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Home Porting Stock Cylinder Heads


Figure 1 - Understanding the Intake Port

Terms and Definitions

Bowl: The area below the valve seat (looks like a bowl!), sometimes called the pocket.

Carbide bit:
A cutting tool used for grinding metal- pretty much a necessity for working on Ford exhaust ports.

Cartridge roll: A cylindrical sandpaper roll used on grinding tools for porting. They come in different grits (roughness) like regular sandpaper sheets.

Floor and Roof:
With the head sitting as installed on block, the bottom surface of the port is the floor and the top surface is the roof.

Gasket match:
Using an intake or exhaust gasket to create marks around the port to use as a guide for increasing the size of the port opening.

Port:
This refers to the complete passage in the head (intake or exhaust) from the mating surface to the valve head. Sometimes will only refer to the opening in the mating surface.

Runner: Essentially the entire length of the port, from opening to bowl.

Throat: The area just under the valve seat in the bowl. It will typically have a sharp edge or lip left over from machining of the seat.

Thermactor bump (air injection bump, smog bump): In the exhaust ports of '68 and later heads there is a raised portion in the roof with a small hole in it leading to the air injection ports at the ends of the heads. I have seen heads with the bump but no hole in it. Needless to say, this bump is a BIG restriction.

Short turn radius: The part of the floor that turns to the valve seat. In the intake port it has the shape of a ski-jump.

Shrouding: When a valve is too close to a combustion chamber wall or cylinder wall the air flow is impeded and the valve is said to be shrouded.

Valve guide boss: The raised material that surrounds the valve guide.

Venturi effect: The physical property of moving air in a passage (pipe or port) increasing velocity as the diameter of the passage decreases. A true venturi actually increases in size after the restriction. Those cool velocity stacks on multiple Weber carb intakes operate on this principle, as do the boosters in Holley carburetors.

Intake Ports
Lets start with some concepts before grabbing the grinder. First, the Venturi effect. A true venturi is a short piece of tube with a restriction in the middle that accelerates the gas flowing through it. We are concerned with the change in size, preferably big to small. The opposite happens if you increase the size of the pipe or port, i.e. the flow velocity decreases. So, if we look at the drawing of the intake port cross section from the top, we see that the stock port is smaller at the opening and larger at the valve head. The incursion caused by the pushrod hole gives some venturi effect, but is really too far from the valve to be helpful. By opening up the port and cutting back the wall by the pushrod hole, we can at least make the port straighter, if not truly taper down, as it leads to the valve.

Once again, begin with scribing the gasket boundaries. Unfortunately it is harder to align the intake gasket for marking the ports since the bolts go in at an angle. The best way to determine where an intake gasket will sit when the manifold is torqued down, is to mock it up on the motor. Place the heads on the block, secured with a couple of head bolts hand-tight. Then lay the intake gaskets in place, don't use any sealer for this mock up, and don't the cork end seals (you should never use these anyway. Use silicone for the final installation.) Lay the intake manifold down, and torque it to spec. You will now be able to see the edge of the intake gasket stick up along the head. Place a mark on the head to indicate the top of the intake gasket. (It's also a good idea to note and mark the position of the intake manifold at this time, so you can gasket match it also.) Disassemble the mock up, and tape the intake gasket to the head using the marks you made for alignment. Then proceed to mark and scribe the gasket openings around the ports. The Fel-Pro 1250's "Printoseal" is a good template for the 289 and 5.0 (E7) heads, having dimensions of 1.20"x2.00". For the 351 head, or for a larger port, the Fel Pro 1262 offers 1.28"x2.10".

You should also check your intake manifold ports to be sure they are not going to be larger than your head ports (not likely with stock intakes, unless yours has been modified.) That is the worst mismatch, since the air/fuel mixture coming through the intake runner will hit the smaller head port. The opposite effect, a smaller intake port leading to a larger head port, is not a problem, and is in fact is how the intake and heads come from the factory.

After marking you can begin grinding to match your template. Keep the grinder moving up and down and in circles to ensure smooth, flat walls. Take material off of the bump caused by the pushrod hole. Use calipers to keep the width consistent top to bottom and port to port. Clean out casting ridges as far back as you can reach (the Dremel flex shaft tool works well here.)

Once all eight port entries are finished you can move onto the valve guide bosses. The intake valve guide boss stands out at 90 degrees to the roof surface, has a flat top with sharp edges, and is WAY too tall. Begin grinding the front edge of the boss keeping in mind three goals:

1. You want to lay the front side back to about 45 degrees,
2. You want to decrease the height by at least ,
3. The leading edge needs to be narrower than stock, but not sharp edged, more like a neutral lift foil (think bottle nosed dolphin or the leading edge of a sailboat keel.)

The short turn radius is another critical area. Again, you don't want to change it's as-cast shape, just remove ridges and any bumps. The turn allows the airflow to change direction gradually as it turns to go out the valve. The idea of a ski-jump is apt. If the radius is too small the flow will separate at the valve seat and become very turbulent, reducing flow. Professional porters can change this shape to improve flow, but since we aren't professionals, our advice is to not change its shape!

With the boss looking good the bowl is all that's left. Just take out the sharp ridges and remove the bulge in the side wall. Leave the intake surfaces at about an 80 grit finish. It will help keep the fuel in suspension. There is really no need to have a high polish in the intake port, it can actually cause fuel puddling


Finally, the combustion chamber can be cleaned up with possibly a little de-shrouding work. Looking at the diagram of flow through the intake and you can see how the valve head turns the flow to almost 90 degrees from its original direction down the bowl. As the valve opens the flow is moving out, towards the side of the combustion chamber, not down into the cylinder. If the combustion chamber wall is too close to the valve seat and standing up at 90 degrees from the chamber floor, the flow hits the wall and literally "stacks-up" until the valve is opened far enough for the flow to go past the wall. If the valve is big enough and the cylinder bore small enough, the cylinder wall itself will cause shrouding.Some porters like to smooth the chamber to a mirror smooth finish, others only to the same finish as the intake port. Removing sharp edges will help reduce the chance of detonation. If you want to try and open up the area around the intake valve (good idea on early 289 heads w/ 54cc chambers) the technique is to lay the wall back at a 45degree angle without changing where it meets the chamber floor. Don't be too aggressive in the combustion chamber, since any material you remove also increases the volume of the combustion chamber, resulting in lower compression. If you do extensive de-shrouding, be sure to check the volume of each chamber to achieve consistancy.


(Exhaust Port )

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Cross Sectional top view of the intake port. Notice how the push-rod hole pinches into the runner.
 

Intake Before
 

Intake After


If you look down the intake port and try to see the valve opening on a bare head it blocks most of the view. Looking down the port on the ported heads and you can easily see the valve opening.
 
Porting tips from Joe Mondello:

1. Shape the intake valve guide boss so it curves with the port and points towards the port opening.

2. Make the back of the boss (towards the bowl) taper to a point, like a teardrop. Look at the TFS head below for a perfect example.

TFS Twisted Wedge

 

 


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