The general rule of thumb, back in the day, was that 2 litres, by way of 4 cyl engine size, was the limit for relative smoothness, acceptable smoothness. That was when balance shafts were not common, and when engines were oversquare... or at least when somewhat performance-oriented engines were oversquare. The 2.5 in my Mazda6 has, I believe, over 4" in the way of stroke. 'Real long; long strokes exacerbate roughness characteristics. Yes, we have balance shafts, but to the contrary - we have aluminum blocks... light weight, less mass. I keep going back to my '99 Camry 2.2 L4 5speed manual. Iron block, balance shafts, smooth as velvet. Only about 135 hp I believe, but honestly, the car drives so, so well (to the extent that I can't part with it). To many folks it's garbage by comparison. There's hardly a car made that I cannot make friends with... each has its own area of excellence in one or more areas or aspects. And believe me, I really am picky. And it's unfair to look at a late '90's Toyota, the Camry, in its (relatively) golden days and compare it to a modern-day car (for quality, anyways).