The plenum has several restrictions to air flow:
1. The two openings in the front of the plenum are obstructed by excess material on
either outside edge and on the inner edges.
2. There are two small "ledges" that protrude from the floor just behind the TB openings
that obstruct air flow.
3. The runner openings are rough and rather abrupt and hinder air as it turns to flow into
the runners.
The runners are difficult to remove even with good tools. These are useful:
1. Metric box-end and open-end wrences through 20mm for plenum & fuel rails.
2. Low profile Torx T40 socket for 3/8" drive for lower runners.
3. Long Torx bits with T10 bit for TB throttle stop..
4. 3/16" ball driver for lower runners (yes they are a real PITA to access).
5. Magnetic wand for replacing lower runner Torx bolts.
The plenum and runners must be removed from the engine along to perform the porting.
1. Disconnect the throttle and TV cables from the TB and the plenum.
2. Remove the intake ducting from the front of the TB.
3. Unbolt the TB from the front of the plenum, remove all cables and vacuum hoses and
set it aside.
4. Unbolt the plenum from upper runners, disconnect all cables and vacuum hoses and
remove it.
5. Unbolt the runners from the intake base (It is necessary to remove the distribut cap to a
to access all runner Torx bolts.) and remove them.
6. Clean gasket material from the plenum, runners and intake base, taking care not to
get debris into the intake base.
The TB, plenum, runners and base will be coated with carbon buildup which should be removed before porting begins.
1. Use TB cleaner to clean the TB if it has a Teflon coating.
2. Carb cleaner and lacquer thinner may be used to clean the plenum and runners.
3. Carb cleaner on a cloth can be used to clean the base openings.
Air flows most easily if the transitions between chambers is as smooth as possible.
1. The TB gasket should match the TB openings or be slightly larger.
2. The TB openings in the front of the plenum should match those in the TB and gasket.
3. The openings in the plenum sides should match those in the runners.
4. The gaskets involved are used to size and align the respective openings.
Air flow through the stock runners can be improved by fluting the upper ends and removing the "dimples" inside the lower runners..
1. If stock size plenum runner gaskets are used, carefully open their radii by
about 2-3mm with a fine sanding drum on a Dremel tool.
2. Apply a thin coat of paint (Prussian Blue is prefered) to the upper runner
mounting surface around the openings.
3. Lay a modified plenum runner gasket onto the runner mounting flang and
hold it in place with the plenum mounting bolts.
4. With a sharp point (pin, Xacto blade, etc) carefully scribe the outline of each
gasket opening onto the runner mounting flang.
5. Remove the gasket and use a rough sanding drum on a Dremel to remove
material inside the runner mounting flang out to the gasket outline.
6. Now remove material from withing each runner to smooth the air flow path
down to the actual runner tubes.
7. When the rough work is finished, go over the surfaces with a fine sanding
drum on a Dremel.
8. The sanded surface can be smoothed further with Scotch Brite and then
with Aluminum polish.
9. Lay the lower runner tube on a piece of wood and lay a 5/8" socket on a 1/2"
extension on one of the "dimples".
10. With a small hammer tap on the extension until the "dimple" is flattened.
11. Use a fine sanding drum on the Dremel to smooth any uneveness inside the
lower runner openings.
NOTE: Frequently inspect your progress to assure a good gasket match.
1. If necessary open the TB gasket to match the TB throttle bores.
2. Paint the front surface of the the plenum.
3. Carefully align the gasket with the bolt holes and scribe the opening outline into
the plenum front surface.
4. Use a cutter bit in the Dremel to cut the plenum bores to the gasket outline.
5. Cut the two ledges from the plenum floor.
6. Cut back the passenger side inner wall as much as possible and blend with the bore.
7. Cut back the driver side inner wall as much as possible and blend with the bore.
8. Cut the center ridge down to remove obstrution to air flow.
9. Look into the plenum TB bore openings to assure that air obstructions cannot be
seen or have been mimized.
10. Paint the runner flang sides of the plenum.
11. Carefully align the proper gaket (they are marked L & R) with the plenum bolt
holes and scribe the gasket outline.
12. Use a cutter bit in the Dremel to cut the openings to match the scribes.
13. Flute the inner openings.
14. Use coarse sanding drums to smooth and blend all machined surfaces.