In general engine designs a square is the best all round design. Any up or down from square produces plus and minus to the extreme. Longer and shorter rods and the piston pin has to shift up or down in the piston. the perfect combustion design would be a Full Hemi. What is a full hemi you ask. That would be a dished piston and dished head design. The trouble with this is low compression. Flat pistons are cheaper and give better compression. The perfect design would be a clam shell hemi with a shallow dished piston and a shallow dished hemi head to get the compression up. the object is to keep the ignition and force as close to center and exsert the most even pressure in a 360 degree radius and not have the glow plug hit the piston.