I've got 2 OS .55AXs. I've had them mounter in several planes, always sideways. They run great. As a previous poster said, they're the best OS ever made. Just don't set it too lean on takeoff. If you're leaving a slight smoke trail, that's perfect. It will lean out during flight as fuel burns down.
I do recommend a castor blend fuel for OS ABC (or whatever) engines. I use Omega 10%.
They also respond well to a Jett tuned muffler. I gained 1,700 rpm on an 11-6 APC. 1,500 on the 11-7.
Edit: Correction, I did have a .55 mounted upright. I had one on a Tango, the one in the Avatar. Great little airplane.
Last edited by Ed_Moorman; 08-14-2013 at 12:02 PM.