A cacophony of reasons I'm sure, but mostly we just know this from experience. Subaru's engine's (particularly the EJ) need to run a good 0.1 Lambda richer to stave off detonation. MOST of the contributing factors don't have anything to do with the cylinder layout... except one: boxer engines do not typically have a very long stroke and the rod ratios are larger which means the TDC dwell times are longer. In addition to the shorter stroke, the valves are usually larger reducing intake port velocity which also reduces fuel mixing in the cylinders. All of this means that EJs are a bit more prone to detonation and require more fuel to reduce charge temps and run happy.Why do they need to be tuned richer? I can't think of any reason boxer layout would need to run richer than say an i4 or v6. Is it just inadequate or late response from sensors?
Once you know this, it's pretty obvious why Subaru chose to design the FA differently - longer stroke, lower rod ratio, direct injection, and smaller valves/high velocity ports improve cylinder mixing. Piston oil squirters probably would have also helped. Subaru hasn't used oil squirters since the EJ20G in the 90s. It's a bit head-scratching why they continue to omit these. I suppose they have deemed the notch on the rod big-end sufficient for lubrication and cooling.
Correct. The heat issue was more about the NOx emissions. I feel that Subaru should have engineered oil squirters into the block. It's a curious omission. The G16E-GTS in the GR Yaris/Corolla has 9 piston oil squirters - 3 for each cylinder!The RA thing is definitely ironic. I can see hot pistons causing a nox problem but it shouldn't cause a HC problem. Far as I know, the only way to get oil jets is an HKS shortblock. IAG and outfront don't have oil jets listed on their site.