Intake Ports for a Non-Piped Engine
I am trying to build a 100cc twin cylinder opposed/boxer style model airplane engine. Because this engine is design to direct drive a propeller the rpms are fairly low (around 4000) so a tuned pipe would be pretty long and I want to keep the design simple (avoid having to build a pipe and to keep the engine cool). I was wondering about the differences in the porting of a piped vs. a non-piped engine. Since this is a mildly tuned engine I have read about using a restrictive exhaust. Since I don't have a pipe to help with scavenging I was thinking that the boost port (port directly opposite exhaust port) should be smaller to prevent fuel from escaping but this is just a hunch.
In the model airplane world, where tuned pipes are used, the common porting is that the intake timing is kept "normal" when adapting to a pipe (typically 130-145 degrees). However, the exhaust timing is cranked up from 165 or so to 180 or even 190 degrees. As far as port sizes, it always seems that the more port area you can safely use, the more power you can make. This is limited by how well the rings are supported in the bore.
I would keep all the porting numbers on the low side to begin with as you are looking at low rpms. Typically 130 or so degrees for the side ports and 135- 140 for the for the boost port. Work on the angle of the top of the port, trying to achieve as much vertical angle as you can. Keep the exhaust timing at 155 degrees or so. Then, of course, you need to look at crank timing, unlless you are going to go with a reed valve.
Good luck with your project
Originally Posted by durabol
Shoot me an e-mail if you like, and we can talk port geometry and timing.
Thanks for the responses. I think I will go with pretty typical "scooter" timing. For the title I should have wrote "Transfer Ports for a Non-Piped Engine" as that was what I was thinking about. I have an email group at http://groups.yahoo.com/group/DIY_2S/ for those who may be interested.