It's likely the open chamber (no quench) causing the problems more than the compression.
Wayne Smothers DVD on how to supertune your engine talks about the same issue. It is no so much the compression ratio, but the open chamber is allowing multiple flame fronts which collide causing pinging.

Link to Video on Amazon:
http://www.amazon.com/Performance-Engine-Building-Carburetor-Tuning/dp/B002QWJ1DM