Well, it depends a lot on how the porting is done and who is doing it. There are varying degrees to porting a head. Nearly any engine can benifit from bowl work and combustion chamber work as these areas tend to suffer from mismatched machining or casting issues the most.
I sometimes work at a shop that does this sort of thing so let me see if I can sort of give a basic "porting 101" here. Mind you, the owner of the shop might see things differently, but this is how I understand it.
In the ports, you are typically interested in two things. Flow and velocity. On a 4 valve head you see 'tumble' into the cylinder rather than 'swirl' so I won't go into port design considerations from a cylinder mixture viewpoint.
You want to see the most of both with an emphasis on one or the other depending on your application. On a street engine, you want to concentrate on velocity as it is the air speed that does you the most good. Air has mass, or weight, and as such can be used to 'push' itself into the cylinder. While the engine is running, the air is not constantly moving, it is stopping and starting as the valves open and close. The faster the air moves through the port, the better it will keep that momentum and 'stack up' behind the valve when it shuts. The next time it opens, there could actually be a little bit of extra pressure there waiting to enter the cylinder. On a high performance racing engine this can be used(along with intake and exhaust runner tuning) to exceed 100% VE in the cylinder. Most street engines are lucky to see 80% VE(volumetric efficency, or the ability to fill the cylinder completely with air and fuel). Anything you can do to increase flow without decreasing velocity should get you more power at any rpm. This is where bowl work and combustion chamber work comes in. The bowl is where the air turns the corner right at the valve. Often times, this area has mismatches between the port and valve seat, or machining ridges, or some other issue that causes a loss of flow. These can be smoothed and shaped ever so carefully to net nice gains without slowing the airspeed down. One must be very careful here, especially on the short side radius as a mistake can cause a loss of performance. In the chamber, one might find that the as cast shape blocks the airs ability to get around the valve perimiter into the cylinder. This is referred to as shrouding. When the valves are unshrouded, some material is removed around the seats in the chamber to aid cylinder filling. This also increases flow without hurting port velocity. Same story on the exhaust.
If you were to get a full port, then you would have the entire port worked on. Depending on the goals for the engine, this could mean several things. Could just mean a light smoothing of the walls to eliminate casting flash and excessive roughness, or raising the roof of the port or a complete reshaping and even relocation. All depends on what else is being done and the environment the engine is being designed to operate in.
Typically, you would not want the intake port polished. The reason for this is fuel atomization. If you notice the rain on your windsheild, it collects in droplets that merge into larger droplets and then blow off in big blobs. Imagine that is fuel in your port. Doesn't do much for mixing it well. Now imagine the rain on a concrete wall. It tends to spread out more evenly into a thin sheet and is then able to get picked up by the passing air more easily and evenly. It's a small difference in power and efficency, but measurable.
Polishing the exhaust is perfectly ok, and even a good idea. It is less likely to hold carbon deposits, reflects heat back into the exhaust which serves to keep exhaust gas velocity higher which in turn allows for better scavenging of spent gasses out of the cylinder, and helps keep heat out of the head which has to be removed by the cooling system.
Polishing the combustion chamber helps reflect heat back into the cylinder and increases thermal efficiency. Since only about 30 to 40% of the heat energy our engines create actually gets used to drive the wheels, any help here is benificial. The rest of the heat goes out the exhaust or is removed through the cooling system. Polishing the chamber also helps keep heat out of the head which helps keep the oil cooler theoretically helps the engine live longer.
This heat management is the reason thermal coatings are becoming so popular. It is now even an OEM process for several auto manufacturers to thermal coat pistons and other parts.
So what you do with your head is your decision, but if you decide to get it ported, your porter should be asking you a whole lot of questions about what you want to achieve and what other mods you are doing. If they don't, find someone else. As with most performance modifications, one size does not fit all. Also, as Wedge and Tercel GTS touched on, you would see better results in conjunction with other mods. If you have a bone stock exhaust and intake tract you might not be able to feel any substantial gains with porting since the biggest restriction might be before or after the head.
At this point I'll plug that shop I work for...
www.replikamaschinen.com
By the way, I'm the one called "Electric Bob" wearing a red shirt at the booth with Don in the pic on the home page. I'm so famous...
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