David,
Please check out this:http://www.rcuniverse.com/forum/m_64...tm.htm#6451209.
I am not a expert in motor tuning so I could not explain the sub-induction porting problem. I remember reading about it when tuned pipes became popular. I do remember that the old flow-through mufflers were used for the .23's and folks had problems when "closed" mufflers became mandatory. Maybe some of our more learned friends here could explain this better than I ever could!
My experience with the .18 has been with a Herr Extra and I was very disappointed with a 8/4 prop. It runs ok, just not very powerful. For $10 more, I could have bought an OS .15CVA. However, since you only paid $30 for yours, try it on a stand and see if yours has the power that you need.
Keep up the good work!
Frank