Jack,

You've had a lot of excellent suggestions so far. May I add one or two?

Depends on what you want to use the engine for, for one thing... Extreme power is usually pretty harsh on an engine, and durability will likely suffer. If you're going to use the engine for something flat-out, where the increased wear and fuel consumption are normal factors to live with, "flowing" the port passages might help.

If you want to have a reliable, powerful engine that will stay with you a while, perhaps the most extreme cutting isn't where you should go. Maximum-flow capable engines can get very twitchy and unpredictable to operate. If you want to fuel up, hit the prop with the starter and fly, a pretty near plain vanilla engine will do you a fine job.

Also, excess flow capacity IS possible. For certain uses, keeping the flow velocity high - to keep the fuel and air well mixed and turbulent for an optimal 'burn' - can be more important than the theoretical possibility of getting maximum volume up to the chamber. ...which doesn't help if it isn't swirling hard enough to burn really well. To keep the charge velocity high, smaller passages and some surface roughness inside them help.

As one responder mentioned, if you do carve on the passages, settle for smooth, not highly polished.

You mentioned that the case passages were smaller than the cylinder ports? the 'edge' where the flow opens out through the port might maintain some part of the desired turbulence...

If you have the owners' manual, check out the rated power @ RPM info. The cylinder ports are not the only elements in an engine's volumetric efficiency. Changing only one part of the system may or may not make a positive, beneficial change. From what I've seen, few recent engines are port-limited - schneurle porting allows MUCH more charge transfer than the old crossflow baffled piston layout. Almost 3/4 of the sleeve diameter is used for charge transfer, and exhaust uses much of the remainder. Flushing that much of the cylinder with cool fresh charge is also a great aid to cooling... Used to be only about 90° (each) of cylinder circumference used for bypass and exhaust ports...