How to Port Your Plenum

Last Updated - November 28th, 2004

Porting your plenum is one of the cheap do-it-yourself mods that will give you power you can feel.  You'll notice a difference when you see your RPMs climb beyond 3500 and feel your car continuing to pull hard.  Below is an example of what Mike did to the plenum on his IROC.  This is only one way to do it and represents one of the more extreme portings out there.  A variety of tools could be used.  There are also many opinions with respect to what to and not to do.  That in mind, look at the pics, and make your own decisions for how to port your own.


Time Investment (Effort):  6 - 10 hours
Time Investment (Project Duration):  2 -3 days due to drying time for epoxy and paint

Go to Pictures

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Click on a picture to enlarge it.
After the screws are removed and the halves are pried apart, your plenum will look like this.
Closeup of the top of the plenum
In the top of the plenum, the ports neck down a little as indicated in yellow.  This can be opened up a little, and the center divider can be removed.  Removing the ribs on the top is fine as long as you don't make the top too thin.  In that case, it will flex and could fail.
Closeup of the bottom of the plenum
On the bottom, the center divider can be removed and the openings to each port can be smoothed out.  The neck also has a bump in it that can be removed.
If' you're doing anything more than a mild smoothing, you may well cut through somewhere.  JB Weld epoxy can be used to build up the anticipated thin spots before porting.
One great way to build molds for the JB Weld is out of masking tape.  Rig it up so that the sticky side of the tape comes into contact with the JB Weld.  Doing it this way will allow you to easily peel the tape off later.
Mike built up V shaped barriers in order to create a lot more material under the ports of the lower pleum.  This was inspired by "Nitetrain's Wild Porting Pics" that you may have seen elsewhere on the net.
Here's a closeup of one port and the V build up
Because Mike was going all out, he decided to build up blocks around the top ports as well.  The only problem with this is that air bubbles get into the JB Weld giving you a less than smooth finish.
The neck gets a little thin on the bottom where it bumps up and can also be build up with JB Weld.
Here's a top shot of the upper plenum.  Note that the top of the neck has also been filled with JB Weld.
After the application of JB Weld, an orbital sander was used to put a nice smooth finish on the flanges.  Note that when porting, the edges of the circular opening should not be flattened.  The trasition to the lower runners should be as smooth as possible. 
Sanding drums of various sizes are good for porting however you can't get too many different grits.  Mike discovered that you can tap a band of sand paper directly to the drum and then coil the paper around the drum.  This works great.
Here's the finished inside of the top half of the upper plenum.
Here's the bottom half.
Here's a closeup of one of the upper ports
Here's another upper port.  All of the ports should be similar in shape as possible.
This is one of the lower ports.  All should be ported as equally as possible.  Note the gray patch where Mike cut through into the JB Weld.
Here's the finsihed painted plenum installed on the car


1.  Remove the plenum from the car.
You may want to cover the openings of the lower plenum so that nothing accidentally falls in.  It is truely amazing how easy it is to drop a bolt down one of those runners when you aren't trying to do it.  If you're going to have the upper plenum off for awhile, you also don't want critters getting in there.  A paper towel can be ripped in half and each half then folded and stuffed into one runner.  Alternatively, you could cover each side with aluminum foil.

2.  Remove the screws from the bottom of the plenum that hold the halves together.  Be careful not to strip the screws.

3.  Separate the halves of the uppper plenum. 
To do this, you'll want to have two or three flat blade screw drivers of different sizes.  You do not want to damage the plenum but do need to get a screw driver between the halves so you can start prying it apart.  RTV is all that holds this thing together once the screws are removed.  It's probably best to start between the ports on one of the sides where there is plenty of sealing surface area.   Once you get one screw driver in, slowly work your way around until the top and bottom pop apart.  Don't pry so hard on any one area that you bend the plunum out of shape.

4.  Figure out what and where to port.
Your goal is to visualize air flowing through the neck and to the ports.  You want air to be able to enter the ports as easily as possible and you want the same amount of air to go into each port, otherwise some cylinders would be leaner than others.  We could get into a lot of theory here, but the bottom line is just to think smooth.  You don't want any rough lips or bumps to impede air flow and you want air to easily be sucked through the upper plenum into the lower runners.

Center Divider
Several years ago some people got excited about removing the center divider and reported that they lost some low end torque.  If this is truely the case, the losses are minimal and more than compensated for by the gains on the high end.  Mike didn't notice any losses when he removed the divider on his IROC and would deem it foolish to go to the trouble of porting your plenum without removing the divider.  If you're still really paranoid about removing it, at the very least, you should knife edge the ends and the front and rear of the plenum.

