Introduction: Acrylic Air Intake Baffle Plug

About: A jack-of-all-trades, proteus, vagrant. If there is a job for it, he's probably done it.
In order to follow this instructable, you must have access to the following items:
  • Acrylic
  • A drill
  • A lathe
  • Screws
  • Gouges and skews
If you don't know what any of these tools are, just get a drain plug, or have someone who does have access to these tools to make it for you. I am willing to make these for people who are interested, but I will be charging through PayPal for materials and my time, so they will run about $40 a pop for a basic plug for a 3" intake port. Send me a message if you need one made and we'll work something out.

I (as of this instructable) drive a 96 Caprice. It's a retired cop car, and I love to tinker.

If you look under the hood of a Caprice, more often than not you'll see a big black "home plate" shaped piece of plastic on top of the engine.

This is an air baffle. Air baffles normally do a few things. First off, they quiet down the airflow a little bit (which is more noticeable when you have a turbo installed). After that, they tend to slow down the airflow into your engine. Slower airflow means it takes longer for particulates to collect and build up on your air filter, but it also means you have a little less power (generally speaking, the more air in and out of your engine, the more power you're going to get). The home plate really only serves one purpose though, to quiet out noise from the intake. There will always be debate on whether or not it robs you of performance, but even if it did, the difference would be so negligible that you probably wouldn't even notice it on a drag strip. (A good discussion on this topic can be found on the 9c1 forum post here:

Since I hated the look of the baffle, and I'm never one to turn away easy mods, I pulled the baffle and installed a drain plug from Home Depot ($3, if you're looking to save on coin).

It's basically two metal discs with a large rubber ring sandwiched between them, and a bolt running through to squeeze the rubber ring between the discs (causing it to expand). This worked fine, but I still wasn't pleased with the aesthetics.

Enter bullet resistant acrylic from TAP Plastics.

Step 1: Mark Out Your Circle

Using a straight edge (or whatever handy piece of flat material you have on hand), mark out lines from corner to corner on your acrylic block. Repeat on the other side. This will help you center the circle you want to draw, and help with centering the project for mounting on the lathe.

Next, measure out the diameter of your intake port that you will be plugging. Mine is a 3" intake port, so I will need to set my compass to the radius, 1 1/2". Push the point of your compass into the intersecting lines on the acrylic block, and draw out your circle. Now, add about a 1/4" to the radius measurement on your compass, we'll want a lip to prevent the plug from sinking all the way into the intake. Mark out the circle the same way. Flip the acrylic block over, and make the same circle on the other side, this (and the lines) will help you center the mounting plate on the acrylic.

Mount your plate onto the acrylic (I use a drill to make some pilot holes for the mounting screws) and jump to the next step to handle cutting off excess material.

Step 2: Cut It Down to Size

Now would be a great time to grab some safety goggles, because bits of acrylic are going to start flying everywhere. Don't bother taking them off until you're done with the project at this point, because it's only going to get worse.

Mark out some straight lines with your handy-dandy piece of flat material, at the corners of the acrylic. Obviously, don't mark them through your circles. Step over to a band saw and trim the corners off.

Removing the excess acrylic helps speed up the process of shaping it, and also helps prevent chipping and chattering since the corners are less abrupt. 

Step 3: Get Shredded Acrylic EVERYWHERE

Remember how you put those goggles on? Yeah. Now it's REALLY going to pay off. Mount your piece to the lathe and set up a tool rest next to it. Spin the piece by hand and move the rest fairly close in, but not so close that it hits any corners of the acrylic. I like to get about 1/4" from the biggest corner.

Grab a gouge, whichever size you feel comfortable with. If you're a beginner, a bigger gouge will help you keep a relatively flat surface as you shave material off. Hold the handle of the gouge tight against the side of your body and under your arm. Switch on that lathe and lets start working!

Slow and steady wins this race. If you try to take off too much material at the beginning, while it's still relatively flat, you may chip the acrylic or even catch the gouge and stop the whole thing from spinning, leaving a nasty mark on your piece. Be patient. You want to shave the whole material down to your outer circle you made for the lip of the plug. Watch the video below to see what the final rounding looks like, but let me warn you first: When you see me running my fingers along the mounting plate to get rid of acrylic shavings? Don't do that unless you're familiar with reattaching fingertips. I'm reasonably comfortable with with cleaning the spindle while it's still running because I know where and how to place my fingers without breaking them off. If you're new to lathe work, DON'T DO IT.

Ok, now it's time to cut down your material to the intake port diameter. Remember that inner circle you drew? You want to make a section of your plug that size. I used a skew to make a divide between the plug and the top, then switched to a point to cut off bits of material at a time. Watch the video to see how it's done.

Here's an example of the process halfway through getting the plug-end done:

Once you've gotten the plug-end smoothed out, you can either stop using a gouge now, or fiddle around a bit to make the basic plug look however you want it to. I usually add a bevel to the ends of the acrylic, and to both sides of the lip of the plug.

Now that you're really finished with cutting the shape, jump to the next step.

Step 4: Finishing and Parts List/cost

Once you've lathed out your plug to the correct dimensions, you're nearly done. Now it's time to polish it. Here's a secret for anyone who's never polished acrylic before: toothpaste.

That's right, toothpaste will work wonders for polishing acrylic, so there's no need to go out and spend money on polishing compound if you're not doing something like this often.
Grab a piece of scrap cloth, and put a dab of toothpaste on. It helps to wad up the cloth, since this is friction polishing there will be heat.

At this point, it would be a good idea to remove the tool rest, or move it out of the way on your lathe, You could end up catching the acrylic wrong and the last thing you want to do is slam your fingers into a piece of metal.

With the lathe spinning, hold the cloth firmly against the acrylic, making sure to follow the direction your piece is spinning. Remember, this will get hot , so watch for steam coming up from the cloth. If the toothpaste gets dry, or rubbed off completely, stop and put more on the cloth. Use a different section of the cloth if you have to. Again, patience here is a virtue. Watch the video to get an idea of how it's done.

Once you're done polishing, brush off the acrylic you've accumulated on yourself. You're done! You can take those goggles off now.

Remove the mounting plate from the lathe, and unscrew the acrylic from the mounting plate. Go ahead and clean off the excess toothpaste, and if you're ready to use it, pull off the paper protecting the finished sides. Mounting it into your intake shouldn't be any problem. Just loosen the hose clamp around the baffle port and plug it in! Take a minute to admire your work, then get out and drive. Brag to your friends. Or just be generally awesome. Or specifically awesome, if that's your cup of tea.

Parts list:
  • 1x Drain plug, ~$3.00 from Home Depot.
  • 1x 4"x4"x1.25" Bullet resistant acrylic block, $10.00 +shipping from TAP Plastics.
  • 1x 4"x4"x2" Super thick acrylic block, ~$13.50 +shipping from TAP Plastics.
Links to purchase
Drain Plug (Found on Amazon & Home Depot as "test plug"):

Acrylic block: - (bullet resistant) - (regular, thicker for more working area on a lathe.

If you want an intake plug made, but don't have the tools or knowhow, I'm willing to make custom plugs starting at $40 +shipping. Or you could check around and see if you know anyone locally who can do this for you. Happy modding!

Update: Here's the (basically) finished plug from the steps in this Instructable: