Excerpts from article by: Paul M. Pitcher
taken without permission
Original article available here
Porting Small Block Heads
The impetus to consider modifications to the ports of modern small block heads comes from the need to match performance manifolds to them. As a footnote, it is interesting that production exhaust manifolds on 1985 and newer M bodies (Diplomat/Gran Fury) have openings that are similar in size to the 340 HP manifolds of '68- '70 or '71. Thus, when these or 340 HP exhaust manifolds are mated to the 4323302 heads, a huge "step" exists at the transition between the head and the exhaust manifold.
Other performance-enhancing modifications besides port-matching manifolds include 1) removing the "steps" at the transition between the valve seat and the combustion chamber and at the transition between the valve seat and bowl, 2) smoothing the transition between the exhaust bowl and runner, and 3) polishing the exhaust bowl and runner. Each of these modifications will be described. A cautionary note is in order: modifications to the port configuration will not necessarily result in performance gains. Unless the home-porter has access to a flow bench for evaluation of modifications, attempts to extensively modify the port configuration should be avoided. Instead, simple line-of-sight smoothing and removal of gross obstructions to flow should be the objective. Keep in mind the direction of gas flow and consider inertia of the flowing gas as cuts are planned. That said, for intake ports, bigger is generally better.
One other easily solved problem arises when mating modern heads to early applications. That is the air injection port present on the late heads. Simply tap this opening with a 1/4"-20 NC tap and thread in a socket-head set screw to plug it.
Materials
The equipment needed by the home-porter is readily available and not prohibitive in cost. A pneumatic die grinder is essential. The "mini" designs, such as Ingersoll-Rand 307A offer greater maneuverability and control. Die grinders consume a lot of air, and a higher capacity air compressor is desirable. A selection of carbide burrs is required. The most useful configurations are: 3/8" cylindrical, 3/8" round nose, taper nose, 3/8" ball, and 1/4" round nose. One bit of each type is sufficient to do several heads. Sandpaper "tootsie rolls" are also needed to polish the ports after cutting is completed. These are offered in taper and straight cylindrical shapes. About 15 each coarse and fine grade taper rolls and 5 of each grade straight rolls are needed for each pair of heads. A mandrel is needed to mount the rolls in the grinder. Good lighting is essential. A dust mask and goggles are recommended as safety equipment. The total cost of this equipment is between $100 and $200.
Porting Procedure
Before investing many hours in grinding and polishing, take
the heads to a machine shop for cleaning and magnafluxing to
detect cracks. It makes no sense to invest a lot of time on a
cracked head, unless you're just doing it for practice. If you
end up with a cracked head, don't just scrap it, have it sliced
up with a power hack saw through the ports so that you can
observe the thickness of the walls in critical locations.
The first procedure for the uninitiated should be to remove
casting flash on the outside of each head to be modified. This
will provide the porter with familiarity with the equipment and
its characteristics. Use the 3/8" cylindrical burr
exclusively for this procedure, and save the wear on the other
burrs. It will take between 1 and 2 hours to remove all the
flash, including the flash adjacent to the pushrod holes, at the
"window" between the rocker gear side and the block
side of the head. There are 3 rules in using a die grinder: 1)
use two hands on the grinder at all times, 2) keep it moving to
avoid gouging the metal, and 3) take light cuts at high RPM for
the smoothest surface texture and best control. Be aware that
using the end of the cutter can result in the grinder spinning
out of control and damaging a valve seat, for example.
Next, the manifolds to be used should be matched to the port openings of the head. First, clean up the openings on the manifolds, simply straightening the edges of the openings and removing minor casting flash. Then bolt up the manifolds to the heads. Two bolts in each manifold-head junction are sufficient. To port match the heads to the manifolds, you'll need a shop vacuum sweeper, duct tape, and a can of spray paint. The procedure described here is an easier substitute for traditional gasket matching. Tape off each valve seat, except the one for the port to be marked. Wrap duct tape around the nozzle of the vacuum sweeper as a sort of gasket so that the nozzle will seal well to to the valve seat of the port to be marked. Now, turn on the vacuum, place the nozzle in the valve seat, and when flow is established, spritz some spray paint in either the exhaust outlet, or the carb mounting flange. Just a 2 or 3 second burst is sufficient. Proceed to mark all 16 ports the same way. Let the paint set for a few minutes, then unbolt everything. You should see nice, clear impressions of the inside edge of each manifold opening around each port opening on the head. You should also see that the "roof" of each port will not need to be cut very much at all to match it to the manifold.
To port match the head, simply cut to this line of paint. Now, blend back into each port about an inch to create a smooth transition between the port opening and the port runner. Use the 3/8" round nose cutter for this job. Do not be concerned about the large amount of metal that needs to be removed from the lower edge of the exhaust ports. However, do not try to create a straight line from port opening to the valve stem hole. It is likely that the water jacket will be cut into if this is attempted. Chrysler small block heads do not flow exhaust gas particularly well, due to a retrograde flow along the floor of the exhaust ports. This "riptide" sets up turbulence in the port and impedes flow. To minimize this phenomenon, leave as much metal as possible on the floor of the exhaust ports, particularly about 1" in from the exhaust manifold mounting flange. This will create a sort of dam which will help prevent the retrograde flow. When the experts modify small block heads for maximum performance, some metal may even be added in this area by brazing.
There are several obstructions present in the ports which can be dealt with to varying degrees. On the intake side, there is a bump in the roof of each port near the opening. This is a cast-in boss for one of the valve cover bolts. It can be completely removed without problems unless the bolt holes are inaccurately drilled.
