[Fhakya]

I would tend to believe JoeC's explantation as most plausible thus far. When I 1st read the post, I was formulating a guess that the overbore did 2 possible things (both emissions related) but purely speculation: 1. Larger bore unshrouds the valves and contributes to more efficient combustion, thusly lower emissions? 2. Larger bore helps to create a larger combustion chamber for lower compression? (this 2nd one seems flawed though considering the year implemented...and probably a few other factors.)

Just guessing,
-Mark