Lower Ports
For the lower ports, you want to open up the material leading to the port and provide a nice smooth bend down into the lower runner.  Make certain that you do not flatten the sides of the port opening as that will actually impede flow.  Unless you are also porting the openings of the lower runners to match, resist the urge to make the upper plenum port openings any wider.  Those points in mind, look at the port and consider how air would flow through it and how you can ease that flow.

Upper Ports
The upper ports are not surrounded by much material however you can always build up the thin areas with JB Weld.  You don't have to go to the extreme shown in the pictures above.  The upper ports are slightly restricted where the top of the plenum meets the port.  That area can be opened up a little.

Plenum Neck
If you upgrade to a 58mm Throttle Body, you'll need to open up the neck to take advantage of it.  There isn't much material to remove in a few places along the neck so be sure to build it up with JB Weld before you try to enlarge it.  With the smaller throttle bodies, you can just smooth it out.  There is a bit of a bump on the bottom half of the neck that can be smoothed.

Upper Ribs
Removing the upper ribs may or may not gain you anything.  You may wish to remove them all or remove them in strategic areas only.  If you remove them, make sure to be careful not to make the top any thinner than it already is.  One guy found that after porting his plenum and also sanding the Chrysler logo off the top that this plenum actually flexed when he hit and let off the throttle.

5.  Clean up the outside using a drill with a wire brush.

6.  Build up expected thin areas with JB Weld or some equivalent steel epoxy material.
Build barriers for any place that you want to build up out of masking tape.  Rig it up so that a layer of tape has it's sticky side toward the JB Weld.  Otherwise you won't be able to get the tape off the JB Weld later.  Mix the epoxy on a piece of cardboard (like the JBWeld package) and then pour/scoop/drizzle/spread it where you want it.  Let it harden over night in a reasonably warm area (not your garage in the winter).

7.  Grind out the center divider and upper ribs (if desired) with an angle grinder. 
Be VERY careful not to gouge the top of the plenum or the bottom trough.  If you gouge it too badly, you'll be buying another plenum.  Your goal is to remove this material only down to the surface below it.  Do NOT think for 1 second that you are going to do this with a Dremel or equivalent rotary tool.  If you insist, you'll be going out to buy a grinder 6 hours later so that you can fisnish the remaining 80% of the job.  This is going to make a lot of dust so you may want to consider coating everything with a liberal amount of WD40. 

8.  Open up the ports. 
There are a number of things you can get to open up the ports.  This is where a dremel or a die grinder will come in handy.  Multiple 3 or 4 dremel cut off wheels can be put on a Demel at the same time.  This gives you a wide blade that you can use to quickly remove material.  Another option is to use an electric drill with sanding drums.  Sanding drums come in 1/2", 3/4", and 1" sizes which allow you to sand out material around the port failry easily.  For polishing later, you can tape a strip of fine grit sand paper to the drum and coil the paper around such that the coil tightens when the drum spins.  This is a very handy technique.

To finish the ports, take one of those felt Dremel discs and cut a circle of sand paper so that it hanges off by about 1mm.  Put the Dremel screw through the sand paper and felt disc and use that to smooth/clean all of the curved areas of the ports.  You'll go through a lot of sand paper circles, but the results look very good.

9.   Smooth or port out the neck.
Smaller flap wheels work very good on the neck. 

10.  Smooth out the trough in the lower half of the plenum.
Large flap wheels work great for this.  Work with the plenum in one direction and then turn it around after a bit of sanding.  Try to avoid letting the flapwheel rest against one side of the trough otherwise you'll end up with a "W" shape in the bottom.

11.  Smooth out the top of the plenum.
Assuming you cut out the ribs and the divider, you can hit this with an orbital sander.  That will do everything but the corners.  For the corners use the felt pad/sand paper trick metnioned in the ports section.

12.  Clean up the mating surface between the halves of the plenum with an orbital sander.
The goal is just to remove any existing RTV and make everything smooth for re-sealing.

13.  Clean all of the dust and debris from the plenum and dry it. 
Soapy water actually works just fine for this.  Just make sure you let it dry out before proceeding.

14.  Apply RTV sealant all the way around the perimeter of the plenum.
Standard blue RTV will work fine here. 

15.  Screw the plenum back together and let the RTV dry.

16.  Paint the plenum if desired.

17.  Reinstall the plenum and enjoy your new found power.

Tools & Supplies:

You can do this job with a wide variety of different tools.  If for instance you have access to a metal shop, there are far better machines available for the job.  If you're someone sitting at home with a common set of tools at your disposal, the methods listed above work pretty well.