If the holes are not centered over the web between adjacent intake ports, it is likely that the hole will be penetrated attempts are made to completely remove the boss. There is a "dog-leg" in the intake ports resulting from the pushrod hole. The wall of this obstruction is only about 1/8" thick on the '85 up heads, so it cannot be completely removed. However, some of it can be cut off to widen the port at this point. Do not be too aggressive, and eyeball the pushrod hole to get an idea of where you are. On earlier heads with the small pushrod hole, more of this obstruction can be safely ground off. For intake ports, the floor is critical to flow. Take care to clean casting defects from this area and the walls. Do not polish intake ports, the roughness helps to keep gasoline vapor in suspension by creating turbulence. On the exhaust side, there is a bump in the roof of each port. Do NOT try to remove this obstruction completely, or the water jacket will be penetrated. Similar to the intake dogleg, however, some metal can be removed from the highest point of this bump to make the exhaust port taller. Do not be too aggressive here either.
Now that you've spent 8-10 hours on the grinder, you're developing some real skill in controlling it. You're ready to start cutting in the bowls themselves. This is the touchiest part of home- porting because of the supreme control and concentration needed to avoid gouging a valve seat and ruining the head. Make sure the burr is fully inserted PAST the valve seat before powering up the grinder. In other words, do not try to pass a spinning burr through the valve seat opening.
There are several areas that need cutting in the bowls and chamber. The heaviest cutting is to be done at the sides of the exhaust bowls, where they transition into the runners. First, feel the transition at the inside edges of the two exhaust bowls nearest the center of the head. If you were to cut the head in two, making two halves with two chambers on each half, the areas referred to here would be immediately adjacent to that cut. Compare the feel of these ideal profiles to the transition between bowl and runner in the rest of the the bowls. Feel the difference? The rest of the bowls have rather large bumps at this transition which may or may not be removed, using any burr with a round nose. My opinion is that these bumps represent obstructions to exhaust flow and should be removed, but this opinion is NOT based on flow bench results. Next, spend some time in the intake bowls with the 3/8" ball burr to remove edges where the valve guide was machined. Do NOT try to remove this obstruction, just round the corners off.
Probably the greatest single improvement to flow that can be made is to cut off the parting line that runs through each bowl. This parting line is parallel to the deck surface of the head, and is present at the transition between valve seat and bowl. There is a prominent corner where the outside edge of the exhaust valve seat transitions directly into the floor of the exhaust port. Cut this back so that the profile is a smooth curve. Not much metal is to be removed, but removal of this corner is one of the most important things you can do to improve the flow characteristics of small block Chrysler heads. Be VERY careful to not damage the valve seat during this procedure. The cylindrical burrs or the taper nose burr work best for this cut.
The final cut to be made is the most dangerous to the valve seats, and should be reserved until the porter's skills are maximized and NOT be tried at the end of a long day. A tiny amount of metal is removed in this cut, so fine control of the cutter is essential.
The cut is made to remove the step that exists between the valve seat and the chamber itself. Some heads have very little metal to be removed in this area, other heads have a 0.040" to 0.050" step. The step is most detrimental to flow in the shrouded areas of the valve (302 heads). Use the taper or 1/4" round nose burr for this cut and do not try to remove the entire step with the cutter, come back with a sandpaper roll to take off the last few thousandths and finish the surface. During this cut, smooth the vertical parting line which exists at the front and back edge of each chamber. In high performance applications with radical cams, it may not be advantageous to remove this step from the intake side, because it restricts flow at low valve lift which is beneficial. If there is some restriction of flow at low valve lifts, intake charge velocity and inertial filling will be greater as the intake valve continues to lift, resulting in a greater intake charge overall. Since CFM is very small at low valve lifts, a change in flow at this point has a smaller potential effect on the total intake charge than a change in inertial filling at higher valve lifts.
OK, now the cutting has been completed, you're ready to switch over to sandpaper rolls to finish the surfaces. Start with the exhaust port runners to get a feel for how the grinder behaves. Use a coarse cylindrical roll to finish roof, floor, and sides, and a coarse taper roll to finish the corners of each exhaust port runner. You can do some slight smoothing of the intake port at the port opening, just to take off your mill marks from the previous cutting. Proceed to finishing the exhaust bowls, using taper rolls, or cylindrical rolls that have a round end as a result of use in the runners. Take off enough metal to completely remove the cast texture from the mold. Watch the valve seat as you polish to give the best avoidance of contact of the sandpaper rolls with the valve seats. After you've done everything with coarse paper, switch to fine and start over. Expect to spend almost as much time polishing as was spent cutting.
The last thing to be polished is the transition between the valve seat and the chamber. You won't be able to polish in the shrouded areas of the chamber, but try to take off mill marks from previous cutting in the rest of the chamber, and take off any sharp corners that may exist. The margin of each chamber can be rounded off slightly to prevent hot spots and preignition.
OK, now that you are finished porting, all that remains is to have the valve seats ground. The three-angle valve job is a standard in performance heads. You might need to come back with sandpaper rolls and remove any edges that arise from the seat grinding procedure, but for all intents and purposes, you're ready to hit the streets and kick some Brand X butt! The beauty of this procedure is that it is completely inapparent unless the engine is torn down. The Mopar Engines book says that the home porter can expect improvements from 5-10 HP, but that the potential exists to get 50 additional HP out of the 302 heads!