Extrudabody ITB system!

Also because this was for a Racecar I chose not to spend the extra time sanding and filling the Part so I chose to go with a Black Wrinkle Finish!

We hope this helps someone with a Project. Again the more time you take and the more you read before the Project starts the better you project will come out. It may take you a couple of tries to get it right, but it will come together in the end!