K, what is the difference between a 3, 5, 8 port engine?

As far as I know the 3 port design is generally a easily tuned and high torque engine, low grunt mostly used in RTR's and MT's for those reasons. The 5,8, and higher ports are generally used in racing applications due to their high tunability factor, you can generally adjust the tune on those kind of engines thru a 3 needle type carb. That allows you to tune the high end, mid range, and low end of the engines powerband and torque for the specific racing application. The number represents the actuall number of ports that are cut into the piston sleeve. 3port will naturally only have 3 holes in the piston sleeve where a 8 will have 8. The number of ports doesnt always mean more is better either, so dont base this info on that. There are many different engines out there that will range from 2 port configuration up to 11 ports as well. The ports are designed to transfer air from the crankcase, when the piston travels down, into the cylinder then the combustion cycle takes place. In the 3 port you will only have one transfer port for that pupose and one port for the air intake, where once the air is force into the cylinder the crank then pulls more air into the crankcase. And a 3rd port for the exhaust once combustion has taken place and the piston starts to travel down these gases will expell out that port. Now none of the ports are opened exactly at the same time, this is where the engine timing comes into play on when one is opened/close per the others, but I won't go into all that, but I will say that a greatly timed 3 port can outperform and poorly timed 8 port so don't assume more is better. Just some of your higher port engines such as the Italian ones tend to run on the higher side of the rpm/torque scale, does it mean they're better? It's just preferances, but remember this, the more ports means less cylinder wall which will equate to less time between rebuilds. I hope this didn't further confuse you.

[X(]Great answer!