Next the valves are enshrouded....
Next the valves are enshrouded. On an SR20, this makes a big difference on the exhaust side of the combustion chamber. First, a machinist-marking die like Dyechem is painted on the head. You could probably also use a Sharpie.
The high-port SR20 head like ours has pretty decent port angles. The highly placed intake port has a straight shot into the cylinder. To pick up major gains in flow, extensive work has to be done up in the port splitter area where the intake port divides into two passages to miss the valvetrain's hydraulic lash adjuster. This is a major weakness in the older SR's intake port. Unfortunately, this is a difficult area to port and it's best left to an experienced pro. Fortunately, good gains in port flow can still be had with some easy to do porting tricks that generally work well on any head. Although we're working an SR20 head, these tricks can be applied to any cylinder head with a great deal of success. Typically, these tricks can net you 5 to 15 hp on a four-cylinder engine.
To do this, we want to round off the floor humps in the port. Usually, there's a sharp edge on the short side radius of the port, as it turns downward to the valve seat. The idea here is to break this edge and make it smoother. On our SR20 high-port motor, the edge is right at the valve seat. With the low port, it's further down in the actually turn of the port itself. You can easily feel this sharp edge if you touch the port. Use a slightly longer shank die grinder bit and break the edge. Go slow, stop frequently, and feel what you are doing. You should only take out about 0.06 to 0.08 inch of material. Go too far and you can make things worse. You want to keep the basic shape of the port but just to make the edge a smooth radius. The port will still look stock. Repeat this process on the exhaust port.
A head gasket is placed on...
A head gasket is placed on the head.
The next step is to blend the valve seat into the port itself to remove the step between the seat and the port. Using your die grinder, blend the steel valve seat smoothly into the port wall. This usually slightly opens up the valve seat. Once again don't take off too much material; it doesn't have to look perfect or be perfectly gapless. Be very careful that you don't over cut the soft aluminum and undercut the valve seats. When you're beginning to learn how to port, it's better to take out too little than too much, remember you can't put the metal back in. Typically on an SR20, the exhaust port valve seat has a more ragged match to the port itself and requires a little more grinding to make a smooth transition.
This next step is optional. Take your intake manifold gasket and scribe a line into the head of the outline of the port shape. Do the same on the exhaust. Now you can blend the port into this scribed line on the intake port and about 0.03 inch inside the line on the exhaust port. Blend the port at least an inch down into the port. This is called match porting. Match porting doesn't help flow that much but it doesn't hurt.
The border of the fire ring...
The border of the fire ring is scribed onto the head surface.
Once you've finished in the ports, lay an old head gasket on the deck surface of the head and scribe a circle around to mark where the head gasket's fire ring sits. Now in the area of the head where the edge of the combustion chamber comes within a quarter of the diameter of the valve, you want to remove material from here to the edge of the gasket fire ring or until you reach a quarter of the diameter of the valve. This operation is called valve unshrouding and it helps flow coming into and going out of the ports. On the SR20 this helps a lot, especially on the exhaust side of the head. The trick is again to take the least amount of material out as possible because this also reduces compression. On an SR20, this is only about 0.05 to 0.06 inch of material.
 The scribed line shows how...  The scribed line shows how far you can remove material to unshroud the valves before you undercut the head gasket. |  The combustion chamber is...  The combustion chamber is opened up to the gasket line. |  Here's what the chamber looks...  Here's what the chamber looks like after it is unshrouded. Remember, this reduces your compression ratio so you don't want to go overboard and grind more than necessary. |
 After grinding, you might...  After grinding, you might want to polish the ports and combustion chambers. To do this, use 220-grit cartridge rolls and a mandrel with your die grinder. |  Polish and smooth the por...  Polish and smooth the ports. | 
This looks nice! |